CODEWITHSUNDEEP
c++function-pointerspointer-to-member
Alex Suo
Alex Suo

Reputation: 3129

How to check a virtual function on an object instance is a certain implementation

I am trying to check if the virtual function on an certain object instance is from a certain implementation. Intuitively it looks like the following code segment:

#include <cstdio>
using namespace std;

class A
{
public:

    virtual void method()
    {
            printf("This is from A \n");
    }
};

class B : public A
{
public:
    virtual void method()
    {
            printf("This is from B \n");
    }
};

int main()
{
    B b;

    b.method();
    if (b.&method == &B::method)
    {
            printf("Horray! simple. \n");
    }


    return 0;
}

But obviously the line if (b.&method == &B::method) doesn't work.

Can you kindly suggest how this should work? Thanks.

Upvotes: 1

Views: 364

Answers (1)

eerorika
eerorika

Reputation: 238421

Can you kindly suggest how this should work?

In this trivial example, the member function pointer of an instance of B obviously has the address of the member function of the class B there is no point in testing this.

In general, if we had a pointer or reference to an instance of an unknown derived class, such test couldn't be written in standard C++. There is no way to get the address of the function to which a virtual call resolves at run time.

However, GCC does have an extension that allows you to do exactly that.

 typedef int (*fptr)(A *);
 fptr p = (fptr)(a.*fp);

Or, if the compiler documents the ABI that it uses, you may be able to use the specification to extract the address from the member function pointer.

Upvotes: 1

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