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comparison between pointer and integer这个问题
weixin_42106225
2021-04-01 09:04:30
warning: comparison between pointer and integer
i +=strlen(&remain_text[index][0]);
这是报警告的代码段,要怎么改正才能消除警告啊
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comparison between pointer and integer这个问题
warning: comparison between pointer and integer i +=strlen(&remain_text[index][0]); 这是报警告的代码段,要怎么改正才能消除警告啊
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源代码大师
2021-05-03
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C和C++完整教程:https://blog.csdn.net/it_xiangqiang/category_10581430.html C和C++算法完整教程:https://blog.csdn.net/it_xiangqiang/category_10768339.html
赵4老师
2021-04-02
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i +=strlen(remain_text[index][0]);
自信男孩
2021-04-02
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remain_tex这个是怎么定义的?建议提供更多信息吧
remain_tex的类型是不是是int类型?
forever74
2021-04-02
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题目所述疑似不是这行的问题, 而这行正确与否取决于数组的定义。
qzjhjxj
2021-04-01
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i +=strlen (remain _text [index]);
Itanium Architecture For Programm
er
s
Preface Acknowledgments Trademarks Chapt
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1. Architecture and Implementation Section 1.1. Analogy: Piano Architecture Section 1.2. Types of Comput
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Languages Section 1.3. Why Study Assembly Language? Section 1.4. Prefixes for Binary Multiples Section 1.5. Instruction Set Architectures Section 1.6. The Life Cycle of Comput
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Architectures Section 1.7. SQUARES: A First Programming Example Section 1.8. Review of Numb
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Systems Summary Ref
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ences Ex
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cises Chapt
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2. Comput
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Structures and Data Representations Section 2.1. Comput
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Structures Section 2.2. Instruction Execution Section 2.3. Classes of Instruction Set Architectures Section 2.4. Migration to 64-Bit Architectures Section 2.5. Itanium Information Units and Data Types Summary Ref
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ences Ex
er
cises Chapt
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3. The Program Assembl
er
and Debugg
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Section 3.1. Programming Environments Section 3.2. Program Development Steps Section 3.3. Comparing Variants of a Source File Section 3.4. Assembl
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Statement Types Section 3.5. The Functions of a Symbolic Assembl
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Section 3.6. The Assembly Process Section 3.7. The Linking Process Section 3.8. The Program Debugg
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Section 3.9. Conventions for Writing Programs Summary Ref
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ences Ex
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cises Chapt
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4. Itanium Instruction Formats and Addressing Section 4.1. Ov
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view of Itanium Instruction Formats Section 4.2.
Integ
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Arithmetic Instructions Section 4.3. Bit Encoding for Itanium Instructions Section 4.4. HEXNUM: Using Arithmetic Instructions Section 4.5. Data Access Instructions Section 4.6. Oth
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ALU Instructions Section 4.7. DOTPROD: Using Data Access Instr
Microsoft Library MSDN4DOS.zip
MSL 即 Microsoft Library 是 DOS 版的 "WinHelp",也就是现代版 Help View
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的始祖。 安装目录下有个 ini 文件,用来指定图书的路径,它即是目录。 文件来源自 http://wdl2.winworldpc.com/Abandonware%20SDKs/Microsoft Programm
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's Library 1.3.7z Microsoft Programm
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's Library 1.3.iso 这就是 DOS 版的 MSDN!使用 DOSBOX 就可以运行此库。此库含一大古董级MS官方编程参考材料,主要针对 Windows 3.0 平台,真可谓之应用尽有: MS Windows 3.0 SDK Guide to Programming MS Windows 3.0 SDK Install. & Update Guide MS Windows 3.0 SDK Programm
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's Ref
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ence Vol. 1 MS Windows 3.0 SDK Programm
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ence Vol. 2 MS Windows 3.0 SDK Tools MS Windows 3.0 SDK Articles All MS Windows 3.0 SDK Manuals MS Windows 3.0 DDK Install. & Update Guide MS Windows 3.0 DDK Adaptation Guide MS Windows 3.0 DDK Virtual Device Adapt. Guide MS Windows 3.0 DDK Print
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& Font Kit All MS Windows 3.0 DDK Manuals MS Online Us
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's Guide Programming MS Windows MS Windows Sample Code MS KnowledgeBase - MS Windows 以及 Options => Library 菜单下提供的 9 个重要的参考资料,其中就有 C 和 MASM 这些重要的参考资料。这些是已安装的目录部分,鉴于 MASM 的重要性,特将其添加到压缩包内,免CD运行: Windows Ref
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ences OS/S Ref
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ences Network Ref
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ences MS-DOS Ref
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ences MS Systems Journal Hardware Ref
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ences C Ref
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ences MASM Ref
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ences BASIC Ref
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ences Pascal Ref
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ences FORTUAN Ref
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ences 其中 C Ref
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ences 和 MASM Ref
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ences 包含: Installing and Using MS MASM 6.0 MS MASM 6.0 Ref
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ence MS MASM 6.0 Programm
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's Guide MS MASM 6.0 White Pap
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QuickAssembl
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2.01 Programm
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's Guide MS Mixed-Language Programming Guide CodeView & Utilities Us
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's Guide MS Editor Us
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's Guide MS OnLine Us
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's Guide MASM Sample Code MS KnowledgeBase - MASM MS C 6.0 Advanced Programming Techniques MS C 6.0 Installing and Using the P.D.S. MS C 6.0 Ref
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ence MS C 6.0 Run-Time Library Ref
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ence MS C 6.0 Develop
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's Toolkit Ref
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ence QuickC 2.5 Tool Kit QuickC 2.5 C for Yourself QuickC 2.5 Up and Running QuickC 2.5 Update MS Professional
SAX符号化序列范例源码
SAX符号化序列范例源码 -------------------- times
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ies2symbol.m: -------------------- This function takes in a time s
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ies and conv
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t it to string(s). Th
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e are two options: 1. Conv
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t the entire time s
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ies to ONE string 2. Use sliding windows, extract the subsequences and conv
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t these subsequences to strings For the first option, simply ent
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the length of the time s
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ies as "N" ex. We have a time s
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ies of length 32 and we want to conv
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t it to a 8-symbol string, with alphabet size 3: times
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ies2symbol(data, 32, 8, 3) For the second option, ent
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the desired sliding window length as "N" ex. We have a time s
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ies of length 32 and we want to extract subsequences of length 16 using sliding windows, and conv
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t the subsequences to 8-symbol strings, with alphabet size 3: times
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ies2symbol(data, 16, 8, 3) Input: data is the raw time s
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ies. N is the length of sliding window (use the length of the raw time s
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ies instead if you don't want to have sliding windows) n is the numb
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of symbols in the low dimensional approximation of the sub sequence. alphabet_size is the numb
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of discrete symbols. 2 <= alphabet_size > mindist_demo sax_v
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sion_of_A = 3 4 2 1 1 3 4 2 sax_v
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sion_of_B = 1 1 3 4 3 1 1 4 euclidean_distance_A_and_B = 10.9094 ans = 5.3600 ---> This is the mindist ----------------- symbolic_visual.m ----------------- This demo presents a visual
comparison
between SAX and PAA and shows how SAX can represent data in fin
er
granularity while using the same, if not less, amount of space as PAA. The input paramet
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[data] is optional. The default # of PAA segments is 16, and the alphabet size is 4. -------- Examples: -------- You can type this up in your matlab: Recall that th
er
e are two options for times
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ies2symbol. The first option is demonstrated in sax_demo.m Now h
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e is an example of the latt
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. We are going to conv
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t time s
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ies of length 50, with a sliding window of 32, into 8 symbols, with and alphabet size of 3. >> [symbolic_data,
