found only via adl, C++ (vc++)

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#include <iostream>

#ifdef _MSC_VER
    #define NAME_OF_THIS_FUNCTION __FUNCSIG__ // Microsoft
#elif defined __GNUG__
    #define NAME_OF_THIS_FUNCTION __PRETTY_FUNCTION__ // GNU or compatible
#else
    #define NAME_OF_THIS_FUNCTION __func__ // fall back to standard C++
#endif // _MSC

namespace my
{
    struct A
    {
        friend void foo( A& a ) ; // friend declaration in the class body

friend void bar( A& a ) // friend defined within the class body; and not declared at namespace scope
                                // a. the function is not a member function of this class
                                // b. the function is impliciltly inline
                                // c. the function is a member of the enclosing namespace of A
                                //                             (in this case the namespace my)
                                // d. however, its name won't be found via normal name look up
                                // e. however, the function can be found via ADL (argument-dependent lookup aka koenig lookup)
        {
            a.v += 7 ;
            std::cout << NAME_OF_THIS_FUNCTION << " is a friend of A\n" ;
        }

friend void foobar( A& a ) // friend defined within the class body; and declared at namespace scope
                                   // a. the function is not a member function of this class
                                   // b. the function is impliciltly inline
                                   // c. the function is a member of the enclosing namespace of A
                                   // d. like foo, and unlike bar, its name can be found via normal name look up
        {
            a.v += 7 ;
            std::cout << NAME_OF_THIS_FUNCTION << " is a friend of A\n" ;
        }

private: int v = 0 ;
    };

void foo( A& a ) // friend function defined in the enclosing namespace
                     // foo is a normal function which is at namespace scope
    {
        a.v += 7 ;
        std::cout << NAME_OF_THIS_FUNCTION << " is a friend of A\n" ;
    }

void foobar( A& a ) ; // friend function declared in the enclosing namespace
}

int main()
{
    my::A a ;

foo(a) ; // fine: prints 'void my::foo(my::A&) is a friend of A'
    my::foo(a) ; // also fine: prints 'void my::foo(my::A&) is a friend of A'

bar(a) ; // fine, name found via ADL prints 'void my::bar(my::A&) is a friend of A'
             // (this is the only way in which the name my::bar can be found)

// my::bar(a) ; // *** error: name my::bar not found

// my::A::bar(a) ; // *** error: bar is not a member of my::A

foobar(a) ; // fine: prints 'void my::foobor(my::A&) is a friend of A'
    my::foobar(a) ; // also fine: prints 'void my::foobar(my::A&) is a friend of A'
}

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Compilation time: 6,4 sec, absolute running time: 0,21 sec, absolute service time: 6,62 sec