实例化和名称绑定点

Question

I am confused about the point of instantiation with the following example:

#include <iostream>

void f(int){std::cout<<"int"<<std::endl;}//3

template <typename T>
void g(T t)
{
    f(t);//4
}

void f(double){std::cout<<"double"<<std::endl;}

int main()
{
    g<int>(1);//1.point of instantiation for g<int>
    g<double>(1.1);//2.point of instantiation for g<double>, so f(double) is visible from here?
    return 0;
}

I though f is a dependent name and 1. is the point of instantiation for g< int > and 2. is the point of instantiation for g< double >, so f(double) is visible for g(1.1), however the output is

int
int

and if I comment the declaration of f(int) at 3, gcc reports an error (not surprise) and points out f(t) at 4 is the point of instantiation(surprised!!).

test.cpp: In instantiation of ‘void g(T) [with T = int]’:
test.cpp:16:10:   required from here
test.cpp:9:5: error: ‘f’ was not declared in this scope, and no    declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
  f(t);
     ^

Can anyone clear the concept of point of instantiation and name binding for me please?

Answer 1

f(t) is a dependent unqualified function call expression, so only functions found within the definition context and those found via ADL are candidates. f(int) is visible within the definition context, but not f(double) , so overload resolution resolves to f(int) for both calls.

f(double) cannot be found by ADL because built-in types have no associated classes or namespaces. If you passed in an argument of class type, and there was an overload of f taking this type, ADL will be able to find it. For example:

void f(int);

template <typename T>
void g(T t)
{
    f(t);
}

class A {};
void f(double);
void f(A);

int main()
{
    g(1);   // calls f(int)
    g(1.1); // calls f(int)
    g(A{}); // calls f(A)
}

f(A) is called because it is located in the global namespace, and A 's associated namespace set is the global namespace.