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How to ensure two template member functions specificated with same type?
noho
2002-12-30 03:53:04
class A
{
template<typename T> foo(T *);
template<typename T> bar(T *);
};
except for using RTTI technology.
Any suggestion are appreciated.
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How to ensure two template member functions specificated with same type?
class A { template foo(T *); template bar(T *); }; except for using RTTI technology. Any suggestion are appreciated.
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noho
2003-01-06
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//nod
有人从理论上证明了compile-time check是不可能的。
结贴。
rushman
2003-01-04
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--如果foo不是foo,而是class A的构造函数呢?
如果foo是class A的构造函数,那就需要将这个类进行封装(使用这个类的实例)。既然这样,为什么不把它设计成模板?如果需要,可以先设计一个虚基类,再派生出来这个模板。这样一般来说,可以实现与你的类相同的功能。
反过来,也说明你在类的设计上先天就有问题。有时候,不一定非要在墙上打个洞再钻过去,退一步,也许就会发现另有通途。^_^
noho
2003-01-04
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如果foo不是foo,而是class A的构造函数呢?
rushman
2003-01-01
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Happy new year..........
rushman
2003-01-01
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既然能在编译期进行检查,就说明这两个函数的调用是由程序员安排的,所以我认为关键应该在于调用时的处理。
为了不至于在调用时产生混乱,应该将对这两个函数的调用部分封装起来。
首先,将这两个函数保护(protected)起来,然后,将调用这两个函数的代码放在一起,外部只能通过这些接口来访问。
//----------------------------------------------
另写一个模板函数当作跳板来调用这两个函数。
templact<class T>
void function(T * ){
foo(...);
bar(...);
}
//----------------------------------------------
把两个函数变成一个函数。从软件工程的角度,这是个馊主意。从面向对象的角度,这叫基于消息???^_^
template<class T>
void function(T*p,int msg)
{
switch(msg){
case FOO:....
case BAR:....
......
}
}
//---------------------------------------------
将这两个函数调用的接口设计成类,灵活性、封装和实用性更好。
template<class T>
class X{
public:
//使其界面象一个函数
void operator()(int msg,T*)
{
switch(msg){
case FOO:....
case BAR:....
......
}
}
};
noho
2002-12-31
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我找到了一个方法,可惜是在运行时检查,不知道哪位能提供编译时检查的版本。
template<typename T>
struct TypeInfo
{
static void constraints(void){};
};
class A
{
template<typename T> foo(T *)
{
constraints = &TypeInfo<T>::constraints;
}
template<typename T> bar(T *)
{
assert(constraints == &TypeInfo<T>::constraints);
}
void (*constraints)();
};
foo一定先于bar调用。
qing_li73
2002-12-31
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concern
zfluo
2002-12-31
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template <class T>
class A
{
void foo(T*);
void bar(T*);
};
Google C++ Style Guide(Google C++编程规范)高清PDF
Table of Contents Header Files The #define Guard Header File Dependencies Inline
Function
s The -inl.h Files
Function
Parameter Ordering Names and Order of Includes Scoping Namespaces Nest
ed
Classes Non
member
, Static
Member
, and Global
Function
s Local Variables Static and Global Variables Classes Doing Work in Constructors Default Constructors Explicit Constructors Copy Constructors Structs vs. Classes Inheritance Multiple Inheritance Interfaces Operator Overloading Access Control Declaration Order Write Short
Function
s Google-
Specifi
c Magic Smart Pointers cpplint Other C++ Features Reference Arguments
Function
Overloading Default Arguments Variable-Length Arrays and alloca() Friends Exceptions Run-Time
Type
Information (RTTI) Casting Streams Preincrement and Pr
ed
ecrement Use of const Integer
Type
s 64-bit Portability Preprocessor Macros 0 and NULL sizeof Boost C++0x Naming General Naming Rules File Names
Type
Names Variable Names Constant Names
Function
Names Namespace Names Enumerator 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 Depre
cat
ion Comments Formatting Line Length Non-ASCII Characters Spaces vs. Tabs
Function
Declarations and Definitions
Function
Calls Conditionals Loops and Switch Statements Pointer and Reference Expressions Boolean Expressions Return Values Variable and Array Initialization Preprocessor Directives Class Format Constructor Initializer Lists Namespace Formatting Horizontal Whitespace Vertical 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 mark
ed
by the triangle icon, which you see here on your left. Click it now. You should see "Hooray" appear below. Hooray! Now you know you can expand points to get more details. Alternatively, there's an "expand all" at the top of this document. Background C++ is the main development language us
ed
by many of Google's open-source projects. As every C++ programmer knows, the language has many powerful features, but this power brings with it complexity, which in turn can make code more bug-prone and harder 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 coders to use C++ language features productively. Style, also known as readability, is what we call the conventions that govern our C++ code. The term Style is a bit of a misnomer, since these conventions cover far more than just source file formatting. One way in which we keep the code base manageable is by enforcing consistency. It is very important that any programmer be able to look at another's code and quickly understand it. Maintaining a uniform style and following conventions means that we can more easily use "pattern-matching" to infer what various symbols are and what invariants are true about them. Creating common, requir
ed
idioms and patterns makes code much easier to understand. In some cases there might be good arguments for changing certain style rules, but we nonetheless keep things as they are in order to preserve consistency. Another issue this guide addresses is that of C++ feature bloat. C++ is a huge language with many advanc
ed
features. In some cases we constrain, or even ban, use of certain features. We do this to keep code simple and to avoid the various common errors and problems that these features can cause. This guide lists these features and explains why their use is restrict
ed
. Open-source projects develop
ed
by Google conform to the requirements in this guide. Note that this guide is not a C++ tutorial: we assume that the reader is familiar with the language. Header Files In general, every .cc file should have an associat
ed
.h file. There are some common exceptions, such as unittests and small .cc files containing just a main()
function
. Correct use of header files can make a huge difference to the readability, size and performance of your code. The following rules will guide you through the various pitfalls of using header files. The #define Guard link ▶All header files should have #define guards to prevent multiple inclusion. The format of the symbol name should be ___H_. To guarantee uniqueness, they should be bas
ed
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_ Header File Dependencies link ▶Don't use an #include when a forward declaration would suffice. When you include a header file you introduce a dependency that will cause your code to be recompil
ed
whenever the header file changes. If your header file includes other header files, any change to those files will cause any code that includes your header to be recompil
ed
. Therefore, we prefer to minimize includes, particularly includes of header files in other header files. You can significantly minimize the number of header files you ne
ed
