Pointer to data member address

Question

I have read (Inside C++ object model) that address of pointer to data member in C++ is the offset of data member plus 1?
I am trying this on VC++ 2005 but i am not getti

Answer 1

To complement AndreyT's answer: Try running this code on your compiler.

void test()
{  
    using namespace std;

    int X::* pm = NULL;
    cout << "NULL pointer to member: "
        << " value = " << pm 
        << ", raw byte value = 0x" << hex << *(unsigned int*)&pm << endl;

    pm = &X::a;
    cout << "pointer to member a: "
        << " value = " << pm 
        << ", raw byte value = 0x" << hex << *(unsigned int*)&pm << endl;

    pm = &X::b;
    cout << "pointer to member b: "
        << " value = " << pm 
        << ", raw byte value = 0x" << hex << *(unsigned int*)&pm << endl;
}

On Visual Studio 2008 I get:

NULL pointer to member:  value = 0, raw byte value = 0xffffffff
pointer to member a:  value = 1, raw byte value = 0x0
pointer to member b:  value = 1, raw byte value = 0x4

So indeed, this particular compiler is using a special bit pattern to represent a NULL pointer and thus leaving an 0x0 bit pattern as representing a pointer to the first member of an object.

This also means that wherever the compiler generates code to translate such a pointer to an integer or a boolean, it must be taking care to look for that special bit pattern. Thus something like if(pm) or the conversion performed by the << stream operator is actually written by the compiler as a test against the 0xffffffff bit pattern (instead of how we typically like to think of pointer tests being a raw test against address 0x0).