| 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 | |
| 16 | // IBM xlC 8 can't parse the template syntax |
| 17 | #if !defined(__IBMCPP__) |
| 18 | |
| 19 | #include "wx/meta/if.h" |
| 20 | |
| 21 | namespace wxPrivate |
| 22 | { |
| 23 | |
| 24 | // These templates are used to implement wxADD_FLAG macro below. |
| 25 | // |
| 26 | // The idea is that we want to trigger *compilation* error if the flags |
| 27 | // overlap, not just runtime assert failure. We can't implement the check |
| 28 | // using just a simple logical operation, we need checks equivalent to this |
| 29 | // code: |
| 30 | // |
| 31 | // mask = wxFLAG_1; |
| 32 | // assert( (mask & wxFLAG_2) == 0 ); // no overlap |
| 33 | // mask |= wxFLAG_3; |
| 34 | // assert( (mask & wxFLAG_3) == 0 ); // no overlap |
| 35 | // mask |= wxFLAG_3; |
| 36 | // ... |
| 37 | // |
| 38 | // This can be done at compilation time by using templates metaprogramming |
| 39 | // technique that makes the compiler carry on the computation. |
| 40 | // |
| 41 | // NB: If any of this doesn't compile with your compiler and would be too |
| 42 | // hard to make work, it's probably best to disable this code and replace |
| 43 | // the macros below with empty stubs, this isn't anything critical. |
| 44 | |
| 45 | template<int val> struct FlagsHaveConflictingValues |
| 46 | { |
| 47 | // no value here - triggers compilation error |
| 48 | }; |
| 49 | |
| 50 | template<int val> struct FlagValue |
| 51 | { |
| 52 | enum { value = val }; |
| 53 | }; |
| 54 | |
| 55 | // This template adds its template parameter integer 'add' to another integer |
| 56 | // 'all' and produces their OR-combination (all | add). The result is "stored" |
| 57 | // as constant SafelyAddToMask<>::value. Combination of many flags is achieved |
| 58 | // by chaining parameter lists: the 'add' parameter is value member of |
| 59 | // another (different) SafelyAddToMask<> instantiation. |
| 60 | template<int all, int add> struct SafelyAddToMask |
| 61 | { |
| 62 | // This typedefs ensures that no flags in the list conflict. If there's |
| 63 | // any overlap between the already constructed part of the mask ('all') |
| 64 | // and the value being added to it ('add'), the test that is wxIf<>'s |
| 65 | // first parameter will be non-zero and so Added value will be |
| 66 | // FlagsHaveConflictingValues<add>. The next statement will try to use |
| 67 | // AddedValue::value, but there's no such thing in |
| 68 | // FlagsHaveConflictingValues<> and so compilation will fail. |
| 69 | typedef typename wxIf<(all & add) == 0, |
| 70 | FlagValue<add>, |
| 71 | FlagsHaveConflictingValues<add> >::value |
| 72 | AddedValue; |
| 73 | |
| 74 | enum { value = all | AddedValue::value }; |
| 75 | }; |
| 76 | |
| 77 | } // wxPrivate namespace |
| 78 | |
| 79 | |
| 80 | |
| 81 | // This macro is used to ensure that no two flags that can be combined in |
| 82 | // the same integer value have overlapping bits. This is sometimes not entirely |
| 83 | // trivial to ensure, for example in wxWindow styles or flags for wxSizerItem |
| 84 | // that span several enums, some of them used for multiple purposes. |
| 85 | // |
| 86 | // By constructing allowed flags mask using wxADD_FLAG macro and then using |
| 87 | // this mask to check flags passed as arguments, you can ensure that |
| 88 | // |
| 89 | // a) if any of the allowed flags overlap, you will get compilation error |
| 90 | // b) if invalid flag is used, there will be an assert at runtime |
| 91 | // |
| 92 | // Example usage: |
| 93 | // |
| 94 | // static const int SIZER_FLAGS_MASK = |
| 95 | // wxADD_FLAG(wxCENTRE, |
| 96 | // wxADD_FLAG(wxHORIZONTAL, |
| 97 | // wxADD_FLAG(wxVERTICAL, |
| 98 | // ... |
| 99 | // 0))...); |
| 100 | // |
| 101 | // And wherever flags are used: |
| 102 | // |
| 103 | // wxASSERT_VALID_FLAG( m_flag, SIZER_FLAGS_MASK ); |
| 104 | |
| 105 | #define wxADD_FLAG(f, others) \ |
| 106 | ::wxPrivate::SafelyAddToMask<f, others>::value |
| 107 | |
| 108 | #else |
| 109 | #define wxADD_FLAG(f, others) (f | others) |
| 110 | #endif |
| 111 | |
| 112 | // Checks if flags value 'f' is within the mask of allowed values |
| 113 | #define wxASSERT_VALID_FLAGS(f, mask) \ |
| 114 | wxASSERT_MSG( (f & mask) == f, \ |
| 115 | "invalid flag: not within " #mask ) |
| 116 | |
| 117 | #endif // _WX_PRIVATE_FLAGSCHECK_H_ |