to include in your own header files by using forward declarations. For example, if your header file uses the File class in ways that do not require access to the declaration of the File class, your header 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 header file without access to its definition? We can declare data
member
s of
type
Foo* or Foo&. We can declare (but not define)
function
s 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 ne
ed
the full definition to support automatic
type
conversion.) We can declare static data
member
s of
type
Foo. This is because static data
member
s are defin
ed
outside the class definition. On the other hand, you must include the header file for Foo if your class subclasses Foo or has a data
member
of
type
Foo. Sometimes it makes sense to have pointer (or better, scop
ed
_ptr)
member
s instead of object
member
s. However, this compli
cat
es code readability and imposes a performance penalty, so avoid doing this transformation if the only purpose is to minimize includes in header files. Of course, .cc files typically do require the definitions of the classes they use, and usually have to include several header files. Note: If you use a symbol Foo in your source file, you should bring in a definition for Foo yourself, either via an #include or via a forward declaration. Do not depend on the symbol being brought in transitively via headers not directly includ
ed
. One exception is if Foo is us
ed
in myfile.cc, it's ok to #include (or forward-declare) Foo in myfile.h, instead of myfile.cc. Inline
Function
s link ▶Define
function
s inline only when they are small, say, 10 lines or less. Definition: You can declare
function
s in a way that allows the compiler to expand them inline rather than calling them through the usual
function
call mechanism. Pros: Inlining a
function
can generate more efficient object code, as long as the inlin
ed
function
is small. Feel free to inline accessors and mutators, and other short, performance-critical
function
s. Cons: Overuse of inlining can actually make programs slower. Depending on a
function
's size, inlining it can cause the code size to increase or decrease. Inlining a very small accessor
function
will usually decrease code size while inlining a very large
function
can dramatically increase code size. On modern processors smaller code usually runs faster due to better 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 longer than they appear because of implicit
member
- and base-destructor calls! Another useful rule of thumb: it's typically not cost effective to inline
function
s with loops or switch statements (unless, in the common case, the loop or switch statement is never execut
ed
). It is important to know that
function
s are not always inlin
ed
even if they are declar
ed
as such; for example, virtual and recursive
function
s are not normally inlin
ed
. Usually recursive
function
s should not be inline. The main reason for making a virtual
function
inline is to place its definition in the class, either 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
function
s when ne
ed
ed
. The definition of an inline
function
ne
ed
s to be in a header file, so that the compiler has the definition available for inlining at the call sites. However, implementation code properly belongs in .cc files, and we do not like to have much actual code in .h files unless there is a readability or performance advantage. If an inline
function
definition is short, with very little, if any, logic in it, you should put the code in your .h file. For example, accessors and mutators should certainly be inside a class definition. More complex inline
function
s may also be put in a .h file for the convenience of the implementer and callers, 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 includ
ed
where necessary. Another use of -inl.h files is for definitions of
function
template
s. This can be us
ed
to keep your
template
definitions easy to read. Do not forget that a -inl.h file requires a #define guard just like any other header file.
Function
Parameter Ordering link ▶When defining a
function
, parameter order is: inputs, then outputs. Parameters to C/C++
function
s are either input to the
function
, output from the
function
, or both. Input parameters are usually values or const references, while output and input/output parameters will be non-const pointers. When ordering
function
parameters, put all input-only parameters before any output parameters. In particular, do not add new parameters to the end of the
function
just because they are new; place new input-only parameters before the output parameters. This is not a hard-and-fast rule. Parameters that are both input and output (often classes/structs) muddy the waters, and, as always, consistency with relat
ed
function
s may require you to bend the rule. Names and Order of Includes link ▶Use standard order for readability and to avoid hidden dependencies: C library, C++ library, other libraries' .h, your project's .h. All of a project's header files should be list
ed
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 includ
ed
as #include "base/logging.h" In dir/foo.cc, whose main purpose is to implement or test the stuff in dir2/foo2.h, order your includes as follows: dir2/foo2.h (preferr
ed
lo
cat
ion — see details below). C system files. C++ system files. Other libraries' .h files. Your project's .h files. The preferr
ed
ordering r
ed
uces hidden dependencies. We want every header file to be compilable on its own. The easiest way to achieve this is to make sure that every one of them is the first .h file #includ
ed
in some .cc. dir/foo.cc and dir2/foo2.h are often in the same directory (e.g. base/basic
type
s_test.cc and base/basic
type
s.h), but can be in different directories too. Within each section it is nice to order the includes alphabetically. For example, the includes in google-awesome-project/src/foo/internal/fooserver.cc might look like this: #include "foo/public/fooserver.h" // Preferr
ed
lo
cat
ion. #include #include #include #include #include "base/basic
type
s.h" #include "base/commandlineflags.h" #include "foo/public/bar.h" Scoping Namespaces link ▶Unnam
ed
namespaces in .cc files are encourag
ed
. With nam
ed
namespaces, choose the name bas
ed
on the project, and possibly its path. Do not use a using-directive. Definition: Namespaces subdivide the global scope into distinct, nam
ed
scopes, and so are useful for preventing name collisions in the global scope. Pros: Namespaces provide a (hierarchical) axis of naming, in addition to the (also hierarchical) name axis provid
ed
by classes. For example, if two different 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 (hierarchical) axis of naming, in addition to the (also hierarchical) name axis provid
ed
by classes. Use of unnam
ed
spaces in header files can easily cause violations of the C++ One Definition Rule (ODR). Decision: Use namespaces according to the policy describ
ed
below. Unnam
ed
Namespaces Unnam
ed
namespaces are allow
ed
and even encourag
ed
in .cc files, to avoid runtime naming conflicts: namespace { // This is in a .cc file. // The content of a namespace is not indent
ed
enum { kUnus
ed
, kEOF, kError }; // Commonly us
ed
tokens. bool AtEof() { return pos_ == kEOF; } // Uses our namespace's EOF. } // namespace However, file-scope declarations that are associat