point
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s] = times
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ies2symbol(long_time_s
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ies,32,8,alphabet_size) symbolic_data = 1 1 3 3 3 3 1 1 1 2 3 3 3 2 1 1 1 3 3 3 3 1 1 1 2 3 3 3 2 1 1 1 3 3 3 3 1 1 1 1 3 3 3 2 1 1 1 2 3 3 3 1 1 1 1 3 3 3 2 1 1 1 2 3 3 3 1 1 1 1 3 3 3 2 1 1 1 2 3 3
point
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s = 1 2 5 6 9 10 13 14 17 18 Note that each row corresponds to a subsequence (with ov
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lap) The SAX word at 3 and 4 w
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e omitted, since they wh
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e the same as the word at 2, same for 7 and 8, which w
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e the same as 6 etc (look at the
point
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s) It might be helpful to view the data this way >> [
point
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s symbolic_data ] ans = 1 1 1 3 3 3 3 1 1 2 1 2 3 3 3 2 1 1 5 1 3 3 3 3 1 1 1 6 2 3 3 3 2 1 1 1 9 3 3 3 3 1 1 1 1 10 3 3 3 2 1 1 1 2 13 3 3 3 1 1 1 1 3 14 3 3 2 1 1 1 2 3 17 3 3 1 1 1 1 3 3 18 3 2 1 1 1 2 3 3 So the first word is (1 1 3 3 3 3 1 1) , the 9th word is (3 3 3 3 1 1 1 1) , the 14 word is (3 3 2 1 1 1 2 3)
Google C++ Style Guide(Google C++编程规范)高清PDF
Table of Contents Head
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Files The #define Guard Head
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File Dependencies Inline Functions The -inl.h Files Function Paramet
er
Ord
er
ing Names and Ord
er
of Includes Scoping Namespaces Nested Classes Nonmemb
er
, Static Memb
er
, and Global Functions Local Variables Static and Global Variables Classes Doing Work in Constructors Default Constructors Explicit Constructors Copy Constructors Structs vs. Classes Inh
er
itance Multiple Inh
er
itance Int
er
faces Op
er
ator Ov
er
loading Access Control Declaration Ord
er
Write Short Functions Google-Specific Magic Smart
Point
er
s cpplint Oth
er
C++ Features Ref
er
ence Arguments Function Ov
er
loading Default Arguments Variable-Length Arrays and alloca() Friends Exceptions Run-Time Type Information (RTTI) Casting Streams Preincrement and Predecrement Use of const
Integ
er
Types 64-bit Portability Preprocessor Macros 0 and NULL sizeof Boost C++0x Naming Gen
er
al Naming Rules File Names Type Names Variable Names Constant Names Function Names Namespace Names Enum
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ator Names Macro Names Exceptions to Naming Rules Comments Comment Style File Comments Class Comments Function Comments Variable Comments Implementation Comments Punctuation, Spelling and Grammar TODO Comments Deprecation Comments Formatting Line Length Non-ASCII Charact
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s Spaces vs. Tabs Function Declarations and Definitions Function Calls Conditionals Loops and Switch Statements
Point
er
and Ref
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ence Expressions Boolean Expressions Return Values Variable and Array Initialization Preprocessor Directives Class Format Constructor Initializ
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Lists Namespace Formatting Horizontal Whitespace V
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tical Whitespace Exceptions to the Rules Existing Non-conformant Code Windows Code Important Note Displaying Hidden Details in this Guide link ▶This style guide contains many details that are initially hidden from view. They are marked by the triangle icon, which you see h
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e on your left. Click it now. You should see "Hooray" appear below. Hooray! Now you know you can expand
point
s to get more details. Alt
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natively, th
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e's an "expand all" at the top of this document. Background C++ is the main development language used by many of Google's open-source projects. As ev
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y C++ programm
er
knows, the language has many pow
er
ful features, but this pow
er
brings with it complexity, which in turn can make code more bug-prone and hard
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to read and maintain. The goal of this guide is to manage this complexity by describing in detail the dos and don'ts of writing C++ code. These rules exist to keep the code base manageable while still allowing cod
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s to use C++ language features productively. Style, also known as readability, is what we call the conventions that gov
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n our C++ code. The t
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m Style is a bit of a misnom
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, since these conventions cov
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far more than just source file formatting. One way in which we keep the code base manageable is by enforcing consistency. It is v
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y important that any programm
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be able to look at anoth
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's code and quickly und
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stand it. Maintaining a uniform style and following conventions means that we can more easily use "patt
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n-matching" to inf
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what various symbols are and what invariants are true about them. Creating common, required idioms and patt
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ns makes code much easi
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to und
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stand. In some cases th
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e might be good arguments for changing c
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tain style rules, but we nonetheless keep things as they are in ord
er
to pres
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ve consistency. Anoth
er
issue this guide addresses is that of C++ feature bloat. C++ is a huge language with many advanced features. In some cases we constrain, or even ban, use of c
er
tain features. We do this to keep code simple and to avoid the various common
er
rors and problems that these features can cause. This guide lists these features and explains why their use is restricted. Open-source projects developed by Google conform to the requirements in this guide. Note that this guide is not a C++ tutorial: we assume that the read
er
is familiar with the language. Head
er
Files In gen
er
al, ev
er
y .cc file should have an associated .h file. Th
er
e are some common exceptions, such as unittests and small .cc files containing just a main() function. Correct use of head
er
files can make a huge diff
er
ence to the readability, size and p
er
formance of your code. The following rules will guide you through the various pitfalls of using head
er
files. The #define Guard link ▶All head
er
files should have #define guards to prevent multiple inclusion. The format of the symbol name should be ___H_. To guarantee uniqueness, they should be based on the full path in a project's source tree. For example, the file foo/src/bar/baz.h in project foo should have the following guard: #ifndef FOO_BAR_BAZ_H_ #define FOO_BAR_BAZ_H_ ... #endif // FOO_BAR_BAZ_H_ Head
er
File Dependencies link ▶Don't use an #include when a forward declaration would suffice. When you include a head
er
file you introduce a dependency that will cause your code to be recompiled whenev
er
the head
er
file changes. If your head
er
file includes oth
er
head
er
files, any change to those files will cause any code that includes your head
er
to be recompiled. Th
er
efore, we pref
er
to minimize includes, particularly includes of head
er
files in oth
er
head
er
files. You can significantly minimize the numb
er
of head
er
files you need to include in your own head
er
files by using forward declarations. For example, if your head
er
file uses the File class in ways that do not require access to the declaration of the File class, your head
er
file can just forward declare class File; instead of having to #include "file/base/file.h". How can we use a class Foo in a head
er
file without access to its definition? We can declare data memb
er
s of type Foo* or Foo&. We can declare (but not define) functions with arguments, and/or return values, of type Foo. (One exception is if an argument Foo or const Foo& has a non-explicit, one-argument constructor, in which case we need the full definition to support automatic type conv
er
sion.) We can declare static data memb
er
s of type Foo. This is because static data memb
er
s are defined outside the class definition. On the oth
er
hand, you must include the head
er
file for Foo if your class subclasses Foo or has a data memb
er
of type Foo. Sometimes it makes sense to have
point
er
(or bett
er
, scoped_ptr) memb
er
s instead of object memb
er
s. Howev
er
, this complicates code readability and imposes a p
er
formance penalty, so avoid doing this transformation if the only purpose is to minimize includes in head
er
files. Of course, .cc files typically do require the definitions of the classes they use, and usually have to include sev
er
al head
er
files. Note: If you use a symbol Foo in your source file, you should bring in a definition for Foo yourself, eith
er
via an #include or via a forward declaration. Do not depend on the symbol being brought in transitively via head
er
s not directly included. One exception is if Foo is used in myfile.cc, it's ok to #include (or forward-declare) Foo in myfile.h, instead of myfile.cc. Inline Functions link ▶Define functions inline only when they are small, say, 10 lines or less. Definition: You can declare functions in a way that allows the compil
er
to expand them inline rath
er
than calling them through the usual function call mechanism. Pros: Inlining a function can gen
er
ate more efficient object code, as long as the inlined function is small. Feel free to inline accessors and mutators, and oth
er
short, p
er
formance-critical functions. Cons: Ov
er
use of inlining can actually make programs slow
er
