include/wx/private/flagscheck.h

   1 /////////////////////////////////////////////////////////////////////////////
   2 // Name:        wx/private/flagscheck.h
   3 // Purpose:     helpers for checking that (bit)flags don't overlap
   4 // Author:      Vaclav Slavik
   5 // Created:     2008-02-21
   6 // RCS-ID:      $Id$
   7 // Copyright:   (c) 2008 Vaclav Slavik
   8 // Licence:     wxWindows licence
   9 /////////////////////////////////////////////////////////////////////////////
  10
  11 #ifndef _WX_PRIVATE_FLAGSCHECK_H_
  12 #define _WX_PRIVATE_FLAGSCHECK_H_
  13
  14 #include "wx/debug.h"
  15 #include "wx/meta/if.h"
  16
  17 namespace wxPrivate
  18 {
  19
  20 // These templates are used to implement wxADD_FLAG macro below.
  21 //
  22 // The idea is that we want to trigger *compilation* error if the flags
  23 // overlap, not just runtime assert failure. We can't implement the check
  24 // using just a simple logical operation, we need checks equivalent to this
  25 // code:
  26 //
  27 //   mask = wxFLAG_1;
  28 //   assert( (mask & wxFLAG_2) == 0 ); // no overlap
  29 //   mask |= wxFLAG_3;
  30 //   assert( (mask & wxFLAG_3) == 0 ); // no overlap
  31 //   mask |= wxFLAG_3;
  32 //   ...
  33 //
  34 // This can be done at compilation time by using templates metaprogramming
  35 // technique that makes the compiler carry on the computation.
  36 //
  37 // NB: If any of this doesn't compile with your compiler and would be too
  38 //     hard to make work, it's probably best to disable this code and replace
  39 //     the macros below with empty stubs, this isn't anything criticial.
  40
  41 template<int val> struct FlagsHaveConflictingValues
  42 {
  43     // no value here - triggers compilation error
  44 };
  45
  46 template<int val> struct FlagValue
  47 {
  48     enum { value = val };
  49 };
  50
  51 // This template adds its template parameter integer 'add' to another integer
  52 // 'all' and produces their OR-combination (all | add). The result is "stored"
  53 // as constant SafelyAddToMask<>::value. Combination of many flags is achieved
  54 // by chaining parameter lists: the 'add' parameter is value member of
  55 // another (different) SafelyAddToMask<> instantiation.
  56 template<int all, int add> struct SafelyAddToMask
  57 {
  58     // This typedefs ensures that no flags in the list conflict. If there's
  59     // any overlap between the already constructed part of the mask ('all')
  60     // and the value being added to it ('add'), the test that is wxIf<>'s
  61     // first parameter will be non-zero and so Added value will be
  62     // FlagsHaveConflictingValues<add>. The next statement will try to use
  63     // AddedValue::value, but there's no such thing in
  64     // FlagsHaveConflictingValues<> and so compilation will fail.
  65     typedef typename wxIf<(all & add) == 0,
  66                           FlagValue<add>,
  67                           FlagsHaveConflictingValues<add> >::value
  68             AddedValue;
  69
  70     enum { value = all | AddedValue::value };
  71 };
  72
  73 } // wxPrivate namespace
  74
  75
  76
  77 // This macro is used to ensure that no two flags that can be combined in
  78 // the same integer value have overlapping bits. This is sometimes not entirely
  79 // trivial to ensure, for example in wxWindow styles or flags for wxSizerItem
  80 // that span several enums, some of them used for multiple purposes.
  81 //
  82 // By constructing allowed flags mask using wxADD_FLAG macro and then using
  83 // this mask to check flags passed as arguments, you can ensure that
  84 //
  85 // a) if any of the allowed flags overlap, you will get compilation error
  86 // b) if invalid flag is used, there will be an assert at runtime
  87 //
  88 // Example usage:
  89 //
  90 //   static const int SIZER_FLAGS_MASK =
  91 //       wxADD_FLAG(wxCENTRE,
  92 //       wxADD_FLAG(wxHORIZONTAL,
  93 //       wxADD_FLAG(wxVERTICAL,
  94 //       ...
  95 //       0))...);
  96 //
  97 // And wherever flags are used:
  98 //
  99 //   wxASSERT_VALID_FLAG( m_flag, SIZER_FLAGS_MASK );
 100
 101 #define wxADD_FLAG(f, others) \
 102     ::wxPrivate::SafelyAddToMask<f, others>::value
 103
 104 // Checks if flags value 'f' is within the mask of allowed values
 105 #define wxASSERT_VALID_FLAGS(f, mask)                   \
 106     wxASSERT_MSG( (f & mask) == f,                      \
 107                   "invalid flag: not within " #mask )
 108
 109 #endif // _WX_PRIVATE_FLAGSCHECK_H_