ed
with a particular class may be declar
ed
in that class as
type
s, static data
member
s or static
member
function
s rather than as
member
s of an unnam
ed
namespace. Terminate the unnam
ed
namespace as shown, with a comment // namespace. Do not use unnam
ed
namespaces in .h files. Nam
ed
Namespaces Nam
ed
namespaces should be us
ed
as follows: Namespaces wrap the entire source file after includes, gflags definitions/declarations, and forward declarations of classes from other 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
function
s is within scope of the namespace. void MyClass::Foo() { ... } } // namespace mynamespace The typical .cc file might have more complex detail, including the ne
ed
to reference classes in other 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 undefin
ed
behavior, i.e., not portable. To declare entities from the standard library, include the appropriate header 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 anywhere in a .cc file, and in
function
s, 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 allow
ed
anywhere in a .cc file, anywhere inside the nam
ed
namespace that wraps an entire .h file, and in
function
s and methods. // Shorten access to some commonly us
ed
names in .cc files. namespace fbz = ::foo::bar::baz; // Shorten access to some commonly us
ed
names (in a .h file). namespace librarian { // The following alias is available to all files including // this header (in namespace librarian): // alias names should therefore 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 everyone #including that file, so public headers (those available outside a project) and headers transitively #includ
ed
by them, should avoid defining aliases, as part of the general goal of keeping public APIs as small as possible. Nest
ed
Classes link ▶Although you may use public nest
ed
classes when they are part of an interface, consider a namespace to keep declarations out of the global scope. Definition: A class can define another class within it; this is also call
ed
a
member
class. class Foo { private: // Bar is a
member
class, nest
ed
within Foo. class Bar { ... }; }; Pros: This is useful when the nest
ed
(or
member
) class is only us
ed
by the enclosing class; making it a
member
puts it in the enclosing class scope rather than polluting the outer scope with the class name. Nest
ed
classes can be forward declar
ed
within the enclosing class and then defin
ed
in the .cc file to avoid including the nest
ed
class definition in the enclosing class declaration, since the nest
ed
class definition is usually only relevant to the implementation. Cons: Nest
ed
classes can be forward-declar
ed
only within the definition of the enclosing class. Thus, any header file manipulating a Foo::Bar* pointer will have to include the full class declaration for Foo. Decision: Do not make nest
ed
classes public unless they are actually part of the interface, e.g., a class that holds a set of options for some method. Non
member
, Static
Member
, and Global
Function
s link ▶Prefer non
member
function
s within a namespace or static
member
function
s to global
function
s; use completely global
function
s rarely. Pros: Non
member
and static
member
function
s can be useful in some situations. Putting non
member
function
s in a namespace avoids polluting the global namespace. Cons: Non
member
and static
member
function
s may make more sense as
member
s of a new class, especially if they access external 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 either a static
member
or a non
member
function
. Non
member
function
s should not depend on external variables, and should nearly always exist in a namespace. Rather than creating classes only to group static
member
function
s which do not share static data, use namespaces instead.
Function
s defin
ed
in the same compilation unit as production classes may introduce unnecessary coupling and link-time dependencies when directly call
ed
from other compilation units; static
member
function
s are particularly susceptible to this. Consider extracting a new class, or placing the
function
s in a namespace possibly in a separate library. If you must define a non
member
function
and it is only ne
ed
ed
in its .cc file, use an unnam
ed
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 anywhere 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 easier for the reader to find the declaration and see what
type
the variable is and what it was initializ
ed
to. In particular, initialization should be us
ed
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 another 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; There is one caveat: if the variable is an object, its constructor is invok
ed
every time it enters scope and is creat
ed
, and its destructor is invok
ed
every time it goes out of scope. // Inefficient implementation: for (int i = 0; i < 1000000; ++i) { Foo f; // My ctor and dtor get call
ed
1000000 times each. f.DoSomething(i); } It may be more efficient to declare such a variable us
ed
in a loop outside that loop: Foo f; // My ctor and dtor get call
ed
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 indeterminate order of construction and destruction. Objects with static storage duration, including global variables, static variables, static class
member
variables, and
function
static variables, must be Plain Old Data (POD): only ints, chars, floats, or pointers, or arrays/structs of POD. The order in which class constructors and initializers for static variables are call
ed
is only partially
specifi
ed
in C++ and can even change from build to build, which can cause bugs that are difficult to find. Therefore in addition to banning globals of class
type
, we do not allow static POD variables to be initializ
ed
with the result of a
function
, unless that
function
(such as getenv(), or getpid()) does not itself depend on any other globals. Likewise, the order in which destructors are call
ed
is defin
ed
to be the reverse of the order in which the constructors were call
ed
. Since constructor order is indeterminate, so is destructor order. For example, at program-end time a static variable might have been destroy
ed
, but code still running -- perhaps in another thread -- tries to access it and fails. Or the destructor for a static 'string' variable might be run prior to the destructor for another variable that contains a reference 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 ne
ed
a static or global variable of a class
type
, consider initializing a pointer (which will never be fre
ed
), from either your main()
function
or from pthread_once(). Note that this must be a raw pointer, not a "smart" pointer, since the smart pointer's destructor will have the order-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 general, constructors should merely set
member
variables to their initial values. Any complex initialization should go in an explicit Init() method. Definition: It is possible to perform initialization in the body of the constructor. Pros: Convenience in typing. No ne
ed
to worry about whether the class has been initializ
ed