. Depending on a function's size, inlining it can cause the code size to increase or decrease. Inlining a v
er
y small accessor function will usually decrease code size while inlining a v
er
y large function can dramatically increase code size. On mod
er
n processors small
er
code usually runs fast
er
due to bett
er
use of the instruction cache. Decision: A decent rule of thumb is to not inline a function if it is more than 10 lines long. Beware of destructors, which are often long
er
than they appear because of implicit memb
er
- and base-destructor calls! Anoth
er
useful rule of thumb: it's typically not cost effective to inline functions with loops or switch statements (unless, in the common case, the loop or switch statement is nev
er
executed). It is important to know that functions are not always inlined even if they are declared as such; for example, virtual and recursive functions are not normally inlined. Usually recursive functions should not be inline. The main reason for making a virtual function inline is to place its definition in the class, eith
er
for convenience or to document its behavior, e.g., for accessors and mutators. The -inl.h Files link ▶You may use file names with a -inl.h suffix to define complex inline functions when needed. The definition of an inline function needs to be in a head
er
file, so that the compil
er
has the definition available for inlining at the call sites. Howev
er
, implementation code prop
er
ly belongs in .cc files, and we do not like to have much actual code in .h files unless th
er
e is a readability or p
er
formance advantage. If an inline function definition is short, with v
er
y little, if any, logic in it, you should put the code in your .h file. For example, accessors and mutators should c
er
tainly be inside a class definition. More complex inline functions may also be put in a .h file for the convenience of the implement
er
and call
er
s, though if this makes the .h file too unwieldy you can instead put that code in a separate -inl.h file. This separates the implementation from the class definition, while still allowing the implementation to be included wh
er
e necessary. Anoth
er
use of -inl.h files is for definitions of function templates. This can be used to keep your template definitions easy to read. Do not forget that a -inl.h file requires a #define guard just like any oth
er
head
er
file. Function Paramet
er
Ord
er
ing link ▶When defining a function, paramet
er
ord
er
is: inputs, then outputs. Paramet
er
s to C/C++ functions are eith
er
input to the function, output from the function, or both. Input paramet
er
s are usually values or const ref
er
ences, while output and input/output paramet
er
s will be non-const
point
er
s. When ord
er
ing function paramet
er
s, put all input-only paramet
er
s before any output paramet
er
s. In particular, do not add new paramet
er
s to the end of the function just because they are new; place new input-only paramet
er
s before the output paramet
er
s. This is not a hard-and-fast rule. Paramet
er
s that are both input and output (often classes/structs) muddy the wat
er
s, and, as always, consistency with related functions may require you to bend the rule. Names and Ord
er
of Includes link ▶Use standard ord
er
for readability and to avoid hidden dependencies: C library, C++ library, oth
er
libraries' .h, your project's .h. All of a project's head
er
files should be listed as descentants of the project's source directory without use of UNIX directory shortcuts . (the current directory) or .. (the parent directory). For example, google-awesome-project/src/base/logging.h should be included as #include "base/logging.h" In dir/foo.cc, whose main purpose is to implement or test the stuff in dir2/foo2.h, ord
er
your includes as follows: dir2/foo2.h (pref
er
red location — see details below). C system files. C++ system files. Oth
er
libraries' .h files. Your project's .h files. The pref
er
red ord
er
ing reduces hidden dependencies. We want ev
er
y head
er
file to be compilable on its own. The easiest way to achieve this is to make sure that ev
er
y one of them is the first .h file #included in some .cc. dir/foo.cc and dir2/foo2.h are often in the same directory (e.g. base/basictypes_test.cc and base/basictypes.h), but can be in diff
er
ent directories too. Within each section it is nice to ord
er
the includes alphabetically. For example, the includes in google-awesome-project/src/foo/int
er
nal/foos
er
v
er
.cc might look like this: #include "foo/public/foos
er
v
er
.h" // Pref
er
red location. #include #include #include #include #include "base/basictypes.h" #include "base/commandlineflags.h" #include "foo/public/bar.h" Scoping Namespaces link ▶Unnamed namespaces in .cc files are encouraged. With named namespaces, choose the name based on the project, and possibly its path. Do not use a using-directive. Definition: Namespaces subdivide the global scope into distinct, named scopes, and so are useful for preventing name collisions in the global scope. Pros: Namespaces provide a (hi
er
archical) axis of naming, in addition to the (also hi
er
archical) name axis provided by classes. For example, if two diff
er
ent projects have a class Foo in the global scope, these symbols may collide at compile time or at runtime. If each project places their code in a namespace, project1::Foo and project2::Foo are now distinct symbols that do not collide. Cons: Namespaces can be confusing, because they provide an additional (hi
er
archical) axis of naming, in addition to the (also hi
er
archical) name axis provided by classes. Use of unnamed spaces in head
er
files can easily cause violations of the C++ One Definition Rule (ODR). Decision: Use namespaces according to the policy described below. Unnamed Namespaces Unnamed namespaces are allowed and even encouraged in .cc files, to avoid runtime naming conflicts: namespace { // This is in a .cc file. // The content of a namespace is not indented enum { kUnused, kEOF, k
Er
ror }; // Commonly used tokens. bool AtEof() { return pos_ == kEOF; } // Uses our namespace's EOF. } // namespace Howev
er
, file-scope declarations that are associated with a particular class may be declared in that class as types, static data memb
er
s or static memb
er
functions rath
er
than as memb
er
s of an unnamed namespace. T
er
minate the unnamed namespace as shown, with a comment // namespace. Do not use unnamed namespaces in .h files. Named Namespaces Named namespaces should be used as follows: Namespaces wrap the entire source file aft
er
includes, gflags definitions/declarations, and forward declarations of classes from oth
er
namespaces: // In the .h file namespace mynamespace { // All declarations are within the namespace scope. // Notice the lack of indentation. class MyClass { public: ... void Foo(); }; } // namespace mynamespace // In the .cc file namespace mynamespace { // Definition of functions is within scope of the namespace. void MyClass::Foo() { ... } } // namespace mynamespace The typical .cc file might have more complex detail, including the need to ref
er
ence classes in oth
er
namespaces. #include "a.h" DEFINE_bool(someflag, false, "dummy flag"); class C; // Forward declaration of class C in the global namespace. namespace a { class A; } // Forward declaration of a::A. namespace b { ...code for b... // Code goes against the left margin. } // namespace b Do not declare anything in namespace std, not even forward declarations of standard library classes. Declaring entities in namespace std is undefined behavior, i.e., not portable. To declare entities from the standard library, include the appropriate head
er
file. You may not use a using-directive to make all names from a namespace available. // Forbidden -- This pollutes the namespace. using namespace foo; You may use a using-declaration anywh
er
e in a .cc file, and in functions, methods or classes in .h files. // OK in .cc files. // Must be in a function, method or class in .h files. using ::foo::bar; Namespace aliases are allowed anywh
er
e in a .cc file, anywh
er
e inside the named namespace that wraps an entire .h file, and in functions and methods. // Shorten access to some commonly used names in .cc files. namespace fbz = ::foo::bar::baz; // Shorten access to some commonly used names (in a .h file). namespace librarian { // The following alias is available to all files including // this head
er
(in namespace librarian): // alias names should th
er
efore be chosen consistently // within a project. namespace pd_s = ::pipeline_diagnostics::sidetable; inline void my_inline_function() { // namespace alias local to a function (or method). namespace fbz = ::foo::bar::baz; ... } } // namespace librarian Note that an alias in a .h file is visible to ev
er
yone #including that file, so public head
er
s (those available outside a project) and head
er
s transitively #included by them, should avoid defining aliases, as part of the gen
er
al goal of keeping public APIs as small as possible. Nested Classes link ▶Although you may use public nested classes when they are part of an int
er
face, consid
er
a namespace to keep declarations out of the global scope. Definition: A class can define anoth
er
class within it; this is also called a memb
er
class. class Foo { private: // Bar is a memb
er
class, nested within Foo. class Bar { ... }; }; Pros: This is useful when the nested (or memb
er
) class is only used by the enclosing class; making it a memb
er
puts it in the enclosing class scope rath
er
than polluting the out
er
scope with the class name. Nested classes can be forward declared within the enclosing class and then defined in the .cc file to avoid including the nested class definition in the enclosing class declaration, since the nested class definition is usually only relevant to the implementation. Cons: Nested classes can be forward-declared only within the definition of the enclosing class. Thus, any head
er
file manipulating a Foo::Bar*
point
er
will have to include the full class declaration for Foo. Decision: Do not make nested classes public unless they are actually part of the int
er
face, e.g., a class that holds a set of options for some method. Nonmemb
er
, Static Memb
er
, and Global Functions link ▶Pref
er
nonmemb
er
functions within a namespace or static memb
er
functions to global functions; use completely global functions rarely. Pros: Nonmemb
er
and static memb
er
functions can be useful in some situations. Putting nonmemb
er
functions in a namespace avoids polluting the global namespace. Cons: Nonmemb
er
and static memb
er
functions may make more sense as memb
er
s of a new class, especially if they access ext
er
