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virtual UINT Fun() = 0; 什么意思?
liumang9527
2008-03-19 11:07:00
virtual UINT Fun() = 0; 什么意思?
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virtual UINT Fun() = 0; 什么意思?
virtual UINT Fun() = 0; 什么意思?
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abupie
2008-03-20
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[Quote=引用 2 楼 AlwaysSLH 的回复:]
纯虚函数是一种特殊的虚函数,它的一般格式如下:
class <类名>
{
virtual <类型> <函数名> ( <参数表> )=0;
…
};
在许多情况下,在基类中不能对虚函数给出有意义有实现,而把它说明为纯虚函数,它的实现留给该基类的派生类去做。
[/Quote]
正解
yeing2008
2008-03-20
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为其他子类提供界面的函数。
taodm
2008-03-20
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楼主,你需要一本好点的C++教材。
wwwwfly
2008-03-19
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virtual 代表函数为虚函数
typedef unsigned int UINT; UINT为返回类型,无符号整型
=0代表此虚函数为纯虚函数,包含它的类为虚类,不可有实例
AlwaysSLH
2008-03-19
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纯虚函数是一种特殊的虚函数,它的一般格式如下:
class <类名>
{
virtual <类型><函数名>(<参数表>)=0;
…
};
在许多情况下,在基类中不能对虚函数给出有意义有实现,而把它说明为纯虚函数,它的实现留给该基类的派生类去做。
AlwaysSLH
2008-03-19
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定义一个纯虚函数
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
Fun
ctions The -inl.h Files
Fun
ction Parameter Ordering Names and Order of Includes Scoping Namespaces Nested Classes Nonmember, Static Member, and Global
Fun
ctions Local Variables Static and Global Variables Classes Doing Work in Constructors Default Constructors Explicit Constructors Copy Constructors Structs vs. Classes Inheritance Multiple Inheritance
Int
erfaces Operator Overloading Access Control Declaration Order Write Short
Fun
ctions Google-Specific Magic Smart Po
int
ers cppl
int
Other C++ Features Reference Arguments
Fun
ction Overloading Default Arguments Variable-Length Arrays and alloca() Friends Exceptions Run-Time Type Information (RTTI) Casting Streams Preincrement and Predecrement Use of const
Int
eger Types 64-bit Portability Preprocessor Macros 0 and NULL sizeof Boost C++0x Naming General Naming Rules File Names Type Names Variable Names Constant Names
Fun
ction Names Namespace Names Enumerator Names Macro Names Exceptions to Naming Rules Comments Comment Style File Comments Class Comments
Fun
ction Comments Variable Comments Implementation Comments Punctuation, Spelling and Grammar TODO Comments Deprecation Comments Formatting Line Length Non-ASCII Characters Spaces vs. Tabs
Fun
ction Declarations and Definitions
Fun
ction Calls Conditionals Loops and Switch Statements Po
int
er 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 marked 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 po
int
s to get more details. Alternatively, there'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 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 ma
int
ain. 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. Ma
int
aining 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, required 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 advanced 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 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 reader is familiar with the language. Header Files In general, every .cc file should have an associated .h file. There are some common exceptions, such as unittests and small .cc files containing just a main()
fun
ction. 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 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_ Header File Dependencies link ▶Don't use an #include when a forward declaration would suffice. When you include a header file you
int
roduce a dependency that will cause your code to be recompiled 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 recompiled. 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 need 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 members of type Foo* or Foo&. We can declare (but not define)
fun
ctions 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 conversion.) We can declare static data members of type Foo. This is because static data members are defined 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 po
int
er (or better, scoped_ptr) members instead of object members. However, this complicates 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 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
Fun
ctions link ▶Define
fun
ctions inline only when they are small, say, 10 lines or less. Definition: You can declare
fun
ctions in a way that allows the compiler to expand them inline rather than calling them through the usual
fun
ction call mechanism. Pros: Inlining a
fun
ction can generate more efficient object code, as long as the inlined
fun
ction is small. Feel free to inline accessors and mutators, and other short, performance-critical
fun
ctions. Cons: Overuse of inlining can actually make programs slower. Depending on a
fun
ction's size, inlining it can cause the code size to increase or decrease. Inlining a very small accessor
fun
ction will usually decrease code size while inlining a very large
fun
ction 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
fun
ction 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
fun
ctions with loops or switch statements (unless, in the common case, the loop or switch statement is never executed). It is important to know that
fun
ctions are not always inlined even if they are declared as such; for example,
virtual
and recursive
fun
ctions are not normally inlined. Usually recursive
fun
ctions should not be inline. The main reason for making a
virtual
fun
ction 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
fun
ctions when needed. The definition of an inline
fun
ction needs 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
fun
ction 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
fun
ctions 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 included where necessary. Another use of -inl.h files is for definitions of
fun
ction 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 other header file.
