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