[SFML 2] sf::Packet i zabronienie użycia operatora << dla określonego typu

#include <ostream>

template < bool P, typename T >
struct enable_if;

template < typename T >
struct enable_if < true, T > {
    typedef T type;
};

template < typename Base, typename T >
struct is_same_or_base {
    typedef char yes[ 2 ];
    typedef char no[ 1 ];
    static yes * test( Base * );
    static no * test(...);
    static const bool value = sizeof( * test( static_cast < T *>( 0 ) ) ) == sizeof( yes );
};

template < typename Base, typename T >
const bool is_same_or_base < Base, T >::value;

struct Foo {
    operator int() const { return 5; }
};

struct Bar
    : Foo
{
   
};

struct Qux
    : Bar
{
   
};

template < typename T >
typename enable_if < is_same_or_base < Foo, T >::value, std::ostream &>::type operator <<( std::ostream & foo, const T & ) {
    typedef char error_do_not_use_this_function[ sizeof( T ) != 0 ? - 1
        : 1 ];
    error_do_not_use_this_function x;
    static_cast < void >( x );
    return foo;
}

std::ostream & operator <<( std::ostream & foo, const Qux & ) {
    return foo;
}

void foo( std::ostream & stream ) {
    Foo obj;
    stream << obj; // nie
    Bar obj2;
    stream << obj2; // nie
    Qux obj3;
    stream << obj3; // tak
}