nal resources or have significant dependencies. Decision: Sometimes it is useful, or even necessary, to define a function not bound to a class instance. Such a function can be eith
er
a static memb
er
or a nonmemb
er
function. Nonmemb
er
functions should not depend on ext
er
nal variables, and should nearly always exist in a namespace. Rath
er
than creating classes only to group static memb
er
functions which do not share static data, use namespaces instead. Functions defined in the same compilation unit as production classes may introduce unnecessary coupling and link-time dependencies when directly called from oth
er
compilation units; static memb
er
functions are particularly susceptible to this. Consid
er
extracting a new class, or placing the functions in a namespace possibly in a separate library. If you must define a nonmemb
er
function and it is only needed in its .cc file, use an unnamed namespace or static linkage (eg static int Foo() {...}) to limit its scope. Local Variables link ▶Place a function's variables in the narrowest scope possible, and initialize variables in the declaration. C++ allows you to declare variables anywh
er
e in a function. We encourage you to declare them in as local a scope as possible, and as close to the first use as possible. This makes it easi
er
for the read
er
to find the declaration and see what type the variable is and what it was initialized to. In particular, initialization should be used instead of declaration and assignment, e.g. int i; i = f(); // Bad -- initialization separate from declaration. int j = g(); // Good -- declaration has initialization. Note that gcc implements for (int i = 0; i < 10; ++i) correctly (the scope of i is only the scope of the for loop), so you can then reuse i in anoth
er
for loop in the same scope. It also correctly scopes declarations in if and while statements, e.g. while (const char* p = strchr(str, '/')) str = p + 1; Th
er
e is one caveat: if the variable is an object, its constructor is invoked ev
er
y time it ent
er
s scope and is created, and its destructor is invoked ev
er
y time it goes out of scope. // Inefficient implementation: for (int i = 0; i < 1000000; ++i) { Foo f; // My ctor and dtor get called 1000000 times each. f.DoSomething(i); } It may be more efficient to declare such a variable used in a loop outside that loop: Foo f; // My ctor and dtor get called once each. for (int i = 0; i < 1000000; ++i) { f.DoSomething(i); } Static and Global Variables link ▶Static or global variables of class type are forbidden: they cause hard-to-find bugs due to indet
er
minate ord
er
of construction and destruction. Objects with static storage duration, including global variables, static variables, static class memb
er
variables, and function static variables, must be Plain Old Data (POD): only ints, chars, floats, or
point
er
s, or arrays/structs of POD. The ord
er
in which class constructors and initializ
er
s for static variables are called is only partially specified in C++ and can even change from build to build, which can cause bugs that are difficult to find. Th
er
efore in addition to banning globals of class type, we do not allow static POD variables to be initialized with the result of a function, unless that function (such as getenv(), or getpid()) does not itself depend on any oth
er
globals. Likewise, the ord
er
in which destructors are called is defined to be the rev
er
se of the ord
er
in which the constructors w
er
e called. Since constructor ord
er
is indet
er
minate, so is destructor ord
er
. For example, at program-end time a static variable might have been destroyed, but code still running -- p
er
haps in anoth
er
thread -- tries to access it and fails. Or the destructor for a static 'string' variable might be run prior to the destructor for anoth
er
variable that contains a ref
er
ence to that string. As a result we only allow static variables to contain POD data. This rule completely disallows vector (use C arrays instead), or string (use const char []). If you need a static or global variable of a class type, consid
er
initializing a
point
er
(which will nev
er
be freed), from eith
er
your main() function or from pthread_once(). Note that this must be a raw
point
er
, not a "smart"
point
er
, since the smart
point
er
's destructor will have the ord
er
-of-destructor issue that we are trying to avoid. Classes Classes are the fundamental unit of code in C++. Naturally, we use them extensively. This section lists the main dos and don'ts you should follow when writing a class. Doing Work in Constructors link ▶In gen
er
al, constructors should m
er
ely set memb
er
variables to their initial values. Any complex initialization should go in an explicit Init() method. Definition: It is possible to p
er
form initialization in the body of the constructor. Pros: Convenience in typing. No need to worry about wheth
er
the class has been initialized or not. Cons: The problems with doing work in constructors are: Th
er
e is no easy way for constructors to signal
er
rors, short of using exceptions (which are forbidden). If the work fails, we now have an object whose initialization code failed, so it may be an indet
er
minate state. If the work calls virtual functions, these calls will not get dispatched to the subclass implementations. Future modification to your class can quietly introduce this problem even if your class is not currently subclassed, causing much confusion. If someone creates a global variable of this type (which is against the rules, but still), the constructor code will be called before main(), possibly breaking some implicit assumptions in the constructor code. For instance, gflags will not yet have been initialized. Decision: If your object requires non-trivial initialization, consid
er
having an explicit Init() method. In particular, constructors should not call virtual functions, attempt to raise
er
rors, access potentially uninitialized global variables, etc. Default Constructors link ▶You must define a default constructor if your class defines memb
er
variables and has no oth
er
constructors. Oth
er
wise the compil
er
will do it for you, badly. Definition: The default constructor is called when we new a class object with no arguments. It is always called when calling new[] (for arrays). Pros: Initializing structures by default, to hold "impossible" values, makes debugging much easi
er
. Cons: Extra work for you, the code writ
er
. Decision: If your class defines memb
er
variables and has no oth
er
constructors you must define a default constructor (one that takes no arguments). It should pref
er
ably initialize the object in such a way that its int
er
nal state is consistent and valid. The reason for this is that if you have no oth
er
constructors and do not define a default constructor, the compil
er
will gen
er
ate one for you. This compil
er
gen
er
ated constructor may not initialize your object sensibly. If your class inh
er
its from an existing class but you add no new memb
er
variables, you are not required to have a default constructor. Explicit Constructors link ▶Use the C++ keyword explicit for constructors with one argument. Definition: Normally, if a constructor takes one argument, it can be used as a conv
er
sion. For instance, if you define Foo::Foo(string name) and then pass a string to a function that expects a Foo, the constructor will be called to conv
er
t the string into a Foo and will pass the Foo to your function for you. This can be convenient but is also a source of trouble when things get conv
er
ted and new objects created without you meaning them to. Declaring a constructor explicit prevents it from being invoked implicitly as a conv
er
sion. Pros: Avoids undesirable conv
er
sions. Cons: None. Decision: We require all single argument constructors to be explicit. Always put explicit in front of one-argument constructors in the class definition: explicit Foo(string name); The exception is copy constructors, which, in the rare cases when we allow them, should probably not be explicit. Classes that are intended to be transparent wrapp
er
s around oth
er
classes are also exceptions. Such exceptions should be clearly marked with comments. Copy Constructors link ▶Provide a copy constructor and assignment op
er
ator only when necessary. Oth
er
wise, disable them with DISALLOW_COPY_AND_ASSIGN. Definition: The copy constructor and assignment op
er
ator are used to create copies of objects. The copy constructor is implicitly invoked by the compil
er
in some situations, e.g. passing objects by value. Pros: Copy constructors make it easy to copy objects. STL contain
er
s require that all contents be copyable and assignable. Copy constructors can be more efficient than CopyFrom()-style workarounds because they combine construction with copying, the compil
er
can elide them in some contexts, and they make it easi
er
to avoid heap allocation. Cons: Implicit copying of objects in C++ is a rich source of bugs and of p
er
formance problems. It also reduces readability, as it becomes hard to track which objects are being passed around by value as opposed to by ref
er
ence, and th
er
efore wh
er
e changes to an object are reflected. Decision: Few classes need to be copyable. Most should have neith
er
a copy constructor nor an assignment op
er
ator. In many situations, a
point
er
or ref
er
ence will work just as well as a copied value, with bett
er
p
er
formance. For example, you can pass function paramet
er
s by ref
er
ence or
point
er
instead of by value, and you can store
point
er
s rath
er
than objects in an STL contain
er
. If your class needs to be copyable, pref
er
providing a copy method, such as CopyFrom() or Clone(), rath
er
than a copy constructor, because such methods cannot be invoked implicitly. If a copy method is insufficient in your situation (e.g. for p
er
formance reasons, or because your class needs to be stored by value in an STL contain
er
), provide both a copy constructor and assignment op
er
ator. If your class does not need a copy constructor or assignment op
er
ator, you must explicitly disable them. To do so, add dummy declarations for the copy constructor and assignment op
er
ator in the private: section of your class, but do not provide any corresponding definition (so that any attempt to use them results in a link
er
ror). For convenience, a DISALLOW_COPY_AND_ASSIGN macro can be used: // A macro to disallow the copy constructor and op
er
ator= functions // This should be used in the private: declarations for a class #define DISALLOW_COPY_AND_ASSIGN(TypeName) \ TypeName(const TypeName&); \ void op
er
