wxWidgets/include/wx/meta/if.h

/////////////////////////////////////////////////////////////////////////////
// Name:        wx/meta/if.h
// Purpose:     declares wxIf<> metaprogramming construct
// Author:      Vaclav Slavik
// Created:     2008-01-22
// RCS-ID:      $Id$
// Copyright:   (c) 2008 Vaclav Slavik
// Licence:     wxWindows licence
/////////////////////////////////////////////////////////////////////////////

#ifndef _WX_META_IF_H_
#define _WX_META_IF_H_

#include "wx/defs.h"

// NB: This code is intentionally written without partial templates
//     specialization, because some older compilers (notably VC6) don't
//     support it.

namespace wxPrivate
{

template <bool Cond>
struct wxIfImpl

// broken VC6 needs not just an incomplete template class declaration but a
// "skeleton" declaration of the specialized versions below as it apparently
// tries to look up the types in the generic template definition at some moment
// even though it ends up by using the correct specialization in the end -- but
// without this skeleton it doesn't recognize Result as a class at all below
#if defined(__VISUALC__) && !wxCHECK_VISUALC_VERSION(7)
{
    template<typename TTrue, typename TFalse> struct Result
    {
        // unfortunately we also need to define value here because otherwise
        // Result::value is not recognized as a class neither and it has to be
        // complete too -- at least make it unusable because it really, really
        // should never be used
        class value
        {
        private:
            value();
            ~value();
        };
    };
}
#endif // VC++ <= 6
;

// specialization for true:
template <>
struct wxIfImpl<true>
{
    template<typename TTrue, typename TFalse> struct Result
    {
        struct value : TTrue { };
    };
};

// specialization for false:
template<>
struct wxIfImpl<false>
{
    template<typename TTrue, typename TFalse> struct Result
    {
        struct value : TFalse { };
    };
};

} // namespace wxPrivate

// wxIf<> template defines nested type "value" which is the same as 
// TTrue if the condition Cond (boolean compile-time constant) was met and
// TFalse if it wasn't.
//
// See wxVector<T> in vector.h for usage example
template<bool Cond, typename TTrue, typename TFalse>
struct wxIf
{
    // notice that value can't be a typedef, VC6 refuses to use it as a base
    // class in this case
    struct value : wxPrivate::wxIfImpl<Cond>::Result<TTrue, TFalse>::value { };
};

#endif // _WX_META_IF_H_