Fun
ction Parameter Ordering link ▶When defining a
fun
ction, parameter order is: inputs, then outputs. Parameters to C/C++
fun
ctions are either input to the
fun
ction, output from the
fun
ction, or both. Input parameters are usually values or const references, while output and input/output parameters will be non-const po
int
ers. When ordering
fun
ction parameters, put all input-only parameters before any output parameters. In particular, do not add new parameters to the end of the
fun
ction 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 related
fun
ctions 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 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, order your includes as follows: dir2/foo2.h (preferred location — see details below). C system files. C++ system files. Other libraries' .h files. Your project's .h files. The preferred ordering reduces 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 #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 different directories too. Within each section it is nice to order the includes alphabetically. For example, the includes in google-awesome-project/src/foo/
int
ernal/fooserver.cc might look like this: #include "foo/public/fooserver.h" // Preferred 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
int
o distinct, named 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 provided 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 provided by classes. Use of unnamed spaces in header 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, kError }; // Commonly used tokens. bool AtEof() { return pos_ == kEOF; } // Uses our namespace's EOF. } // namespace However, file-scope declarations that are associated with a particular class may be declared in that class as types, static data members or static member
fun
ctions rather than as members of an unnamed namespace. Terminate 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 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
fun
ctions is within scope of the namespace. void MyClass::Foo() { ... } } // namespace mynamespace The typical .cc file might have more complex detail, including the need 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 undefined 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
fun
ctions, methods or classes in .h files. // OK in .cc files. // Must be in a
fun
ction, method or class in .h files. using ::foo::bar; Namespace aliases are allowed anywhere in a .cc file, anywhere inside the named namespace that wraps an entire .h file, and in
fun
ctions 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 header (in namespace librarian): // alias names should therefore be chosen consistently // within a project. namespace pd_s = ::pipeline_diagnostics::sidetable; inline void my_inline_
fun
ction() { // namespace alias local to a
fun
ction (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 #included by them, should avoid defining aliases, as part of the general 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
erface, consider a namespace to keep declarations out of the global scope. Definition: A class can define another class within it; this is also called a member class. class Foo { private: // Bar is a member class, nested within Foo. class Bar { ... }; }; Pros: This is useful when the nested (or member) class is only used 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. 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 header file manipulating a Foo::Bar* po
int
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
erface, e.g., a class that holds a set of options for some method. Nonmember, Static Member, and Global
Fun
ctions link ▶Prefer nonmember
fun
ctions within a namespace or static member
fun
ctions to global
fun
ctions; use completely global
fun
ctions rarely. Pros: Nonmember and static member
fun
ctions can be useful in some situations. Putting nonmember
fun
ctions in a namespace avoids polluting the global namespace. Cons: Nonmember and static member
fun
ctions may make more sense as members 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
fun
ction not bound to a class instance. Such a
fun
ction can be either a static member or a nonmember
fun
ction. Nonmember
fun
ctions should not depend on external variables, and should nearly always exist in a namespace. Rather than creating classes only to group static member
fun
ctions which do not share static data, use namespaces instead.