or not. Cons: The problems with doing work in constructors are: There is no easy way for constructors to signal errors, short of using exceptions (which are forbidden). If the work fails, we now have an object whose initialization code fail
ed
, so it may be an indeterminate state. If the work calls virtual
function
s, these calls will not get dispatch
ed
to the subclass implementations. Future modifi
cat
ion to your class can quietly introduce this problem even if your class is not currently subclass
ed
, causing much confusion. If someone creates a global variable of this
type
(which is against the rules, but still), the constructor code will be call
ed
before main(), possibly breaking some implicit assumptions in the constructor code. For instance, gflags will not yet have been initializ
ed
. Decision: If your object requires non-trivial initialization, consider having an explicit Init() method. In particular, constructors should not call virtual
function
s, attempt to raise errors, access potentially uninitializ
ed
global variables, etc. Default Constructors link ▶You must define a default constructor if your class defines
member
variables and has no other constructors. Otherwise the compiler will do it for you, badly. Definition: The default constructor is call
ed
when we new a class object with no arguments. It is always call
ed
when calling new[] (for arrays). Pros: Initializing structures by default, to hold "impossible" values, makes debugging much easier. Cons: Extra work for you, the code writer. Decision: If your class defines
member
variables and has no other constructors you must define a default constructor (one that takes no arguments). It should preferably initialize the object in such a way that its internal state is consistent and valid. The reason for this is that if you have no other constructors and do not define a default constructor, the compiler will generate one for you. This compiler generat
ed
constructor may not initialize your object sensibly. If your class inherits from an existing class but you add no new
member
variables, you are not requir
ed
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 us
ed
as a conversion. 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 call
ed
to convert 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 convert
ed
and new objects creat
ed
without you meaning them to. Declaring a constructor explicit prevents it from being invok
ed
implicitly as a conversion. Pros: Avoids undesirable conversions. 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 intend
ed
to be transparent wrappers around other classes are also exceptions. Such exceptions should be clearly mark
ed
with comments. Copy Constructors link ▶Provide a copy constructor and assignment operator only when necessary. Otherwise, disable them with DISALLOW_COPY_AND_ASSIGN. Definition: The copy constructor and assignment operator are us
ed
to create copies of objects. The copy constructor is implicitly invok
ed
by the compiler in some situations, e.g. passing objects by value. Pros: Copy constructors make it easy to copy objects. STL containers 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 compiler can elide them in some contexts, and they make it easier to avoid heap allo
cat
ion. Cons: Implicit copying of objects in C++ is a rich source of bugs and of performance problems. It also r
ed
uces readability, as it becomes hard to track which objects are being pass
ed
around by value as oppos
ed
to by reference, and therefore where changes to an object are reflect
ed
. Decision: Few classes ne
ed
to be copyable. Most should have neither a copy constructor nor an assignment operator. In many situations, a pointer or reference will work just as well as a copi
ed
value, with better performance. For example, you can pass
function
parameters by reference or pointer instead of by value, and you can store pointers rather than objects in an STL container. If your class ne
ed
s to be copyable, prefer providing a copy method, such as CopyFrom() or Clone(), rather than a copy constructor, because such methods cannot be invok
ed
implicitly. If a copy method is insufficient in your situation (e.g. for performance reasons, or because your class ne
ed
s to be stor
ed
by value in an STL container), provide both a copy constructor and assignment operator. If your class does not ne
ed
a copy constructor or assignment operator, you must explicitly disable them. To do so, add dummy declarations for the copy constructor and assignment operator 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 error). For convenience, a DISALLOW_COPY_AND_ASSIGN macro can be us
ed
: // A macro to disallow the copy constructor and operator=
function
s // This should be us
ed
in the private: declarations for a class #define DISALLOW_COPY_AND_ASSIGN(
Type
Name) \
Type
Name(const
Type
Name&); \ void operator=(const
Type
Name&) 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; everything 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 us
ed
for passive objects that carry data, and may have associat
ed
constants, but lack any
function
ality other than access/setting the data
member
s. The accessing/setting of fields is done by directly accessing the fields rather than through method invo
cat
ions. Methods should not provide behavior but should only be us
ed
to set up the data
member
s, e.g., constructor, destructor, Initialize(), Reset(), Validate(). If more
function
ality is requir
ed
, 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
member
variables in structs and classes have different naming rules. Inheritance link ▶Composition is often more appropriate than inheritance. When using inheritance, make it public. Definition: When a sub-class inherits from a base class, it includes the definitions of all the data and operations that the parent base class defines. In practice, inheritance is us
ed
in two major ways in C++: implementation inheritance, in which actual code is inherit
ed
by the child, and interface inheritance, in which only method names are inherit
ed
. Pros: Implementation inheritance r
ed
uces code size by re-using the base class code as it specializes an existing
type
. Because inheritance is a compile-time declaration, you and the compiler can understand the operation and detect errors. Interface inheritance can be us
ed
to programmatically enforce that a class expose a particular API. Again, the compiler can detect errors, in this case, when a class does not define a necessary method of the API. Cons: For implementation inheritance, because the code implementing a sub-class is spread between the base and the sub-class, it can be more difficult to understand an implementation. The sub-class cannot override
function
s that are not virtual, so the sub-class cannot change implementation. The base class may also define some data
member
s, so that
specifi
es physical layout of the base class. Decision: All inheritance should be public. If you want to do private inheritance, you should be including an instance of the base class as a
member
instead. Do not overuse implementation inheritance. Composition is often more appropriate. Try to restrict use of inheritance 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 protect
ed
to those
member
function
s that might ne
ed
to be access
ed
from subclasses. Note that data
member
