allocating memory

have this pointer
Char *ptr
i allocate memory as
ptr = (char *) malloc (sizeof(char) * 100);
say later on in the program i may or may not need to store more characters so i'll use realloc to assign more memory. How can i find out how much memory is currently assigned to ptr so i cant find if i need to resize it or not.

>>15
>because all pointers take the same amount of memory

Not necessarily.

#include <iostream>

using namespace std;

class A {
public:
    int foo()
    {
        return 1;
    }
};

int bar()
{
    return 2;
}

int main()
{
    cout << "sizeof(&A::foo) = " << sizeof(&A::foo) << "\n";
    cout << "sizeof(&bar) = " << sizeof(&bar) << "\n";
    return 0;
}