明确选择应使用哪个模板重载

Question

Can we chose which function template overload should be used in this case?

struct X { };

struct A { A(X){} };
struct B { B(X){} };

template<class T>
void fun(T, A) { }

template<class T>
void fun(T, B) { }

int main() {
    /* explicitly choose overload */ fun(1, X());
}

Error:

error: call of overloaded 'fun(int, X)' is ambiguous
     /* explicitly choose overload */ fun(1, X());
                                                ^
    candidate: void fun(T, A) [with T = int]
 void fun(T, A) { }
      ^~~
    candidate: void fun(T, B) [with T = int]
 void fun(T, B) { }
      ^~~

For normal function it looks like this:

void fun(A){}
void fun(B){}

int main() {
    ((void(*)(A))(fun))(X());
}

Is it possible?

Answer 1

改善你的榜样，你可以尝试

 ((void(*)(int, A))(fun))(1, X());

Answer 2

If you choose not to explicitly specify the first parameter type but still want to specify the second you could go along with lambda dedicated for casting purpose (c++14 solution):

struct X { };

struct A { A(X){} };
struct B { B(X){} };

template<class T>
void fun(T, A) { }

template<class T>
void fun(T, B) { }

int main() {
    [](auto v, auto x){ static_cast<void(*)(decltype(v), A)>(fun)(v, x); }(1, X());
}

[live demo]

Answer 3

A low-tech solution to this problem would be to add one extra level of indirection. Add a function like funA , whose sole purpose is to give an explicit name to the first version of fun :

struct X { };

struct A { A(X){} };
struct B { B(X){} };

template<class T>
void fun(T, A) { }

template<class T>
void fun(T, B) { }

template <class T>
void funA(T t, A a) { fun(t, a); }

int main() {
    /* explicitly choose overload */ funA(1, X());
}

However, I wonder why you cannot just change the argument to A(X()) . You will have to change the calling code anyway, so what's the problem?