Question

Here is my code :

#include<iostream>

class foo;
template<typename T> void bar(T a) { std::cout<< a.var; }

class foo { 
    int var;
    public: 
      friend void bar(foo); //here, bar function template is declared in global namescope,
      // deduction can deduce which template instance we're talking about. 
      //Note: If I place <> just after name bar or qualify bar like ::bar , it all works then.
};

int main() {
    foo fooo;
    bar(fooo);
}

error: /home/O1wLF2/cc1xOI20.o: In function ‘main’: prog.cpp:(.text+0x46): undefined reference to ‘bar(foo)’

What I want to know why non-qualified template function’s instance can’t be friend?

Answer 1

You are making a non-template function a friend. You need yo tell the compiler that you want to make a template a friend or one of the instantiations.