s should be private. When r
ed
efining an inherit
ed
virtual
function
, explicitly declare it virtual in the declaration of the deriv
ed
class. Rationale: If virtual is omitt
ed
, the reader has to check all ancestors of the class in question to determine if the
function
is virtual or not. Multiple Inheritance link ▶Only very rarely is multiple implementation inheritance actually useful. We allow multiple inheritance only when at most one of the base classes has an implementation; all other base classes must be pure interface classes tagg
ed
with the Interface suffix. Definition: Multiple inheritance allows a sub-class to have more than one base class. We distinguish between base classes that are pure interfaces and those that have an implementation. Pros: Multiple implementation inheritance may let you re-use even more code than single inheritance (see Inheritance). Cons: Only very rarely is multiple implementation inheritance actually useful. When multiple implementation inheritance seems like the solution, you can usually find a different, more explicit, and cleaner solution. Decision: Multiple inheritance is allow
ed
only when all superclasses, with the possible exception of the first one, are pure interfaces. In order to
ensu
re that they remain pure interfaces, they must end with the Interface suffix. Note: There is an exception to this rule on Windows. Interfaces link ▶Classes that satisfy certain conditions are allow
ed
, but not requir
ed
, to end with an Interface suffix. Definition: A class is a pure interface 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
member
s. It ne
ed
not have any constructors defin
ed
. If a constructor is provid
ed
, it must take no arguments and it must be protect
ed
. If it is a subclass, it may only be deriv
ed
from classes that satisfy these conditions and are tagg
ed
with the Interface suffix. An interface class can never be directly instantiat
ed
because of the pure virtual method(s) it declares. To make sure all implementations of the interface can be destroy
ed
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
ed
ition, section 12.4 for details. Pros: Tagging a class with the Interface suffix lets others know that they must not add implement
ed
methods or non static data
member
s. This is particularly important in the case of multiple inheritance. Additionally, the interface concept is already well-understood by Java programmers. Cons: The Interface suffix lengthens the class name, which can make it harder to read and understand. Also, the interface property may be consider
ed
an implementation detail that shouldn't be expos
ed
to clients. Decision: A class may end with Interface only if it meets the above requirements. We do not require the converse, however: classes that meet the above requirements are not requir
ed
to end with Interface. Operator Overloading link ▶Do not overload operators except in rare, special circumstances. Definition: A class can define that operators such as + and / operate on the class as if it were a built-in
type
. Pros: Can make code appear more intuitive because a class will behave in the same way as built-in
type
s (such as int). Overload
ed
operators are more playful names for
function
s that are less-colorfully nam
ed
, such as Equals() or Add(). For some
template
function
s to work correctly, you may ne
ed
to define operators. Cons: While operator overloading can make code more intuitive, it has several drawbacks: It can fool our intuition into thinking that expensive operations are cheap, built-in operations. It is much harder to find the call sites for overload
ed
operators. Searching for Equals() is much easier than searching for relevant invo
cat
ions of ==. Some operators work on pointers too, making it easy to introduce bugs. Foo + 4 may do one thing, while &Foo + 4 does something totally different. The compiler does not complain for either of these, making this very hard to debug. Overloading also has surprising ramifi
cat
ions. For instance, if a class overloads unary operator&, it cannot safely be forward-declar
ed
. Decision: In general, do not overload operators. The assignment operator (operator=), in particular, is insidious and should be avoid
ed
. You can define
function
s like Equals() and CopyFrom() if you ne
ed
them. Likewise, avoid the dangerous unary operator& at all costs, if there's any possibility the class might be forward-declar
ed
. However, there may be rare cases where you ne
ed
to overload an operator to interoperate with
template
s or "standard" C++ classes (such as operator<<(ostream&, const T&) for logging). These are acceptable if fully justifi
ed
, but you should try to avoid these whenever possible. In particular, do not overload operator== or operator< just so that your class can be us
ed
as a key in an STL container; instead, you should create equality and comparison functor
type
s when declaring the container. Some of the STL algorithms do require you to overload operator==, and you may do so in these cases, provid
ed
you document why. See also Copy Constructors and
Function
Overloading. Access Control link ▶Make data
member
s private, and provide access to them through accessor
function
s as ne
ed
ed
(for technical reasons, we allow data
member
s of a test fixture class to be protect
ed
when using Google Test). Typically a variable would be call
ed
foo_ and the accessor
function
foo(). You may also want a mutator
function
set_foo(). Exception: static const data
member
s (typically call
ed
kFoo) ne
ed
not be private. The definitions of accessors are usually inlin
ed
in the header file. See also Inheritance and
Function
Names. Declaration Order link ▶Use the
specifi
ed
order of declarations within a class: public: before private:, methods before data
member
s (variables), etc. Your class definition should start with its public: section, follow
ed
by its protect
ed
: section and then its private: section. If any of these sections are empty, omit them. Within each section, the declarations generally should be in the following order: Typ
ed
efs and Enums Constants (static const data
member
s) Constructors Destructor Methods, including static methods Data
Member
s (except static const data
member
s) Friend declarations should always be in the private section, and the DISALLOW_COPY_AND_ASSIGN macro invo
cat
ion 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 order, as much as possible. Do not put large method definitions inline in the class definition. Usually, only trivial or performance-critical, and very short, methods may be defin
ed
inline. See Inline
Function
s for more details. Write Short
Function
s link ▶Prefer small and focus
ed
function
s. We recognize that long
function
s are sometimes appropriate, so no hard limit is plac
ed
on
function
s length. If a
function
exce
ed
s about 40 lines, think about whether it can be broken up without harming the structure of the program. Even if your long
function
works perfectly now, someone modifying it in a few months may add new behavior. This could result in bugs that are hard to find. Keeping your
function
s short and simple makes it easier for other people to read and modify your code. You could find long and compli
cat
ed
function
s when working with some code. Do not be intimidat
ed