Fun
ctions defined in the same compilation unit as production classes may
int
roduce unnecessary coupling and link-time dependencies when directly called from other compilation units; static member
fun
ctions are particularly susceptible to this. Consider extracting a new class, or placing the
fun
ctions in a namespace possibly in a separate library. If you must define a nonmember
fun
ction 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
fun
ction's variables in the narrowest scope possible, and initialize variables in the declaration. C++ allows you to declare variables anywhere in a
fun
ction. 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 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 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 invoked every time it enters scope and is created, and its destructor is invoked every 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 indeterminate order of construction and destruction. Objects with static storage duration, including global variables, static variables, static class member variables, and
fun
ction static variables, must be Plain Old Data (POD): only
int
s, chars, floats, or po
int
ers, or arrays/structs of POD. The order in which class constructors and initializers 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. Therefore in addition to banning globals of class type, we do not allow static POD variables to be initialized with the result of a
fun
ction, unless that
fun
ction (such as getenv(), or getpid()) does not itself depend on any other globals. Likewise, the order in which destructors are called is defined to be the reverse of the order in which the constructors were called. Since constructor order is indeterminate, so is destructor order. For example, at program-end time a static variable might have been destroyed, 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 need a static or global variable of a class type, consider initializing a po
int
er (which will never be freed), from either your main()
fun
ction or from pthread_once(). Note that this must be a raw po
int
er, not a "smart" po
int
er, since the smart po
int
er's destructor will have the order-of-destructor issue that we are trying to avoid. Classes Classes are the
fun
damental 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 need to worry about whether the class has been initialized 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 failed, so it may be an indeterminate state. If the work calls
virtual
fun
ctions, these calls will not get dispatched to the subclass implementations. Future modification to your class can quietly
int
roduce 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, consider having an explicit Init() method. In particular, constructors should not call
virtual
fun
ctions, attempt to raise errors, access potentially uninitialized 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 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 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
int
ernal 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 generated 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 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 conversion. For instance, if you define Foo::Foo(string name) and then pass a string to a
fun
ction that expects a Foo, the constructor will be called to convert the string
int
o a Foo and will pass the Foo to your
fun
ction for you. This can be convenient but is also a source of trouble when things get converted and new objects created without you meaning them to. Declaring a constructor explicit prevents it from being invoked 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
int
ended to be transparent wrappers around other classes are also exceptions. Such exceptions should be clearly marked 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 used to create copies of objects. The copy constructor is implicitly invoked 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 allocation. Cons: Implicit copying of objects in C++ is a rich source of bugs and of performance problems. It also reduces readability, as it becomes hard to track which objects are being passed around by value as opposed to by reference, and therefore where changes to an object are reflected. Decision: Few classes need to be copyable. Most should have neither a copy constructor nor an assignment operator. In many situations, a po
int
er or reference will work just as well as a copied value, with better performance. For example, you can pass
fun
ction parameters by reference or po
int
er instead of by value, and you can store po
int
ers rather than objects in an STL container. If your class needs to be copyable, prefer providing a copy method, such as CopyFrom() or Clone(), rather than a copy constructor, because such methods cannot be invoked implicitly. If a copy method is insufficient in your situation (e.g. for performance reasons, or because your class needs to be stored by value in an STL container), provide both a copy constructor and assignment operator. If your class does not need 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 used: // A macro to disallow the copy constructor and operator=
fun
ctions // This should be used in the private: declarations for a class #define DISALLOW_COPY_AND_ASSIGN(TypeName) \ TypeName(const TypeName&); \ void operator=(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; 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 used for passive objects that carry data, and may have associated constants, but lack any
fun
ctionality other than access/setting the data members. The accessing/setting of fields is done by directly accessing the fields rather than through method invocations. Methods should not provide behavior but should only be used to set up the data members, e.g., constructor, destructor, Initialize(), Reset(), Validate(). If more
fun
ctionality 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
fun
ctors 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 used in two major ways in C++: implementation inheritance, in which actual code is inherited by the child, and
int
erface inheritance, in which only method names are inherited. Pros: Implementation inheritance reduces 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.
