Question

I’m trying to check if an operator exists at compile time, if it doesn’t I just want it ignored, is there any way to do that?

example operator:

 template <typename T>
 QDataStream& operator<<(QDataStream& s, const QList<T>& l);

Answer 1

I ended up using a fallback namespace :

namespace operators_fallback {
template <typename T>
inline QDataStream& operator<<(QDataStream& s, const T &) { return s; }

template <typename T>
inline QDataStream& operator>>(QDataStream& s, T &) { return s; }

template <typename T>
inline QDebug operator<<(QDebug d, const T &) { return d; }
};

...
inline void load(QDataStream & s) {
    using namespace operator_fallback;
    s >> item;
}

Also found the proper way to check for operators at compile time (although I’m going with the fallback namespace).

more or less based on this :

namespace private_impl {
    typedef char yes;
typedef char (&no)[2];

struct anyx { template <class T> anyx(const T &); };

no operator << (const anyx &, const anyx &);
no operator >> (const anyx &, const anyx &);


template <class T> yes check(T const&);
no check(no);

template <typename StreamType, typename T>
struct has_loading_support {
    static StreamType & stream;
    static T & x;
    static const bool value = sizeof(check(stream >> x)) == sizeof(yes);
};

template <typename StreamType, typename T>
struct has_saving_support {
    static StreamType & stream;
    static T & x;
    static const bool value = sizeof(check(stream << x)) == sizeof(yes);
};

template <typename StreamType, typename T>
struct has_stream_operators {
    static const bool can_load = has_loading_support<StreamType, T>::value;
    static const bool can_save = has_saving_support<StreamType, T>::value;
    static const bool value = can_load && can_save;
};
}
template<typename T>
struct supports_qdatastream : private_impl::has_stream_operators<QDataStream, T> {};

template<typename T>
struct can_load : private_impl::has_loading_support<QDataStream, T> {};

template<typename T>
struct can_save : private_impl::has_saving_support<QDataStream, T> {};

template<typename T>
struct can_debug : private_impl::has_saving_support<QDebug, T> {};

//edit changed has_stream_operators a bit.

//edit removed the link, apparently the site has some attack javascript.