by modifying existing code: if working with such a
function
proves to be difficult, you find that errors are hard to debug, or you want to use a piece of it in several different contexts, consider breaking up the
function
into smaller and more manageable pieces. Google-
Specifi
c Magic There are various tricks and utilities that we use to make C++ code more robust, and various ways we use C++ that may differ from what you see elsewhere. Smart Pointers link ▶If you actually ne
ed
pointer semantics, scop
ed
_ptr is great. You should only use std::tr1::shar
ed
_ptr under very
specifi
c conditions, such as when objects ne
ed
to be held by STL containers. You should never use auto_ptr. "Smart" pointers are objects that act like pointers but have add
ed
semantics. When a scop
ed
_ptr is destroy
ed
, for instance, it deletes the object it's pointing to. shar
ed
_ptr is the same way, but implements reference-counting so only the last pointer to an object deletes it. Generally speaking, we prefer that we design code with clear object ownership. The clearest object ownership is obtain
ed
by using an object directly as a field or local variable, without using pointers at all. On the other extreme, by their very definition, reference count
ed
pointers are own
ed
by nobody. The problem with this design is that it is easy to create circular references or other strange conditions that cause an object to never be delet
ed
. It is also slow to perform atomic operations every time a value is copi
ed
or assign
ed
. Although they are not recommend
ed
, reference count
ed
pointers are sometimes the simplest and most elegant way to solve a problem. cpplint link ▶Use cpplint.py to detect style errors. cpplint.py is a tool that reads a source file and identifies many style errors. It is not perfect, and has both false positives and false negatives, but it is still a valuable tool. False positives can be ignor
ed
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. Other C++ Features Reference Arguments link ▶All parameters pass
ed
by reference must be label
ed
const. Definition: In C, if a
function
ne
ed
s to modify a variable, the parameter must use a pointer, eg int foo(int *pval). In C++, the
function
can alternatively declare a reference parameter: int foo(int &val). Pros: Defining a parameter as reference avoids ugly code like (*pval)++. Necessary for some appli
cat
ions like copy constructors. Makes it clear, unlike with pointers, that NULL is not a possible value. Cons: References can be confusing, as they have value syntax but pointer semantics. Decision: Within
function
parameter lists all references must be const: void Foo(const string &in, string *out); In fact it is a very strong convention in Google code that input arguments are values or const references while output arguments are pointers. Input parameters may be const pointers, but we never allow non-const reference parameters. One case when you might want an input parameter to be a const pointer is if you want to emphasize that the argument is not copi
ed
, so it must exist for the lifetime of the object; it is usually best to document this in comments as well. STL adapters such as bind2nd and mem_fun do not permit reference parameters, so you must declare
function
s with pointer parameters in these cases, too.
Function
Overloading link ▶Use overload
ed
function
s (including constructors) only if a reader looking at a call site can get a good idea of what is happening without having to first figure out exactly which overload is being call
ed
. Definition: You may write a
function
that takes a const string& and overload it with another that takes const char*. class MyClass { public: void Analyze(const string &text); void Analyze(const char *text, size_t textlen); }; Pros: Overloading can make code more intuitive by allowing an identically-nam
ed
function
to take different arguments. It may be necessary for templatiz
ed
code, and it can be convenient for Visitors. Cons: If a
function
is overload
ed
by the argument
type
s alone, a reader may have to understand C++'s complex matching rules in order to tell what's going on. Also many people are confus
ed
by the semantics of inheritance if a deriv
ed
class overrides only some of the variants of a
function
. Decision: If you want to overload a
function
, consider qualifying the name with some information about the arguments, e.g., AppendString(), AppendInt() rather than just Append(). Default Arguments link ▶We do not allow default
function
parameters, except in a few uncommon situations explain
ed
below. Pros: Often you have a
function
that uses lots of default values, but occasionally you want to override the defaults. Default parameters allow an easy way to do this without having to define many
function
s for the rare exceptions. Cons: People often figure out how to use an API by looking at existing code that uses it. Default parameters are more difficult to maintain because copy-and-paste from previous code may not reveal all the parameters. Copy-and-pasting of code segments can cause major problems when the default arguments are not appropriate for the new code. Decision: Except as describ
ed
below, we require all arguments to be explicitly
specifi
ed
, to force programmers to consider the API and the values they are passing for each argument rather than silently accepting defaults they may not be aware of. One
specifi
c exception is when default arguments are us
ed
to simulate variable-length argument lists. // Support up to 4 params by using a default empty AlphaNum. string Str
Cat
(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 very efficient. Cons: Variable-length arrays and alloca are not part of Standard C++. More importantly, they allo
cat
e a data-dependent amount of stack space that can trigger difficult-to-find memory overwriting bugs: "It ran fine on my machine, but dies mysteriously in production". Decision: Use a safe allo
cat
or instead, such as scop
ed
_ptr/scop
ed
_array. Friends link ▶We allow use of friend classes and
function
s, within reason. Friends should usually be defin
ed
in the same file so that the reader does not have to look in another file to find uses of the private
member
s of a class. A common use of friend is to have a FooBuilder class be a friend of Foo so that it can construct the inner 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 better than making a
member
public when you want to give only one other class access to it. However, most classes should interact with other classes solely through their public
member
s. Exceptions link ▶We do not use C++ exceptions. Pros: Exceptions allow higher levels of an appli
cat
ion to decide how to handle "can't happen" failures in deeply nest
ed
function
s, without the obscuring and error-prone bookkeeping of error codes. Exceptions are us
ed
by most other modern languages. Using them in C++ would make it more consistent with Python, Java, and the C++ that others are familiar with. Some third-party C++ libraries use exceptions, and turning them off internally makes it harder 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 allo
cat
ion 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 callers. Either they must make at least the basic exception safety guarantee, or they must never
