Example on ADL

adl_example.cpp

#include <iostream>

#define LOG_ARG(ARG) \
    do { std::cout << __PRETTY_FUNCTION__ << std::endl; } while (0)

#define LOG LOG_ARG(__PRETTY_FUNCTION__)

class Widget {
    class Toolbox { };
public:
    friend void foo(Widget*) { LOG; }
    friend void bar(Toolbox) { LOG; }
    Toolbox getBox() { return {}; }
};

namespace N
{
    void baz(class X()) { LOG; }
    class X { };
}

int main()
{
    //foo(nullptr); // Error! nullptr has the associated namespace std and
                    // associated class std::nullptr_t, but ADL cannot
                    // find a function named foo.
    foo(static_cast<Widget*>(nullptr)); // OK! If T is a pointer to U or an
                                        // array of U, its associated
                                        // namespaces and classes are those
                                        // associated with U.
    bar(Widget().getBox()); // OK! If T is a class type
                            // (including unions), its
                            // associated classes are:
                            // the class itself; the
                            // class of which it is a
                            // member, if any; [..]
    
    extern N::X norf();
    baz(norf); // OK! If T is a function type,
               // its associated namespaces
               // and classes are those
               // associated with the function
               // parameter types and those
               // associated with the return
               // type.
}