fork(1) download

copy

// How NOT TO DO IT
 
#include <iostream>
using namespace std;
 
template <typename T>
struct Foo
{
    friend ostream& operator<<(ostream &output, const Foo& rhs);
 
};
 
template <class T>
ostream& operator<<(ostream &output, const Foo<T>& rhs)
{
    return output; // don't do anything
}
 
int main()
{
    Foo<int> foo;
    std::cout << foo << std::endl;
}

prog.cpp:9:63: warning: friend declaration 'std::ostream& operator<<(std::ostream&, const Foo<T>&)' declares a non-template function [-Wnon-template-friend]
     friend ostream& operator<<(ostream &output, const Foo& rhs);
                                                               ^
prog.cpp:9:63: note: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here) 
/home/3GzhQ0/ccH29znL.o: In function `main':
prog.cpp:(.text.startup+0x1b): undefined reference to `operator<<(std::ostream&, Foo<int> const&)'
collect2: error: ld returned 1 exit status

Standard output is empty