Appended '= x' after a method declaration in c++.

Question

In C++, when a method is declared, I've noticed that sometime the method may have an assignement appended to it.

Could anyone tell me what this is?

For example:

virtual void MyMethod () = 0;

What doe the '= 0' mean. :)

Thanks everyone !!!

Answer 1

It means it's a pure virtual function, i.e. no actual definition of it is available in this class and it must be overridden in a subclass. It's not actually an assignment as such, zero is the only value you can "assign".

And this is C++ syntax; in C# the same would be accomplished with the abstract keyword.

Answer 2

In C#, that is a syntax error.

If you meant C++, see calmh's answer.

Answer 3

In C++ this means that the method is a pure virtual method.

This means that an instance of this particular class-type cannot be instantiated. You can only create instances of classes derived from this, which override all pure virtual methods in the base class.

A base class with pure virtual methods defines an interface that derived classes have to implement, and is not meant to be used on its own.

---

Contrary to what calmh claims, as far as I know pure virtual functions can be implemented and they can be called explicitly.

#include <cstdio>

class A
{
public:
    virtual void foo() const = 0; //pure virtual
};

void A::foo() const { puts("A::foo"); }

class B: public A
{
public:
    virtual void foo() const { puts("B::foo"); }
};

int main()
{
    //A a;  //this would be an error - main point of having pure virtual functions
    B b;
    b.foo();
    b.A::foo();
}

Usually one wouldn't do this, though, except perhaps if the virtual destructor is pure in a base class (in this case it has to be defined).