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         // unfortunately we also need to define value here because otherwise
  36         // Result::value is not recognized as a class neither and it has to be
  37         // complete too -- at least make it unusable because it really, really
  38         // should never be used
  39         class value
  40         {
  41         private:
  42             value();
  43             ~value();
  44         };
  45     };
  46 }
  47 #endif // VC++ <= 6
  48 ;
  49
  50 // specialization for true:
  51 template <>
  52 struct wxIfImpl<true>
  53 {
  54     template<typename TTrue, typename TFalse> struct Result
  55     {
  56         struct value : TTrue { };
  57     };
  58 };
  59
  60 // specialization for false:
  61 template<>
  62 struct wxIfImpl<false>
  63 {
  64     template<typename TTrue, typename TFalse> struct Result
  65     {
  66         struct value : TFalse { };
  67     };
  68 };
  69
  70 } // namespace wxPrivate
  71
  72 // wxIf<> template defines nested type "value" which is the same as
  73 // TTrue if the condition Cond (boolean compile-time constant) was met and
  74 // TFalse if it wasn't.
  75 //
  76 // See wxVector<T> in vector.h for usage example
  77 template<bool Cond, typename TTrue, typename TFalse>
  78 struct wxIf
  79 {
  80     // notice that value can't be a typedef, VC6 refuses to use it as a base
  81     // class in this case
  82     struct value : wxPrivate::wxIfImpl<Cond>::Result<TTrue, TFalse>::value { };
  83 };
  84
  85 #endif // _WX_META_IF_H_