Int
erface inheritance can be used 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
fun
ctions that are not
virtual
, so the sub-class cannot change implementation. The base class may also define some data members, so that specifies 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 protected to those member
fun
ctions that might need to be accessed from subclasses. Note that data members should be private. When redefining an inherited
virtual
fun
ction, explicitly declare it
virtual
in the declaration of the derived class. Rationale: If
virtual
is omitted, the reader has to check all ancestors of the class in question to determine if the
fun
ction 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
int
erface classes tagged with the
Int
erface suffix. Definition: Multiple inheritance allows a sub-class to have more than one base class. We distinguish between base classes that are pure
int
erfaces 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 allowed only when all superclasses, with the possible exception of the first one, are pure
int
erfaces. In order to ensure that they remain pure
int
erfaces, they must end with the
Int
erface suffix. Note: There is an exception to this rule on Windows.
Int
erfaces link ▶Classes that satisfy certain conditions are allowed, but not required, to end with an
Int
erface suffix. Definition: A class is a pure
int
erface 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 members. 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 derived from classes that satisfy these conditions and are tagged with the
Int
erface suffix. An
int
erface class can never be directly instantiated because of the pure
virtual
method(s) it declares. To make sure all implementations of the
int
erface 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
erface suffix lets others know that they must not add implemented methods or non static data members. This is particularly important in the case of multiple inheritance. Additionally, the
int
erface concept is already well-understood by Java programmers. Cons: The
Int
erface suffix lengthens the class name, which can make it harder to read and understand. Also, the
int
erface property may be considered an implementation detail that shouldn't be exposed to clients. Decision: A class may end with
Int
erface only if it meets the above requirements. We do not require the converse, however: classes that meet the above requirements are not required to end with
Int
erface. 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
int
uitive because a class will behave in the same way as built-in types (such as
int
). Overloaded operators are more playful names for
fun
ctions that are less-colorfully named, such as Equals() or Add(). For some template
fun
ctions to work correctly, you may need to define operators. Cons: While operator overloading can make code more
int
uitive, it has several drawbacks: It can fool our
int
uition
int
o thinking that expensive operations are cheap, built-in operations. It is much harder to find the call sites for overloaded operators. Searching for Equals() is much easier than searching for relevant invocations of ==. Some operators work on po
int
ers too, making it easy to
int
roduce 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 ramifications. For instance, if a class overloads unary operator&, it cannot safely be forward-declared. Decision: In general, do not overload operators. The assignment operator (operator=), in particular, is insidious and should be avoided. You can define
fun
ctions like Equals() and CopyFrom() if you need them. Likewise, avoid the dangerous unary operator& at all costs, if there's any possibility the class might be forward-declared. However, there may be rare cases where you need to overload an operator to
int
eroperate with templates or "standard" C++ classes (such as operator<<(ostream&, const T&) for logging). These are acceptable if fully justified, but you should try to avoid these whenever possible. In particular, do not overload operator== or operator< just so that your class can be used as a key in an STL container; instead, you should create equality and comparison
fun
ctor types when declaring the container. Some of the STL algorithms do require you to overload operator==, and you may do so in these cases, provided you document why. See also Copy Constructors and
Fun
ction Overloading. Access Control link ▶Make data members private, and provide access to them through accessor
fun
ctions as needed (for technical reasons, we allow data members of a test fixture class to be protected when using Google Test). Typically a variable would be called foo_ and the accessor
fun
ction foo(). You may also want a mutator
fun
ction set_foo(). Exception: static const data members (typically called kFoo) need not be private. The definitions of accessors are usually inlined in the header file. See also Inheritance and
Fun
ction Names. Declaration Order link ▶Use the specified order of declarations within a class: public: before private:, methods before data members (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 generally should be in the following order: Typedefs and Enums Constants (static const data members) Constructors Destructor Methods, including static methods Data Members (except static const data members) 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 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 defined inline. See Inline
Fun
ctions for more details. Write Short
Fun
ctions link ▶Prefer small and focused
fun
ctions. We recognize that long
fun
ctions are sometimes appropriate, so no hard limit is placed on
fun
ctions length. If a
fun
ction exceeds about 40 lines, think about whether it can be broken up without harming the structure of the program. Even if your long
fun
ction 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
fun
ctions short and simple makes it easier for other people to read and modify your code. You could find long and complicated
fun
ctions when working with some code. Do not be
int
imidated by modifying existing code: if working with such a
fun
ction 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
fun
ction
int
o smaller and more manageable pieces. Google-Specific 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 Po
int
ers link ▶If you actually need po
int
er semantics, scoped_ptr is great. You should only use std::tr1::shared_ptr under very specific conditions, such as when objects need to be held by STL containers. You should never use auto_ptr. "Smart" po
int
ers are objects that act like po
