Argument-dependent search in C ++

Question

How it works? Is this related to ADL?

#include <iostream> template <typename T> struct A { friend void f(T x) { std::cout << "A\n"; } }; int main() { f(new A<void*>()); } 

Can someone tell me why I can't use something like

 f(A<int>()); 

Answer 1

 f(new A<void*>()); 

Really works due to Koenig argument / search dependent ( ADL ) lookup
In Koenig search mode:

You do not need to qualify a namespace (scope) for functions if one or more argument types are defined in the function namespace.

Consider a simplified example that doesn't use templates, and should help you better understand how ADL works:

 #include <iostream> struct A { friend void f(A x) { std::cout << "A\n"; } }; int main() { f(A()); return 0; } 

Exit:

 A 

When you use f(A<int>()) , it requires f() require an argument of type int , but your structure does not provide any conversion from A to int and therefore errors.

If you provide the appropriate conversion, it will also work. Something like :

 operator int(){return 1;}