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 // Copyright:   (c) 2008 Vaclav Slavik
   7 // Licence:     wxWindows licence
   8 /////////////////////////////////////////////////////////////////////////////
   9
  10 #ifndef _WX_PRIVATE_FLAGSCHECK_H_
  11 #define _WX_PRIVATE_FLAGSCHECK_H_
  12
  13 #include "wx/debug.h"
  14
  15 // IBM xlC 8 can't parse the template syntax
  16 #if !defined(__IBMCPP__)
  17
  18 #include "wx/meta/if.h"
  19
  20 namespace wxPrivate
  21 {
  22
  23 // These templates are used to implement wxADD_FLAG macro below.
  24 //
  25 // The idea is that we want to trigger *compilation* error if the flags
  26 // overlap, not just runtime assert failure. We can't implement the check
  27 // using just a simple logical operation, we need checks equivalent to this
  28 // code:
  29 //
  30 //   mask = wxFLAG_1;
  31 //   assert( (mask & wxFLAG_2) == 0 ); // no overlap
  32 //   mask |= wxFLAG_3;
  33 //   assert( (mask & wxFLAG_3) == 0 ); // no overlap
  34 //   mask |= wxFLAG_3;
  35 //   ...
  36 //
  37 // This can be done at compilation time by using templates metaprogramming
  38 // technique that makes the compiler carry on the computation.
  39 //
  40 // NB: If any of this doesn't compile with your compiler and would be too
  41 //     hard to make work, it's probably best to disable this code and replace
  42 //     the macros below with empty stubs, this isn't anything critical.
  43
  44 template<int val> struct FlagsHaveConflictingValues
  45 {
  46     // no value here - triggers compilation error
  47 };
  48
  49 template<int val> struct FlagValue
  50 {
  51     enum { value = val };
  52 };
  53
  54 // This template adds its template parameter integer 'add' to another integer
  55 // 'all' and produces their OR-combination (all | add). The result is "stored"
  56 // as constant SafelyAddToMask<>::value. Combination of many flags is achieved
  57 // by chaining parameter lists: the 'add' parameter is value member of
  58 // another (different) SafelyAddToMask<> instantiation.
  59 template<int all, int add> struct SafelyAddToMask
  60 {
  61     // This typedefs ensures that no flags in the list conflict. If there's
  62     // any overlap between the already constructed part of the mask ('all')
  63     // and the value being added to it ('add'), the test that is wxIf<>'s
  64     // first parameter will be non-zero and so Added value will be
  65     // FlagsHaveConflictingValues<add>. The next statement will try to use
  66     // AddedValue::value, but there's no such thing in
  67     // FlagsHaveConflictingValues<> and so compilation will fail.
  68     typedef typename wxIf<(all & add) == 0,
  69                           FlagValue<add>,
  70                           FlagsHaveConflictingValues<add> >::value
  71             AddedValue;
  72
  73     enum { value = all | AddedValue::value };
  74 };
  75
  76 } // wxPrivate namespace
  77
  78
  79
  80 // This macro is used to ensure that no two flags that can be combined in
  81 // the same integer value have overlapping bits. This is sometimes not entirely
  82 // trivial to ensure, for example in wxWindow styles or flags for wxSizerItem
  83 // that span several enums, some of them used for multiple purposes.
  84 //
  85 // By constructing allowed flags mask using wxADD_FLAG macro and then using
  86 // this mask to check flags passed as arguments, you can ensure that
  87 //
  88 // a) if any of the allowed flags overlap, you will get compilation error
  89 // b) if invalid flag is used, there will be an assert at runtime
  90 //
  91 // Example usage:
  92 //
  93 //   static const int SIZER_FLAGS_MASK =
  94 //       wxADD_FLAG(wxCENTRE,
  95 //       wxADD_FLAG(wxHORIZONTAL,
  96 //       wxADD_FLAG(wxVERTICAL,
  97 //       ...
  98 //       0))...);
  99 //
 100 // And wherever flags are used:
 101 //
 102 //   wxASSERT_VALID_FLAG( m_flag, SIZER_FLAGS_MASK );
 103
 104 #define wxADD_FLAG(f, others) \
 105     ::wxPrivate::SafelyAddToMask<f, others>::value
 106
 107 #else
 108     #define wxADD_FLAG(f, others) (f | others)
 109 #endif
 110
 111 // Checks if flags value 'f' is within the mask of allowed values
 112 #define wxASSERT_VALID_FLAGS(f, mask)                   \
 113     wxASSERT_MSG( (f & mask) == f,                      \
 114                   "invalid flag: not within " #mask )
 115
 116 #endif // _WX_PRIVATE_FLAGSCHECK_H_