int
ers but have added semantics. When a scoped_ptr is destroyed, for instance, it deletes the object it's po
int
ing to. shared_ptr is the same way, but implements reference-counting so only the last po
int
er to an object deletes it. Generally speaking, we prefer that we design code with clear object ownership. The clearest object ownership is obtained by using an object directly as a field or local variable, without using po
int
ers at all. On the other extreme, by their very definition, reference counted po
int
ers are owned 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 deleted. It is also slow to perform atomic operations every time a value is copied or assigned. Although they are not recommended, reference counted po
int
ers are sometimes the simplest and most elegant way to solve a problem. cppl
int
link ▶Use cppl
int
.py to detect style errors. cppl
int
.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 ignored by putting // NOL
INT
at the end of the line. Some projects have instructions on how to run cppl
int
.py from their project tools. If the project you are contributing to does not, you can download cppl
int
.py separately. Other C++ Features Reference Arguments link ▶All parameters passed by reference must be labeled const. Definition: In C, if a
fun
ction needs to modify a variable, the parameter must use a po
int
er, eg
int
foo(
int
*pval). In C++, the
fun
ction can alternatively declare a reference parameter:
int
foo(
int
&val). Pros: Defining a parameter as reference avoids ugly code like (*pval)++. Necessary for some applications like copy constructors. Makes it clear, unlike with po
int
ers, that NULL is not a possible value. Cons: References can be confusing, as they have value syntax but po
int
er semantics. Decision: Within
fun
ction 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 po
int
ers. Input parameters may be const po
int
ers, but we never allow non-const reference parameters. One case when you might want an input parameter to be a const po
int
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 adapters such as bind2nd and mem_
fun
do not permit reference parameters, so you must declare
fun
ctions with po
int
er parameters in these cases, too.
Fun
ction Overloading link ▶Use overloaded
fun
ctions (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 called. Definition: You may write a
fun
ction 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
int
uitive by allowing an identically-named
fun
ction to take different arguments. It may be necessary for templatized code, and it can be convenient for Visitors. Cons: If a
fun
ction is overloaded by the argument types alone, a reader may have to understand C++'s complex matching rules in order to tell what's going on. Also many people are confused by the semantics of inheritance if a derived class overrides only some of the variants of a
fun
ction. Decision: If you want to overload a
fun
ction, consider qualifying the name with some information about the arguments, e.g., AppendString(), Append
Int
() rather than just Append(). Default Arguments link ▶We do not allow default
fun
ction parameters, except in a few uncommon situations explained below. Pros: Often you have a
fun
ction 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
fun
ctions 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 ma
int
ain 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 described below, we require all arguments to be explicitly specified, 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 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 very 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 trigger difficult-to-find memory overwriting bugs: "It ran fine on my machine, but dies mysteriously in production". Decision: Use a safe allocator instead, such as scoped_ptr/scoped_array. Friends link ▶We allow use of friend classes and
fun
ctions, within reason. Friends should usually be defined in the same file so that the reader does not have to look in another file to find uses of the private members 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
int
eract with other classes solely through their public members. Exceptions link ▶We do not use C++ exceptions. Pros: Exceptions allow higher levels of an application to decide how to handle "can't happen" failures in deeply nested
fun
ctions, without the obscuring and error-prone bookkeeping of error codes. Exceptions are used 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
int
ernally makes it harder to
int
egrate with those libraries. Exceptions are the only way for a constructor to fail. We can simulate this with a factory
fun
ction 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
fun
ction, you must examine all of its transitive callers. Either they must make at least the basic exception safety guarantee, or they must never catch 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 catches, 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:
fun
ctions may return in places you don't expect. This causes ma
int
ainability and debugging difficulties. You can minimize this cost via some rules on how and where exceptions can be used, but at the cost of more that a developer needs to know and understand. Exception safety requires both RAII and different coding practices. Lots of supporting machinery is needed 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
int
o a "commit" phase. This will have both benefits and costs (perhaps where 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 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 need 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
int
roduction of exceptions has implications on all dependent code. If exceptions can be propagated beyond a new project, it also becomes problematic to
int
egrate the new project
int
o 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 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,
int
roduce 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 different if we had to do it all over again from scratch. There is an exception to this rule (no pun
int
ended) 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 created object has the expected 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 need to know the type of an object at runtime, that is often an indication that you should reconsider the design of your class. Decision: Do not use RTTI, except in unittests. If you find yourself in need of writing code that behaves differently based on the class of an object, consider one of the alternatives to querying the type.