cat
ch the exception and be happy with the program terminating as a result. For instance, if f() calls g() calls h(), and h throws an exception that f
cat
ches, g has to be careful or it may not clean up properly. More generally, exceptions make the control flow of programs difficult to evaluate by looking at code:
function
s 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 where exceptions can be us
ed
, but at the cost of more that a developer ne
ed
s to know and understand. Exception safety requires both RAII and different coding practices. Lots of supporting machinery is ne
ed
ed
to make writing correct exception-safe code easy. Further, to avoid requiring readers to understand the entire call graph, exception-safe code must isolate logic that writes to persistent state into a "commit" phase. This will have both benefits and costs (perhaps where you're forc
ed
to obfus
cat
e 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 produc
ed
, increasing compile time (probably slightly) and possibly increasing address space pressure. The availability of exceptions may encourage developers to throw them when they are not appropriate or recover from them when it's not safe to do so. For example, invalid user input should not cause exceptions to be thrown. We would ne
ed
to make the style guide even longer to document these restrictions! Decision: On their face, the benefits of using exceptions outweigh the costs, especially in new projects. However, for existing code, the introduction of exceptions has impli
cat
ions on all dependent code. If exceptions can be propagat
ed
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 prepar
ed
to deal with exceptions, it is comparatively difficult to adopt new code that generates exceptions. Given that Google's existing code is not exception-tolerant, the costs of using exceptions are somewhat greater than the costs in a new project. The conversion process would be slow and error-prone. We don't believe that the available alternatives to exceptions, such as error codes and assertions, introduce a significant burden. Our advice against using exceptions is not pr
ed
i
cat
ed
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 ne
ed
to advise against exceptions in Google open-source projects as well. Things would probably be different if we had to do it all over again from scratch. There is an exception to this rule (no pun intend
ed
) for Windows code. Run-Time
Type
Information (RTTI) link ▶We do not use Run Time
Type
Information (RTTI). Definition: RTTI allows a programmer to query 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 where the test has to verify that a newly creat
ed
object has the expect
ed
dynamic
type
. In rare circumstances, it is useful even outside of tests. Cons: A query of
type
during run-time typically means a design problem. If you ne
ed
to know the
type
of an object at runtime, that is often an indi
cat
ion that you should reconsider the design of your class. Decision: Do not use RTTI, except in unittests. If you find yourself in ne
ed
of writing code that behaves differently bas
ed
on the class of an object, consider one of the alternatives to querying the
type
. Virtual methods are the preferr
ed
way of executing different code paths depending on a
specifi
c subclass
type
. This puts the work within the object itself. If the work belongs outside the object and instead in some processing code, consider a double-dispatch solution, such as the Visitor design pattern. This allows a facility outside the object itself to determine 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 hierarchies with
type
tags. Casting link ▶Use C++ casts like static_cast(). Do not use other cast formats like int y = (int)x; or int y = int(x);. Definition: C++ introduc
ed
a different cast system from C that distinguishes the
type
s of cast operations. Pros: The problem with C casts is the ambiguity of the operation; sometimes you are doing a conversion (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 conversion, or when you ne
ed
to explicitly up-cast a pointer from a class to its superclass. Use const_cast to remove the const qualifier (see const). Use reinterpret_cast to do unsafe conversions of pointer
type
s to and from integer and other pointer
type
s. Use this only if you know what you are doing and you understand the aliasing issues. Do not use dynamic_cast except in test code. If you ne
ed
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 ne
ed
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 either.) Streams have automatic constructors and destructors that open and close the relevant files. Cons: Streams make it difficult to do
function
ality 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 operator reordering (the %1s directive), which is helpful for internationalization. Decision: Do not use streams, except where requir
ed
by a logging interface. Use printf-like routines instead. There are various pros and cons to using streams, but in this case, as in many other cases, consistency trumps the debate. Do not use streams in your code. Extend
ed
Discussion There has been debate on this issue, so this explains the reasoning in greater depth. Recall the Only One Way guiding principle: we want to make sure that whenever we do a certain
type
of I/O, the code looks the same in all those places. Because of this, we do not want to allow users to decide between using streams or using printf plus Read/Write/etc. Instead, we should settle on one or the other. We made an exception for logging because it is a pretty specializ
ed
appli
cat
ion, and for historical reasons. Proponents of streams have argu
ed
that streams are the obvious choice of the two, but the issue is not actually so clear. For every advantage of streams they point out, there is an equivalent disadvantage. The biggest advantage is that you do not ne
ed
to know the
type
of the object to be printing. This is a fair point. But, there is a downside: you can easily use the wrong
type
, and the compiler 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 compiler does not generate an error because << has been overload
ed
. We discourage overloading for just this reason. Some say printf formatting is ugly and hard to read, but streams are often no better. Consider the following two fragments, both with the same typo. Which is easier to discover? cerr << "Error connecting to '"
hostname.first << ":"
hostname.second << ": "
hostname.first, foo->bar()->hostname.second, strerror(errno)); And so on and so forth for any issue you might bring up. (You could argue, "Things would be better with the right wrappers," but if it is true for one scheme, is it not also true for the other? Also, re
member
the goal is to make the language smaller, not add yet more machinery that someone has to learn.) Either path would yield different advantages and disadvantages, and there is not a clearly superior solution. The simplicity doctrine mandates we settle on one of them though, and the majority decision was on printf + read/write. Preincrement and Pr
ed
ecrement link ▶Use prefix form (++i) of the increment and decrement operators with iterators and other
template
objects. Definition: When a variable is increment
ed
(++i or i++) or decrement
ed