ator=(const TypeName&) Then, in class Foo: class Foo { public: Foo(int f); ~Foo(); private: DISALLOW_COPY_AND_ASSIGN(Foo); }; Structs vs. Classes link ▶Use a struct only for passive objects that carry data; ev
er
ything else is a class. The struct and class keywords behave almost identically in C++. We add our own semantic meanings to each keyword, so you should use the appropriate keyword for the data-type you're defining. structs should be used for passive objects that carry data, and may have associated constants, but lack any functionality oth
er
than access/setting the data memb
er
s. The accessing/setting of fields is done by directly accessing the fields rath
er
than through method invocations. Methods should not provide behavior but should only be used to set up the data memb
er
s, e.g., constructor, destructor, Initialize(), Reset(), Validate(). If more functionality is required, a class is more appropriate. If in doubt, make it a class. For consistency with STL, you can use struct instead of class for functors and traits. Note that memb
er
variables in structs and classes have diff
er
ent naming rules. Inh
er
itance link ▶Composition is often more appropriate than inh
er
itance. When using inh
er
itance, make it public. Definition: When a sub-class inh
er
its from a base class, it includes the definitions of all the data and op
er
ations that the parent base class defines. In practice, inh
er
itance is used in two major ways in C++: implementation inh
er
itance, in which actual code is inh
er
ited by the child, and int
er
face inh
er
itance, in which only method names are inh
er
ited. Pros: Implementation inh
er
itance reduces code size by re-using the base class code as it specializes an existing type. Because inh
er
itance is a compile-time declaration, you and the compil
er
can und
er
stand the op
er
ation and detect
er
rors. Int
er
face inh
er
itance can be used to programmatically enforce that a class expose a particular API. Again, the compil
er
can detect
er
rors, in this case, when a class does not define a necessary method of the API. Cons: For implementation inh
er
itance, because the code implementing a sub-class is spread between the base and the sub-class, it can be more difficult to und
er
stand an implementation. The sub-class cannot ov
er
ride functions that are not virtual, so the sub-class cannot change implementation. The base class may also define some data memb
er
s, so that specifies physical layout of the base class. Decision: All inh
er
itance should be public. If you want to do private inh
er
itance, you should be including an instance of the base class as a memb
er
instead. Do not ov
er
use implementation inh
er
itance. Composition is often more appropriate. Try to restrict use of inh
er
itance to the "is-a" case: Bar subclasses Foo if it can reasonably be said that Bar "is a kind of" Foo. Make your destructor virtual if necessary. If your class has virtual methods, its destructor should be virtual. Limit the use of protected to those memb
er
functions that might need to be accessed from subclasses. Note that data memb
er
s should be private. When redefining an inh
er
ited virtual function, explicitly declare it virtual in the declaration of the d
er
ived class. Rationale: If virtual is omitted, the read
er
has to check all ancestors of the class in question to det
er
mine if the function is virtual or not. Multiple Inh
er
itance link ▶Only v
er
y rarely is multiple implementation inh
er
itance actually useful. We allow multiple inh
er
itance only when at most one of the base classes has an implementation; all oth
er
base classes must be pure int
er
face classes tagged with the Int
er
face suffix. Definition: Multiple inh
er
itance allows a sub-class to have more than one base class. We distinguish between base classes that are pure int
er
faces and those that have an implementation. Pros: Multiple implementation inh
er
itance may let you re-use even more code than single inh
er
itance (see Inh
er
itance). Cons: Only v
er
y rarely is multiple implementation inh
er
itance actually useful. When multiple implementation inh
er
itance seems like the solution, you can usually find a diff
er
ent, more explicit, and clean
er
solution. Decision: Multiple inh
er
itance is allowed only when all sup
er
classes, with the possible exception of the first one, are pure int
er
faces. In ord
er
to ensure that they remain pure int
er
faces, they must end with the Int
er
face suffix. Note: Th
er
e is an exception to this rule on Windows. Int
er
faces link ▶Classes that satisfy c
er
tain conditions are allowed, but not required, to end with an Int
er
face suffix. Definition: A class is a pure int
er
face if it meets the following requirements: It has only public pure virtual ("= 0") methods and static methods (but see below for destructor). It may not have non-static data memb
er
s. It need not have any constructors defined. If a constructor is provided, it must take no arguments and it must be protected. If it is a subclass, it may only be d
er
ived from classes that satisfy these conditions and are tagged with the Int
er
face suffix. An int
er
face class can nev
er
be directly instantiated because of the pure virtual method(s) it declares. To make sure all implementations of the int
er
face can be destroyed correctly, they must also declare a virtual destructor (in an exception to the first rule, this should not be pure). See Stroustrup, The C++ Programming Language, 3rd edition, section 12.4 for details. Pros: Tagging a class with the Int
er
face suffix lets oth
er
s know that they must not add implemented methods or non static data memb
er
s. This is particularly important in the case of multiple inh
er
itance. Additionally, the int
er
face concept is already well-und
er
stood by Java programm
er
s. Cons: The Int
er
face suffix lengthens the class name, which can make it hard
er
to read and und
er
stand. Also, the int
er
face prop
er
ty may be consid
er
ed an implementation detail that shouldn't be exposed to clients. Decision: A class may end with Int
er
face only if it meets the above requirements. We do not require the conv
er
se, howev
er
: classes that meet the above requirements are not required to end with Int
er
face. Op
er
ator Ov
er
loading link ▶Do not ov
er
load op
er
ators except in rare, special circumstances. Definition: A class can define that op
er
ators such as + and / op
er
ate on the class as if it w
er
e a built-in type. Pros: Can make code appear more intuitive because a class will behave in the same way as built-in types (such as int). Ov
er
loaded op
er
ators are more playful names for functions that are less-colorfully named, such as Equals() or Add(). For some template functions to work correctly, you may need to define op
er
ators. Cons: While op
er
ator ov
er
loading can make code more intuitive, it has sev
er
al drawbacks: It can fool our intuition into thinking that expensive op
er
ations are cheap, built-in op
er
ations. It is much hard
er
to find the call sites for ov
er
loaded op
er
ators. Searching for Equals() is much easi
er
than searching for relevant invocations of ==. Some op
er
ators work on
point
er
s too, making it easy to introduce bugs. Foo + 4 may do one thing, while &Foo + 4 does something totally diff
er
ent. The compil
er
does not complain for eith
er
of these, making this v
er
y hard to debug. Ov
er
loading also has surprising ramifications. For instance, if a class ov
er
loads unary op
er
ator&, it cannot safely be forward-declared. Decision: In gen
er
al, do not ov
er
load op
er
ators. The assignment op
er
ator (op
er
ator=), in particular, is insidious and should be avoided. You can define functions like Equals() and CopyFrom() if you need them. Likewise, avoid the dang
er
ous unary op
er
ator& at all costs, if th
er
e's any possibility the class might be forward-declared. Howev
er
, th
er
e may be rare cases wh
er
e you need to ov
er
load an op
er
ator to int
er
op
er
ate with templates or "standard" C++ classes (such as op
er
ator<<(ostream&, const T&) for logging). These are acceptable if fully justified, but you should try to avoid these whenev
er
possible. In particular, do not ov
er
load op
er
ator== or op
er
ator< just so that your class can be used as a key in an STL contain
er
; instead, you should create equality and
comparison
functor types when declaring the contain
er
. Some of the STL algorithms do require you to ov
er
load op
er
ator==, and you may do so in these cases, provided you document why. See also Copy Constructors and Function Ov
er
loading. Access Control link ▶Make data memb
er
s private, and provide access to them through accessor functions as needed (for technical reasons, we allow data memb
er
s of a test fixture class to be protected when using Google Test). Typically a variable would be called foo_ and the accessor function foo(). You may also want a mutator function set_foo(). Exception: static const data memb
er
s (typically called kFoo) need not be private. The definitions of accessors are usually inlined in the head
er
file. See also Inh
er
itance and Function Names. Declaration Ord
er
link ▶Use the specified ord
er
of declarations within a class: public: before private:, methods before data memb
er
s (variables), etc. Your class definition should start with its public: section, followed by its protected: section and then its private: section. If any of these sections are empty, omit them. Within each section, the declarations gen
er
ally should be in the following ord
er
: Typedefs and Enums Constants (static const data memb
er
s) Constructors Destructor Methods, including static methods Data Memb
er
s (except static const data memb
er
s) Friend declarations should always be in the private section, and the DISALLOW_COPY_AND_ASSIGN macro invocation should be at the end of the private: section. It should be the last thing in the class. See Copy Constructors. Method definitions in the corresponding .cc file should be the same as the declaration ord
er
, as much as possible. Do not put large method definitions inline in the class definition. Usually, only trivial or p
er
formance-critical, and v
er
y short, methods may be defined inline. See Inline Functions for more details. Write Short Functions link ▶Pref
er
small and focused functions. We recognize that long functions are sometimes appropriate, so no hard limit is placed on functions length. If a function exceeds about 40 lines, think about wheth
er
it can be broken up without harming the structure of the program. Even if your long function works p
er
fectly now, someone modifying it in a few months may add new behavior. This could result in bugs that are hard to find. Keeping your functions short and simple makes it easi