Virtual
methods are the preferred way of executing different 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, 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++
int
roduced a different cast system from C that distinguishes the types 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 need to explicitly up-cast a po
int
er from a class to its superclass. Use const_cast to remove the const qualifier (see const). Use re
int
erpret_cast to do unsafe conversions of po
int
er types to and from
int
eger and other po
int
er types. 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 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 pr
int
f() and scanf(). Pros: With streams, you do not need to know the type of the object you are pr
int
ing. You do not have problems with format strings not matching the argument list. (Though with gcc, you do not have that problem with pr
int
f either.) Streams have automatic constructors and destructors that open and close the relevant files. Cons: Streams make it difficult to do
fun
ctionality like pread(). Some formatting (particularly the common format string idiom %.*s) is difficult if not impossible to do efficiently using streams without using pr
int
f-like hacks. Streams do not support operator reordering (the %1s directive), which is helpful for
int
ernationalization. Decision: Do not use streams, except where required by a logging
int
erface. Use pr
int
f-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. Extended 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 pr
int
f plus Read/Write/etc. Instead, we should settle on one or the other. 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 every advantage of streams they po
int
out, there is an equivalent disadvantage. The biggest advantage is that you do not need to know the type of the object to be pr
int
ing. This is a fair po
int
. 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; // Pr
int
s the address cout << *this; // Pr
int
s the contents The compiler does not generate an error because << has been overloaded. We discourage overloading for just this reason. Some say pr
int
f 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, remember 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 pr
int
f + read/write. Preincrement and Predecrement link ▶Use prefix form (++i) of the increment and decrement operators with iterators and other 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 whether to preincrement (decrement) or postincrement (decrement). Pros: When the return value is ignored, 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 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 easier to read, since the "subject" (i) precedes 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 types, use pre-increment. Use of const link ▶We strongly recommend that you use const whenever it makes sense to do so. Definition: Declared variables and parameters can be preceded by the keyword const to indicate the variables are not changed (e.g., const
int
foo). Class
fun
ctions can have the const qualifier to indicate the
fun
ction 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 used. Allows the compiler to do better type checking, and, conceivably, generate better code. Helps people convince themselves of program correctness because they know the
fun
ctions they call are limited in how they can modify your variables. Helps people know what
fun
ctions are safe to use without locks in multi-threaded programs. Cons: const is viral: if you pass a const variable to a
fun
ction, that
fun
ction must have const in its prototype (or the variable will need a const_cast). This can be a particular problem when calling library
fun
ctions. Decision: const variables, data members, 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
fun
ction does not modify an argument passed by reference or by po
int
er, 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 members, do not call any non-const methods, and do not return a non-const po
int
er or non-const reference to a data member. Consider making data members const whenever they do not need to be modified 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 allowed but is unsafe when used with threads, so thread safety should be carefully considered 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!
