some friend functions don't follow the rule
c++, friend
Solution
"Not visible" is a bit of an over-simplification. With only an in-class definition, a friend function can't be found by qualified or unqualified lookup, which is why the first snippet fails.
However, it can be found by argument-dependent lookup (ADL), so you can call it with an argument involving a type that's scoped in the same namespace as the function.
In this case, the argument type is `B*`, scoped in the global namespace. The friend function is scoped in the namespace containing the class that declares it - also the global namespace. So ADL will look in the global namespace for functions called `f`, find the friend function, and use that.
Problem
For the following snippet: ``` class A{ friend void f(){}; public: A(){f();} //error }; class B{ friend void f(void* ptr){}; public: B(){f(this);} //no error }; ``` According to the rule that although friend functions can be defined inside a class, yet they are not visible until they are declared somewhere outside the class scope, the error in the definition of class A is explained. But I am confused why the snippet for class B doesn't produce the same error as class A's. Please can anyone tell me about this?