Compare member function pointers

Question

Let's say we have a class A which has a method void foo(). I know that I can get the pointer to foo() by using &A::foo, but this pointer only seems to rely on the class type, not on the instantiated object. If we have two objects a1 and a2 of type A, how can we make a difference between the foo() method of a1, and the one of a2 using pointers, member function pointers, or even pointer addresses ? Basically, I want to be able to get a pointer that refers to the void foo() method inside the a1 object, which would be different from a pointer that refers to the void foo() method inside a2. Thank you.

Why would two instances of a class have different member functions? — Cody Gray - on strike, Jun 22 '16 at 18:16
It sounds like you might be looking for function+bind but it's hard to say for sure without more detail in your question. — Mark B, Jun 22 '16 at 18:21
Actually, they don't. Both have the same member function `void foo()`, so that we can do `a1->foo()` and `a2->foo()`. — baboulinet, Jun 22 '16 at 18:22
"I want to be able to get a pointer that refers to the void foo() method inside the a1 object" You must understand that there is no such thing so you cannot get it. There is only one `A::foo` method in the memory that serves all objects of type (class) `A`. That's why you **must** specify object to which you want to apply this method (for example in the form `a1->foo()` - unless that method is static. To answer question "how can we make a difference" - method get's "hidden" parameter `this` by which you know to what instance current call is made — mvidelgauz, Jun 22 '16 at 19:11
Right, that is the point. They are the same, so why would you expect to be able to compare them as if they were different? If you already know that, I don't understand what you are asking here. — Cody Gray - on strike, Jun 23 '16 at 10:06

score 0 · Answer 1 · answered Jun 22 '16 at 19:08

0

I dont think you can make them different. a1->foo() and a2->foo() are basically same.

answered Jun 22 '16 at 19:08

A. Sen

53
4

score 0 · Answer 2 · answered Jun 23 '16 at 13:06

I think you're confused here about the concept of an object (or instance of a class). An object is like a blueprint of a class. You cannot have different members / member functions for each object.

for eg.

class A
{
  int result;

  public: 
  void add(int x, int y)
  {
    this->result = x + y;
  }
}

All the objects of Class A will have member variable result and member function add() in it. So obj1->add() and obj2->add() would call the same add() function, even though the objects itself would have different attributes from each other.

obj1->add(1,2) would result in obj1->result to be 3, whereas

obj2->add(1,3) would result in obj2->result to be 4.

Compare member function pointers

2 Answers2