Int
eger Types link ▶Of the built-in C++
int
eger types, the only one used is
int
. If a program needs a variable of a different size, use a precise-width
int
eger type from , such as
int
16_t. Definition: C++ does not specify the sizes of its
int
eger 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
int
egral types in C++ can vary based on compiler and architecture. Decision: defines types like
int
16_t, u
int
32_t,
int
64_t, etc. You should always use those in preference to short, unsigned long long and the like, when you need a guarantee on the size of an
int
eger. Of the C
int
eger types, only
int
should be used. When appropriate, you are welcome to use standard types like size_t and ptrdiff_t. We use
int
very often, for
int
egers 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 need a 64-bit
int
eger type, use
int
64_t or u
int
64_t. For
int
egers we know can be "big", use
int
64_t. You should not use the unsigned
int
eger types such as u
int
32_t, unless the quantity you are representing is really a bit pattern rather than a number, or unless you need defined twos-complement overflow. In particular, do not use unsigned types to say a number will never be negative. Instead, use assertions for this. On Unsigned
Int
egers Some people, including some textbook authors, recommend using unsigned types to represent numbers that are never negative. This is
int
ended as a form of self-documentation. However, in C, the advantages of such documentation are outweighed by the real bugs it can
int
roduce. Consider: for (unsigned
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 signed and unsigned variables. Basically, C's type-promotion scheme causes unsigned types to behave differently than one might expect. So, document that a variable is non-negative using assertions. Don't use an unsigned type. 64-bit Portability link ▶Code should be 64-bit and 32-bit friendly. Bear in mind problems of pr
int
ing, comparisons, and structure alignment. pr
int
f() specifiers for some types are not cleanly portable between 32-bit and 64-bit systems. C99 defines some portable format specifiers. Unfortunately, MSVC 7.1 does not understand some of these specifiers 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
types.h): // pr
int
f macros for size_t, in the style of
int
types.h #ifdef _LP64 #define __PRIS_PREFIX "z" #else #define __PRIS_PREFIX #endif // Use these macros after a % in a pr
int
f format string // to get correct 32/64 bit behavior, like this: // size_t size = records.size(); // pr
int
f("%"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 po
int
er) %lx %p
int
64_t %qd, %lld %"PRId64" u
int
64_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 compiler. Hence if you are using a non-constant format string, you need to insert the value of the macro
int
o the format, rather than the name. It is still possible, as usual, to include length specifiers, etc., after the % when using the PRI* macros. So, e.g. pr
int
f("x = %30"PRIuS"\n", x) would expand on 32-bit Linux to pr
int
f("x = %30" "u" "\n", x), which the compiler will treat as pr
int
f("x = %30u\n", x). Remember that sizeof(void *) != sizeof(
int
). Use
int
ptr_t if you want a po
int
er-sized
int
eger. You may need to be careful with structure alignments, particularly for structures being stored on disk. Any class/structure with a
int
64_t/u
int
64_t member 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 compilers offer a way to alter structure alignment. For gcc, you can use __attribute__((packed)). MSVC offers #pragma pack() and __declspec(align()). Use the LL or ULL suffixes a
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Codeproject VC++ Forum Q&A
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roduction This article is compilation of Question and Answer, which were asked between 25 April 2005 to8 June 2005 (45 Days) on Visual C++ . Thanks to people for their contributionson th...
C语言面试基础知识之一
第一部分:基本概念及其它问答题1、关键字static的作用是什么?这个简单的问题很少有人能回答完全。在C语言中,关键字static有三个明显的作用:1). 在函数体,一个被声明为静态的变量在这一函数被调用过程中维持其值不变。2). 在模块内(但在函数体外),一个被声明为静态的变量可以被模块内所用函数访问,但不能被模块外其它函数访问。它是一个本地的全局变量。3). 在模块内,一个被声明为静态的函数只...
C试题
试题很多4. static有什么用途?(请至少说明两种)1.限制变量的作用域2.设置变量的存储域7. 引用与指针有什么区别?1) 引用必须被初始化,指针不必。2) 引用初始化以后不能被改变,指针可以改变所指的对象。2) 不存在指向空值的引用,但是存在指向空值的指针。8. 描述实时系统的基本特性在特定时间内完成特定的任务,实时性与可靠性9. 全局变量和局部变量在内存中
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