er
for oth
er
people to read and modify your code. You could find long and complicated functions when working with some code. Do not be intimidated by modifying existing code: if working with such a function proves to be difficult, you find that
er
rors are hard to debug, or you want to use a piece of it in sev
er
al diff
er
ent contexts, consid
er
breaking up the function into small
er
and more manageable pieces. Google-Specific Magic Th
er
e are various tricks and utilities that we use to make C++ code more robust, and various ways we use C++ that may diff
er
from what you see elsewh
er
e. Smart
Point
er
s link ▶If you actually need
point
er
semantics, scoped_ptr is great. You should only use std::tr1::shared_ptr und
er
v
er
y specific conditions, such as when objects need to be held by STL contain
er
s. You should nev
er
use auto_ptr. "Smart"
point
er
s are objects that act like
point
er
s but have added semantics. When a scoped_ptr is destroyed, for instance, it deletes the object it's
point
ing to. shared_ptr is the same way, but implements ref
er
ence-counting so only the last
point
er
to an object deletes it. Gen
er
ally speaking, we pref
er
that we design code with clear object own
er
ship. The clearest object own
er
ship is obtained by using an object directly as a field or local variable, without using
point
er
s at all. On the oth
er
extreme, by their v
er
y definition, ref
er
ence counted
point
er
s are owned by nobody. The problem with this design is that it is easy to create circular ref
er
ences or oth
er
strange conditions that cause an object to nev
er
be deleted. It is also slow to p
er
form atomic op
er
ations ev
er
y time a value is copied or assigned. Although they are not recommended, ref
er
ence counted
point
er
s are sometimes the simplest and most elegant way to solve a problem. cpplint link ▶Use cpplint.py to detect style
er
rors. cpplint.py is a tool that reads a source file and identifies many style
er
rors. It is not p
er
fect, and has both false positives and false negatives, but it is still a valuable tool. False positives can be ignored by putting // NOLINT at the end of the line. Some projects have instructions on how to run cpplint.py from their project tools. If the project you are contributing to does not, you can download cpplint.py separately. Oth
er
C++ Features Ref
er
ence Arguments link ▶All paramet
er
s passed by ref
er
ence must be labeled const. Definition: In C, if a function needs to modify a variable, the paramet
er
must use a
point
er
, eg int foo(int *pval). In C++, the function can alt
er
natively declare a ref
er
ence paramet
er
: int foo(int &val). Pros: Defining a paramet
er
as ref
er
ence avoids ugly code like (*pval)++. Necessary for some applications like copy constructors. Makes it clear, unlike with
point
er
s, that NULL is not a possible value. Cons: Ref
er
ences can be confusing, as they have value syntax but
point
er
semantics. Decision: Within function paramet
er
lists all ref
er
ences must be const: void Foo(const string &in, string *out); In fact it is a v
er
y strong convention in Google code that input arguments are values or const ref
er
ences while output arguments are
point
er
s. Input paramet
er
s may be const
point
er
s, but we nev
er
allow non-const ref
er
ence paramet
er
s. One case when you might want an input paramet
er
to be a const
point
er
is if you want to emphasize that the argument is not copied, so it must exist for the lifetime of the object; it is usually best to document this in comments as well. STL adapt
er
s such as bind2nd and mem_fun do not p
er
mit ref
er
ence paramet
er
s, so you must declare functions with
point
er
paramet
er
s in these cases, too. Function Ov
er
loading link ▶Use ov
er
loaded functions (including constructors) only if a read
er
looking at a call site can get a good idea of what is happening without having to first figure out exactly which ov
er
load is being called. Definition: You may write a function that takes a const string& and ov
er
load it with anoth
er
that takes const char*. class MyClass { public: void Analyze(const string &text); void Analyze(const char *text, size_t textlen); }; Pros: Ov
er
loading can make code more intuitive by allowing an identically-named function to take diff
er
ent arguments. It may be necessary for templatized code, and it can be convenient for Visitors. Cons: If a function is ov
er
loaded by the argument types alone, a read
er
may have to und
er
stand C++'s complex matching rules in ord
er
to tell what's going on. Also many people are confused by the semantics of inh
er
itance if a d
er
ived class ov
er
rides only some of the variants of a function. Decision: If you want to ov
er
load a function, consid
er
qualifying the name with some information about the arguments, e.g., AppendString(), AppendInt() rath
er
than just Append(). Default Arguments link ▶We do not allow default function paramet
er
s, except in a few uncommon situations explained below. Pros: Often you have a function that uses lots of default values, but occasionally you want to ov
er
ride the defaults. Default paramet
er
s allow an easy way to do this without having to define many functions for the rare exceptions. Cons: People often figure out how to use an API by looking at existing code that uses it. Default paramet
er
s are more difficult to maintain because copy-and-paste from previous code may not reveal all the paramet
er
s. Copy-and-pasting of code segments can cause major problems when the default arguments are not appropriate for the new code. Decision: Except as described below, we require all arguments to be explicitly specified, to force programm
er
s to consid
er
the API and the values they are passing for each argument rath
er
than silently accepting defaults they may not be aware of. One specific exception is when default arguments are used to simulate variable-length argument lists. // Support up to 4 params by using a default empty AlphaNum. string StrCat(const AlphaNum &a, const AlphaNum &b = gEmptyAlphaNum, const AlphaNum &c = gEmptyAlphaNum, const AlphaNum &d = gEmptyAlphaNum); Variable-Length Arrays and alloca() link ▶We do not allow variable-length arrays or alloca(). Pros: Variable-length arrays have natural-looking syntax. Both variable-length arrays and alloca() are v
er
y efficient. Cons: Variable-length arrays and alloca are not part of Standard C++. More importantly, they allocate a data-dependent amount of stack space that can trigg
er
difficult-to-find memory ov
er
writing bugs: "It ran fine on my machine, but dies myst
er
iously in production". Decision: Use a safe allocator instead, such as scoped_ptr/scoped_array. Friends link ▶We allow use of friend classes and functions, within reason. Friends should usually be defined in the same file so that the read
er
does not have to look in anoth
er
file to find uses of the private memb
er
s of a class. A common use of friend is to have a FooBuild
er
class be a friend of Foo so that it can construct the inn
er
state of Foo correctly, without exposing this state to the world. In some cases it may be useful to make a unittest class a friend of the class it tests. Friends extend, but do not break, the encapsulation boundary of a class. In some cases this is bett
er
than making a memb
er
public when you want to give only one oth
er
class access to it. Howev
er
, most classes should int
er
act with oth
er
classes solely through their public memb
er
s. Exceptions link ▶We do not use C++ exceptions. Pros: Exceptions allow high
er
levels of an application to decide how to handle "can't happen" failures in deeply nested functions, without the obscuring and
er
ror-prone bookkeeping of
er
ror codes. Exceptions are used by most oth
er
mod
er
n languages. Using them in C++ would make it more consistent with Python, Java, and the C++ that oth
er
s are familiar with. Some third-party C++ libraries use exceptions, and turning them off int
er
nally makes it hard
er
to integrate with those libraries. Exceptions are the only way for a constructor to fail. We can simulate this with a factory function or an Init() method, but these require heap allocation or a new "invalid" state, respectively. Exceptions are really handy in testing frameworks. Cons: When you add a throw statement to an existing function, you must examine all of its transitive call
er
s. Eith
er
they must make at least the basic exception safety guarantee, or they must nev
er
catch the exception and be happy with the program t
er
minating as a result. For instance, if f() calls g() calls h(), and h throws an exception that f catches, g has to be careful or it may not clean up prop
er
ly. More gen
er
ally, exceptions make the control flow of programs difficult to evaluate by looking at code: functions may return in places you don't expect. This causes maintainability and debugging difficulties. You can minimize this cost via some rules on how and wh
er
e exceptions can be used, but at the cost of more that a develop
er
needs to know and und
er
stand. Exception safety requires both RAII and diff
er
ent coding practices. Lots of supporting machin
er
y is needed to make writing correct exception-safe code easy. Furth
er
, to avoid requiring read
er
s to und
er
stand the entire call graph, exception-safe code must isolate logic that writes to p
er
sistent state into a "commit" phase. This will have both benefits and costs (p
er
haps wh
er
e you're forced to obfuscate code to isolate the commit). Allowing exceptions would force us to always pay those costs even when they're not worth it. Turning on exceptions adds data to each binary produced, increasing compile time (probably slightly) and possibly increasing address space pressure. The availability of exceptions may encourage develop
er
s to throw them when they are not appropriate or recov
er
from them when it's not safe to do so. For example, invalid us
er
input should not cause exceptions to be thrown. We would need to make the style guide even long
er
to document these restrictions! Decision: On their face, the benefits of using exceptions outweigh the costs, especially in new projects. Howev
er
, for existing code, the introduction of exceptions has implications on all dependent code. If exceptions can be propagated beyond a new project, it also becomes problematic to integrate the new project into existing exception-free code. Because most existing C++ code at Google is not prepared to deal with exceptions, it is comparatively difficult to adopt new code that gen
er
ates exceptions. Given that Google's existing code is not exception-tol
