include/wx/meta/if.h

   1 /////////////////////////////////////////////////////////////////////////////
   2 // Name:        wx/meta/if.h
   3 // Purpose:     declares wxIf<> metaprogramming construct
   4 // Author:      Vaclav Slavik
   5 // Created:     2008-01-22
   6 // RCS-ID:      $Id$
   7 // Copyright:   (c) 2008 Vaclav Slavik
   8 // Licence:     wxWindows licence
   9 /////////////////////////////////////////////////////////////////////////////
  10
  11 #ifndef _WX_META_IF_H_
  12 #define _WX_META_IF_H_
  13
  14 #include "wx/defs.h"
  15
  16 // NB: This code is intentionally written without partial templates
  17 //     specialization, because some older compilers (notably VC6) don't
  18 //     support it.
  19
  20 namespace wxPrivate
  21 {
  22
  23 template <bool Cond>
  24 struct wxIfImpl
  25
  26 // broken VC6 needs not just an incomplete template class declaration but a
  27 // "skeleton" declaration of the specialized versions below as it apparently
  28 // tries to look up the types in the generic template definition at some moment
  29 // even though it ends up by using the correct specialization in the end -- but
  30 // without this skeleton it doesn't recognize Result as a class at all below
  31 #if defined(__VISUALC__) && !wxCHECK_VISUALC_VERSION(7)
  32 {
  33     template<typename TTrue, typename TFalse> struct Result
  34     {
  35         // intentionally don't define value here, this shouldn't be actually
  36         // used, it's here just to work around a compiler bug
  37     };
  38 }
  39 #endif // VC++ <= 6
  40 ;
  41
  42 // specialization for true:
  43 template <>
  44 struct wxIfImpl<true>
  45 {
  46     template<typename TTrue, typename TFalse> struct Result
  47     {
  48         typedef TTrue value;
  49     };
  50 };
  51
  52 // specialization for false:
  53 template<>
  54 struct wxIfImpl<false>
  55 {
  56     template<typename TTrue, typename TFalse> struct Result
  57     {
  58         typedef TFalse value;
  59     };
  60 };
  61
  62 } // namespace wxPrivate
  63
  64 // wxIf<> template defines nested type "value" which is the same as
  65 // TTrue if the condition Cond (boolean compile-time constant) was met and
  66 // TFalse if it wasn't.
  67 //
  68 // See wxVector<T> in vector.h for usage example
  69 template<bool Cond, typename TTrue, typename TFalse>
  70 struct wxIf : wxPrivate::wxIfImpl<Cond>::template Result<TTrue, TFalse>
  71 {
  72 };
  73
  74 #endif // _WX_META_IF_H_