When to use static member function? [duplicate]

问题:

Possible Duplicates:
Where would you use a friend function vs a static function?
C++: static member functions

When is it appropriate to use a static member function in C++? Please give me a real world example.

回答1:

Good uses of static member functions:

• Meta-programming. Real-world example is template std::char_traits. All member functions are static
• Making it a static member function gives it access to private members of the class, although a friend would suffice here too
• A protected static member function thus is accessible only to the class and classes derived from it.

Note that the last case applies to a protected static member function but not a private one. In the latter case you would just put it into the compilation unit of the class, hiding it away as an implementation detail. For a protected one though you want it to be visible, albeit in a restricted way.

A typical case of that one is "cheating" the lack of inheritance of friendship.

class B
{
   friend class A;
   // lots of private stuff
};

class A
{
protected:
   static void callsSomePrivateMembers( B& b );
};

class AChild : public A
{
   void foo( B& b );
}

void AChild::foo( B& b )
{
      // AChild does not have private access to B as friendship is not inherited
      // but I do have access to protected members of A including the static ones

    callsSomePrivateMembers( b ); // get to call them through a back-door
}

回答2:

The natural place to use it is when you cannot use a free function because you need to access internals of the class. The most typical example of this is a builder function as below. The constructor of Foo is private to make sure that it is not constructed in any other way than with the builder function.

#include <iostream>

class Foo {
public:
  static Foo* createFoo() {return new Foo();}
private: 
  Foo() {}
};

int main() {
  //Foo nonBuiltFoo; //wont compile
  Foo* freshFoo = Foo::createFoo();
  delete freshFoo;
  return 0;
}

A typical use of that is earlier mentioned Singleton pattern. When you do not have to access protected and private parts of the class, static member functions is not necessary (free functions can be used), but there is some that uses static member functions also when it is within the domain of the class but not restricted/logical to use the function on a single instance.

回答3:

A common example you'll find (in a real world example) is when you are creating a thread. The common thread API's (POSIX/pthreads, Boost, and Win32 CreateThread) all require a specific signature. The only way to obtain that signature in a member function is by making the function static.

回答4:

You may want to use the function without an object instantiated. Also if the function is called from another static function it must be static.

回答5:

Please look up a design pattern called singleton.In short, It's one way to restrict creation of object. Thus the only way to create an object is to call a C++ member function which is static.

回答6:

I misread your question and answered when it's appropriate to use static functions.

You meant static member functions. here's an example of when to use a static member function - to wrap a thread call inside a class, so that your thread has access to your class...:

static unsigned WINAPI ArchiveAgent::LogMsgPump(PVOID pData)
{
    ArchiveAgent* pSmith = reinterpret_cast<ArchiveAgent*>(pData);

    if( pSmith )
        pSmith->LogMsgPump();
    else
        return -1;

    return 0;
}

unsigned WINAPI ArchiveAgent::LogMsgPump()
{
    CoInitializeEx(NULL, COINIT_MULTITHREADED);

    // ....

    CoUninitialize();
    return 0;
}

Here was my answer for plain old static functions.. I use static functions where it doesn't make sense for that function to belong to a class.

I generally tend to add these functions to a custom namespace. The following static function sample is part of a namespace I call ShellUtils:

static HRESULT CreateFolder( CString & sPath )
{
// create the destination folder if it doesn't already exist

HRESULT hr      = S_OK;
DWORD   dwError = 0;

if( sPath.GetLength() == 0 || sPath.GetLength() < 2 )                   
    return E_UNEXPECTED;

if( GetFileAttributes( (LPCWSTR) sPath ) == INVALID_FILE_ATTRIBUTES )
{           
    dwError = SHCreateDirectoryEx(NULL, (LPCWSTR)sPath, NULL);

    if (dwError != ERROR_SUCCESS && dwError != ERROR_FILE_EXISTS && dwError != ERROR_ALREADY_EXISTS)
        hr = HRESULT_FROM_WIN32(dwError);
}

return hr;

}

回答7:

A typical example can be a singleton class where the static GetInstance() method returns the singleton instance of the class.

class Singleton
{
    static Singleton instance;

    private Singleton()
    {
    }

    static Singleton & GetInstance()
    {
      if(instance == null)
        instance = new Singleton();

      return instance;
    }
}