er
ant, the costs of using exceptions are somewhat great
er
than the costs in a new project. The conv
er
sion process would be slow and
er
ror-prone. We don't believe that the available alt
er
natives to exceptions, such as
er
ror codes and ass
er
tions, introduce a significant burden. Our advice against using exceptions is not predicated on philosophical or moral grounds, but practical ones. Because we'd like to use our open-source projects at Google and it's difficult to do so if those projects use exceptions, we need to advise against exceptions in Google open-source projects as well. Things would probably be diff
er
ent if we had to do it all ov
er
again from scratch. Th
er
e is an exception to this rule (no pun intended) for Windows code. Run-Time Type Information (RTTI) link ▶We do not use Run Time Type Information (RTTI). Definition: RTTI allows a programm
er
to qu
er
y the C++ class of an object at run time. Pros: It is useful in some unittests. For example, it is useful in tests of factory classes wh
er
e the test has to v
er
ify that a newly created object has the expected dynamic type. In rare circumstances, it is useful even outside of tests. Cons: A qu
er
y of type during run-time typically means a design problem. If you need to know the type of an object at runtime, that is often an indication that you should reconsid
er
the design of your class. Decision: Do not use RTTI, except in unittests. If you find yourself in need of writing code that behaves diff
er
ently based on the class of an object, consid
er
one of the alt
er
natives to qu
er
ying the type. Virtual methods are the pref
er
red way of executing diff
er
ent code paths depending on a specific subclass type. This puts the work within the object itself. If the work belongs outside the object and instead in some processing code, consid
er
a double-dispatch solution, such as the Visitor design patt
er
n. This allows a facility outside the object itself to det
er
mine the type of class using the built-in type system. If you think you truly cannot use those ideas, you may use RTTI. But think twice about it. :-) Then think twice again. Do not hand-implement an RTTI-like workaround. The arguments against RTTI apply just as much to workarounds like class hi
er
archies with type tags. Casting link ▶Use C++ casts like static_cast(). Do not use oth
er
cast formats like int y = (int)x; or int y = int(x);. Definition: C++ introduced a diff
er
ent cast system from C that distinguishes the types of cast op
er
ations. Pros: The problem with C casts is the ambiguity of the op
er
ation; sometimes you are doing a conv
er
sion (e.g., (int)3.5) and sometimes you are doing a cast (e.g., (int)"hello"); C++ casts avoid this. Additionally C++ casts are more visible when searching for them. Cons: The syntax is nasty. Decision: Do not use C-style casts. Instead, use these C++-style casts. Use static_cast as the equivalent of a C-style cast that does value conv
er
sion, or when you need to explicitly up-cast a
point
er
from a class to its sup
er
class. Use const_cast to remove the const qualifi
er
(see const). Use reint
er
pret_cast to do unsafe conv
er
sions of
point
er
types to and from
integ
er
and oth
er
point
er
types. Use this only if you know what you are doing and you und
er
stand the aliasing issues. Do not use dynamic_cast except in test code. If you need to know type information at runtime in this way outside of a unittest, you probably have a design flaw. Streams link ▶Use streams only for logging. Definition: Streams are a replacement for printf() and scanf(). Pros: With streams, you do not need to know the type of the object you are printing. You do not have problems with format strings not matching the argument list. (Though with gcc, you do not have that problem with printf eith
er
.) Streams have automatic constructors and destructors that open and close the relevant files. Cons: Streams make it difficult to do functionality like pread(). Some formatting (particularly the common format string idiom %.*s) is difficult if not impossible to do efficiently using streams without using printf-like hacks. Streams do not support op
er
ator reord
er
ing (the %1s directive), which is helpful for int
er
nationalization. Decision: Do not use streams, except wh
er
e required by a logging int
er
face. Use printf-like routines instead. Th
er
e are various pros and cons to using streams, but in this case, as in many oth
er
cases, consistency trumps the debate. Do not use streams in your code. Extended Discussion Th
er
e has been debate on this issue, so this explains the reasoning in great
er
depth. Recall the Only One Way guiding principle: we want to make sure that whenev
er
we do a c
er
tain type of I/O, the code looks the same in all those places. Because of this, we do not want to allow us
er
s to decide between using streams or using printf plus Read/Write/etc. Instead, we should settle on one or the oth
er
. We made an exception for logging because it is a pretty specialized application, and for historical reasons. Proponents of streams have argued that streams are the obvious choice of the two, but the issue is not actually so clear. For ev
er
y advantage of streams they
point
out, th
er
e is an equivalent disadvantage. The biggest advantage is that you do not need to know the type of the object to be printing. This is a fair
point
. But, th
er
e is a downside: you can easily use the wrong type, and the compil
er
will not warn you. It is easy to make this kind of mistake without knowing when using streams. cout << this; // Prints the address cout << *this; // Prints the contents The compil
er
does not gen
er
ate an
er
ror because << has been ov
er
loaded. We discourage ov
er
loading for just this reason. Some say printf formatting is ugly and hard to read, but streams are often no bett
er
. Consid
er
the following two fragments, both with the same typo. Which is easi
er
to discov
er
? c
er
r << "
Er
ror connecting to '"
hostname.first << ":"
hostname.second << ": "
hostname.first, foo->bar()->hostname.second, str
er
ror(
er
rno)); And so on and so forth for any issue you might bring up. (You could argue, "Things would be bett
er
with the right wrapp
er
s," but if it is true for one scheme, is it not also true for the oth
er
? Also, rememb
er
the goal is to make the language small
er
, not add yet more machin
er
y that someone has to learn.) Eith
er
path would yield diff
er
ent advantages and disadvantages, and th
er
e is not a clearly sup
er
ior solution. The simplicity doctrine mandates we settle on one of them though, and the majority decision was on printf + read/write. Preincrement and Predecrement link ▶Use prefix form (++i) of the increment and decrement op
er
ators with it
er
ators and oth
er
template objects. Definition: When a variable is incremented (++i or i++) or decremented (--i or i--) and the value of the expression is not used, one must decide wheth
er
to preincrement (decrement) or postincrement (decrement). Pros: When the return value is ignored, the "pre" form (++i) is nev
er
less efficient than the "post" form (i++), and is often more efficient. This is because post-increment (or decrement) requires a copy of i to be made, which is the value of the expression. If i is an it
er
ator or oth
er
non-scalar type, copying i could be expensive. Since the two types of increment behave the same when the value is ignored, why not just always pre-increment? Cons: The tradition developed, in C, of using post-increment when the expression value is not used, especially in for loops. Some find post-increment easi
er
to read, since the "subject" (i) precedes the "v
er
b" (++), just like in English. Decision: For simple scalar (non-object) values th
er
e is no reason to pref
er
one form and we allow eith
er
. For it
er
ators and oth
er
template types, use pre-increment. Use of const link ▶We strongly recommend that you use const whenev
er
it makes sense to do so. Definition: Declared variables and paramet
er
s can be preceded by the keyword const to indicate the variables are not changed (e.g., const int foo). Class functions can have the const qualifi
er
to indicate the function does not change the state of the class memb
er
variables (e.g., class Foo { int Bar(char c) const; };). Pros: Easi
er
for people to und
er
stand how variables are being used. Allows the compil
er
to do bett
er
type checking, and, conceivably, gen
er
ate bett
er
code. Helps people convince themselves of program correctness because they know the functions they call are limited in how they can modify your variables. Helps people know what functions are safe to use without locks in multi-threaded programs. Cons: const is viral: if you pass a const variable to a function, that function must have const in its prototype (or the variable will need a const_cast). This can be a particular problem when calling library functions. Decision: const variables, data memb
er
s, methods and arguments add a level of compile-time type checking; it is bett
er
to detect
er
rors as soon as possible. Th
er
efore we strongly recommend that you use const whenev
er
it makes sense to do so: If a function does not modify an argument passed by ref
er
ence or by
point
er
, that argument should be const. Declare methods to be const whenev
er
possible. Accessors should almost always be const. Oth
er
methods should be const if they do not modify any data memb
er
s, do not call any non-const methods, and do not return a non-const
point
er
or non-const ref
er
ence to a data memb
er
. Consid
er
making data memb
er
s const whenev
er
they do not need to be modified aft
er
construction. Howev
er
, do not go crazy with const. Something like const int * const * const x; is likely ov
er
kill, even if it accurately describes how const x is. Focus on what's really useful to know: in this case, const int** x is probably sufficient. The mutable keyword is allowed but is unsafe when used with threads, so thread safety should be carefully consid
er
ed first. Wh
er
e to put the const Some people favor the form int const *foo to const int* foo. They argue that this is more readable because it's more consistent: it keeps the rule that const always follows the object it's describing. Howev
er
, this consistency argument doesn't apply in this case, because the "don't go crazy" dictum eliminates most of the uses you'd have to be consistent with. Putting the const first is arguably more readable, since it follows English in putting the "adjective" (const) before the "noun" (int). That said, while we encourage putting const first, we do not require it. But be consistent with the code around you!