(--i or i--) and the value of the expression is not us
ed
, one must decide whether to preincrement (decrement) or postincrement (decrement). Pros: When the return value is ignor
ed
, the "pre" form (++i) is never 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 iterator or other non-scalar
type
, copying i could be expensive. Since the two
type
s of increment behave the same when the value is ignor
ed
, why not just always pre-increment? Cons: The tradition develop
ed
, in C, of using post-increment when the expression value is not us
ed
, especially in for loops. Some find post-increment easier to read, since the "subject" (i) prec
ed
es the "verb" (++), just like in English. Decision: For simple scalar (non-object) values there is no reason to prefer one form and we allow either. For iterators and other
template
type
s, use pre-increment. Use of const link ▶We strongly recommend that you use const whenever it makes sense to do so. Definition: Declar
ed
variables and parameters can be prec
ed
ed
by the keyword const to indi
cat
e the variables are not chang
ed
(e.g., const int foo). Class
function
s can have the const qualifier to indi
cat
e the
function
does not change the state of the class
member
variables (e.g., class Foo { int Bar(char c) const; };). Pros: Easier for people to understand how variables are being us
ed
. Allows the compiler to do better
type
checking, and, conceivably, generate better code. Helps people convince themselves of program correctness because they know the
function
s they call are limit
ed
in how they can modify your variables. Helps people know what
function
s are safe to use without locks in multi-thread
ed
programs. Cons: const is viral: if you pass a const variable to a
function
, that
function
must have const in its proto
type
(or the variable will ne
ed
a const_cast). This can be a particular problem when calling library
function
s. Decision: const variables, data
member
s, methods and arguments add a level of compile-time
type
checking; it is better to detect errors as soon as possible. Therefore we strongly recommend that you use const whenever it makes sense to do so: If a
function
does not modify an argument pass
ed
by reference or by pointer, that argument should be const. Declare methods to be const whenever possible. Accessors should almost always be const. Other methods should be const if they do not modify any data
member
s, do not call any non-const methods, and do not return a non-const pointer or non-const reference to a data
member
. Consider making data
member
s const whenever they do not ne
ed
to be modifi
ed
after construction. However, do not go crazy with const. Something like const int * const * const x; is likely overkill, 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 allow
ed
but is unsafe when us
ed
with threads, so thread safety should be carefully consider
ed
first. Where 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. However, 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! Integer
Type
s link ▶Of the built-in C++ integer
type
s, the only one us
ed
is int. If a program ne
ed
s a variable of a different size, use a precise-width integer
type
from , such as int16_t. Definition: C++ does not specify the sizes of its integer
type
s. 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
type
s in C++ can vary bas
ed
on compiler and architecture. Decision: defines
type
s like int16_t, uint32_t, int64_t, etc. You should always use those in preference to short, unsign
ed
long long and the like, when you ne
ed
a guarantee on the size of an integer. Of the C integer
type
s, only int should be us
ed
. When appropriate, you are welcome to use standard
type
s like size_t and ptrdiff_t. We use int very often, for integers we know are not going to be too big, e.g., loop counters. 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 ne
ed
a 64-bit integer
type
, use int64_t or uint64_t. For integers we know can be "big", use int64_t. You should not use the unsign
ed
integer
type
s such as uint32_t, unless the quantity you are representing is really a bit pattern rather than a number, or unless you ne
ed
defin
ed
twos-complement overflow. In particular, do not use unsign
ed
type
s to say a number will never be negative. Instead, use assertions for this. On Unsign
ed
Integers Some people, including some textbook authors, recommend using unsign
ed
type
s to represent numbers that are never negative. This is intend
ed
as a form of self-documentation. However, in C, the advantages of such documentation are outweigh
ed
by the real bugs it can introduce. Consider: for (unsign
ed
int i = foo.Length()-1; i >= 0; --i) ... This code will never terminate! Sometimes gcc will notice this bug and warn you, but often it will not. Equally bad bugs can occur when comparing sign
ed
and unsign
ed
variables. Basically, C's
type
-promotion scheme causes unsign
ed
type
s to behave differently than one might expect. So, document that a variable is non-negative using assertions. Don't use an unsign
ed
type
. 64-bit Portability link ▶Code should be 64-bit and 32-bit friendly. Bear in mind problems of printing, comparisons, and structure alignment. printf()
specifi
ers for some
type
s are not cleanly portable between 32-bit and 64-bit systems. C99 defines some portable format
specifi
ers. Unfortunately, MSVC 7.1 does not understand some of these
specifi
ers and the standard is missing a few, so we have to define our own ugly versions in some cases (in the style of the standard include file int
type
s.h): // printf macros for size_t, in the style of int
type
s.h #ifdef _LP64 #define __PRIS_PREFIX "z" #else #define __PRIS_PREFIX #endif // Use these macros after 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 pointer) %lx %p int64_t %qd, %lld %"PRId64" uint64_t %qu, %llu, %llx %"PRIu64", %"PRIx64" size_t %u %"PRIuS", %"PRIxS" C99
specifi
es %zu ptrdiff_t %d %"PRIdS" C99
specifi
es %zd Note that the PRI* macros expand to independent strings which are con
cat
enat
ed
by the compiler. Hence if you are using a non-constant format string, you ne
ed
to insert the value of the macro into the format, rather than the name. It is still possible, as usual, to include length
specifi
ers, etc., after 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 compiler will treat as printf("x = %30u\n", x). Re
member
that sizeof(void *) != sizeof(int). Use intptr_t if you want a pointer-siz
ed
integer. You may ne
ed
to be careful with structure alignments, particularly for structures being stor
ed
on disk. Any class/structure with a int64_t/uint64_t
member
will by default end up being 8-byte align
ed
on a 64-bit system. If you have such structures being shar
ed
on disk between 32-bit and 64-bit code, you will ne
ed
to
ensu
re that they are pack
ed
the same on both architectures. Most compilers offer a way to alter structure alignment. For gcc, you can use __attribute__((pack
ed
)). MSVC offers #pragma pack() and __declspec(align()). Use the LL or ULL suffixes a
Windows Store apps开发[43]C++/CX Part 4 of [n]: Static
Member
Function
s
更多内容请查看下面的帖子 [DevDiv原创]Windows 8 开发Step by Step C++/CX Part 4 of [n]: Static
Member
Function
s See C++/CX Part 0 of [n]: An Introduction for an introduction
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