Ask the C++ Pro 10-Minute Solution(2)

Ask the C++ Pro 10-Minute Solution

Pointing to Class Members

I've received a lot of questions lately such as this about pointers to class methods: "I work with Visual C++ 4.0. I cannot use the pointer of a class method. The compilation error message is: Cannot convert parameter 2 from 'long (unsigned long)'

to 'long (__cdecl *)(unsigned long)'

What should I do?"

Here is the code for one way to solve this problem.//in the header

class CKernel:

{

long (*lpFunc)(DWORD);

long OLESendTC( DWORD dwInfo );

}

//in the cpp File

BOOL CKernel::Init()

{

lpFunc = OLESendTC;

return TRUE;

}

Class methods all have one hidden argument, a pointer to the class object the method is called on. C++ uses this pointer to find the location of any class data that the method may reference. If you try and use a standard function pointer to call a class method, C++ has no way to pass this hidden argument and a conflict results.

To help deal with this and to improve type safety, C++ added three new operators, ::*, .*, and ->*, to allow for safe pointers to members. These pointers to members can point to either member functions or variables.class CTest

{

public:

BOOL Init();

long OLESendTC(DWORD dwInfo);

};

long (CTest::*lpFunc)(DWORD dwInfo) = &CTest::OLESendTC;

int main()

{

CTest test;

(test.*lpFunc)(0);

return 0;

}

long CTest::OLESendTC(DWORD dwInfo)

{

cout << "IN OLESENDTC\n";

return 0;

}

This example shows one use of pointers to members. The code uses the ::* operator to declare lpFunc as a pointer to a CTest member function. Note that, rather than assign a value to this pointer at run time, the pointer is simply initialized in the declaration.

In the main function, this example uses the .* operator to call the method pointed to by lpFunc. If test were a pointer here, you would use the ->* operator instead.

C++ has a lot of things going on under the hood such as hidden arguments to methods. Pointers to members allow you to safely declare a pointer to a class method and call methods through that pointer.