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 #endif // VC++ <= 6
  36 ;
  37
  38 // specialization for true:
  39 template <>
  40 struct wxIfImpl<true>
  41 {
  42     template<typename TTrue, typename TFalse> struct Result
  43     {
  44         typedef TTrue value;
  45     };
  46 };
  47
  48 // specialization for false:
  49 template<>
  50 struct wxIfImpl<false>
  51 {
  52     template<typename TTrue, typename TFalse> struct Result
  53     {
  54         typedef TFalse value;
  55     };
  56 };
  57
  58 } // namespace wxPrivate
  59
  60 // wxIf<> template defines nested type "value" which is the same as
  61 // TTrue if the condition Cond (boolean compile-time constant) was met and
  62 // TFalse if it wasn't.
  63 //
  64 // See wxVector<T> in vector.h for usage example
  65 template<bool Cond, typename TTrue, typename TFalse>
  66 struct wxIf
  67 {
  68     typedef typename wxPrivate::wxIfImpl<Cond>
  69                      ::template Result<TTrue, TFalse>::value
  70             value;
  71 };
  72
  73 #endif // _WX_META_IF_H_