Question

Possible Duplicate:
What’s the scope of inline friend functions?

Consider simple program :

template<typename T> struct foo{
friend void bar(){}
};

int main(){
foo<int>(); foo<float>();
}

Above code breaks the ODR rule, I wonder why? , also where is the scope of function bar ?

Answer 1

friend functions are not member functions; you just declare friendship inside a class, but the function is always a free function. If you define it inside a class template class, you will end up defining it as many times as template instances you have.

I will try to explain that with code. For our purposes, your code is equivalent to this:

template<typename T> struct foo{
};

template<> struct foo<int>{
  friend void bar();
};

void bar() {};

template<> struct foo<double>{
  friend void bar();
};

void bar() {};

int main(){
  foo<int>(); foo<float>();
}