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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.
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