Integ
er
Types link ▶Of the built-in C++
integ
er
types, the only one used is int. If a program needs a variable of a diff
er
ent size, use a precise-width
integ
er
type from , such as int16_t. Definition: C++ does not specify the sizes of its
integ
er
types. Typically people assume that short is 16 bits, int is 32 bits, long is 32 bits and long long is 64 bits. Pros: Uniformity of declaration. Cons: The sizes of integral types in C++ can vary based on compil
er
and architecture. Decision: defines types like int16_t, uint32_t, int64_t, etc. You should always use those in pref
er
ence to short, unsigned long long and the like, when you need a guarantee on the size of an
integ
er
. Of the C
integ
er
types, only int should be used. When appropriate, you are welcome to use standard types like size_t and ptrdiff_t. We use int v
er
y often, for
integ
er
s we know are not going to be too big, e.g., loop count
er
s. Use plain old int for such things. You should assume that an int is at least 32 bits, but don't assume that it has more than 32 bits. If you need a 64-bit
integ
er
type, use int64_t or uint64_t. For
integ
er
s we know can be "big", use int64_t. You should not use the unsigned
integ
er
types such as uint32_t, unless the quantity you are representing is really a bit patt
er
n rath
er
than a numb
er
, or unless you need defined twos-complement ov
er
flow. In particular, do not use unsigned types to say a numb
er
will nev
er
be negative. Instead, use ass
er
tions for this. On Unsigned
Integ
er
s Some people, including some textbook authors, recommend using unsigned types to represent numb
er
s that are nev
er
negative. This is intended as a form of self-documentation. Howev
er
, in C, the advantages of such documentation are outweighed by the real bugs it can introduce. Consid
er
: for (unsigned int i = foo.Length()-1; i >= 0; --i) ... This code will nev
er
t
er
minate! Sometimes gcc will notice this bug and warn you, but often it will not. Equally bad bugs can occur when comparing signed and unsigned variables. Basically, C's type-promotion scheme causes unsigned types to behave diff
er
ently than one might expect. So, document that a variable is non-negative using ass
er
tions. Don't use an unsigned type. 64-bit Portability link ▶Code should be 64-bit and 32-bit friendly. Bear in mind problems of printing,
comparison
s, and structure alignment. printf() specifi
er
s for some types are not cleanly portable between 32-bit and 64-bit systems. C99 defines some portable format specifi
er
s. Unfortunately, MSVC 7.1 does not und
er
stand some of these specifi
er
s and the standard is missing a few, so we have to define our own ugly v
er
sions in some cases (in the style of the standard include file inttypes.h): // printf macros for size_t, in the style of inttypes.h #ifdef _LP64 #define __PRIS_PREFIX "z" #else #define __PRIS_PREFIX #endif // Use these macros aft
er
a % in a printf format string // to get correct 32/64 bit behavior, like this: // size_t size = records.size(); // printf("%"PRIuS"\n", size); #define PRIdS __PRIS_PREFIX "d" #define PRIxS __PRIS_PREFIX "x" #define PRIuS __PRIS_PREFIX "u" #define PRIXS __PRIS_PREFIX "X" #define PRIoS __PRIS_PREFIX "o" Type DO NOT use DO use Notes void * (or any
point
er
) %lx %p int64_t %qd, %lld %"PRId64" uint64_t %qu, %llu, %llx %"PRIu64", %"PRIx64" size_t %u %"PRIuS", %"PRIxS" C99 specifies %zu ptrdiff_t %d %"PRIdS" C99 specifies %zd Note that the PRI* macros expand to independent strings which are concatenated by the compil
er
. Hence if you are using a non-constant format string, you need to ins
er
t the value of the macro into the format, rath
er
than the name. It is still possible, as usual, to include length specifi
er
s, etc., aft
er
the % when using the PRI* macros. So, e.g. printf("x = %30"PRIuS"\n", x) would expand on 32-bit Linux to printf("x = %30" "u" "\n", x), which the compil
er
will treat as printf("x = %30u\n", x). Rememb
er
that sizeof(void *) != sizeof(int). Use intptr_t if you want a
point
er
-sized
integ
er
. You may need to be careful with structure alignments, particularly for structures being stored on disk. Any class/structure with a int64_t/uint64_t memb
er
will by default end up being 8-byte aligned on a 64-bit system. If you have such structures being shared on disk between 32-bit and 64-bit code, you will need to ensure that they are packed the same on both architectures. Most compil
er
s off
er
a way to alt
er
structure alignment. For gcc, you can use __attribute__((packed)). MSVC off
er
s #pragma pack() and __declspec(align()). Use the LL or ULL suffixes a
FlexGraphics_V_1.79_D4-XE10.2_Downloadly.ir
V
er
sion 1.7 ----------- - ADD: Delphi/CBuild
er
10.2 Tokyo now supported. - ADD: Delphi/CBuild
er
10.1 B
er
lin now supported. - ADD: Delphi/CBuild
er
10 Seattle now supported. - ADD: Delphi/CBuild
er
XE8 now supported. - ADD: Delphi/CBuild
er
XE7 now supported. - ADD: Delphi/CBuild
er
XE6 now supported. - ADD: Delphi/CBuild
er
XE5 now supported. - ADD: Delphi/CBuild
er
XE4 now supported. - ADD: Delphi/CBuild
er
XE3 now supported. - ADD: Delphi/CBuild
er
XE2 now supported. - ADD: Delphi/CBuild
er
XE now supported. - ADD: Delphi/CBuild
er
2010 now supported. - ADD: Delphi/CBuild
er
2009 now supported. - ADD: New demo project FlexCADImport. - FIX: The height of the TFlexRegularPolygon object incorrectly changes with its rotation. - FIX: Added division by z
er
o protect in method TFlexControl.MovePathSegment. - FIX: The background beyond docuemnt wasn't filled when TFlexPanel.DocClipping=True. - FIX: In "Windows ClearType" font rend
er
ing mode (OS Windows mode) the "garbage" pixels can appear from the right and from the bottom sides of the painted rectangle of the TFlexText object. - FIX: The result rectangle incorrectly calculated in the TFlexText.GetRefreshRect method. - FIX: Added FPaintCache.rcPaint cleanup in the TFlexPanel.WMPaint method. Now it is possible to define is the drawing take place via WMPaint or via the PaintTo direct call (if rcPaint contain non-empty rectangle then WMPaint in progress). - FIX: The TFlexPanel.FPaintCache field moved in the protected class section. Added rcPaint field in FPaintCache that represents drawing rectangle. - ADD: In the text prcise mode (TFlexText.Precise=True) takes into account the rotation angle (TFlexText.Angle). - FIX: Removed FG_NEWTEXTROTATE directive (the TFlexText Precise mode should be used instead). - FIX: The TFlexRegularPolygon object clones incorrectly drawed in case when TFlexRegularPolygon have alt
er
native brush (gradient, texture). - ADD: Add TFlexPanel.InvalidateControl virtual method which calls from TFle
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