+/////////////////////////////////////////////////////////////////////////////
+// Name: wx/private/flagscheck.h
+// Purpose: helpers for checking that (bit)flags don't overlap
+// Author: Vaclav Slavik
+// Created: 2008-02-21
+// RCS-ID: $Id$
+// Copyright: (c) 2008 Vaclav Slavik
+// Licence: wxWindows licence
+/////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_PRIVATE_FLAGSCHECK_H_
+#define _WX_PRIVATE_FLAGSCHECK_H_
+
+#include "wx/debug.h"
+#include "wx/meta/if.h"
+
+namespace wxPrivate
+{
+
+// These templates are used to implement wxADD_FLAG macro below.
+//
+// The idea is that we want to trigger *compilation* error if the flags
+// overlap, not just runtime assert failure. We can't implement the check
+// using just a simple logical operation, we need checks equivalent to this
+// code:
+//
+// mask = wxFLAG_1;
+// assert( (mask & wxFLAG_2) == 0 ); // no overlap
+// mask |= wxFLAG_3;
+// assert( (mask & wxFLAG_3) == 0 ); // no overlap
+// mask |= wxFLAG_3;
+// ...
+//
+// This can be done at compilation time by using templates metaprogramming
+// technique that makes the compiler carry on the computation.
+//
+// NB: If any of this doesn't compile with your compiler and would be too
+// hard to make work, it's probably best to disable this code and replace
+// the macros below with empty stubs, this isn't anything criticial.
+
+template<int val> struct FlagsHaveConflictingValues
+{
+ // no value here - triggers compilation error
+};
+
+template<int val> struct FlagValue
+{
+ enum { value = val };
+};
+
+// This template adds its template parameter integer 'add' to another integer
+// 'all' and produces their OR-combination (all | add). The result is "stored"
+// as constant SafelyAddToMask<>::value. Combination of many flags is achieved
+// by chaining parameter lists: the 'add' parameter is value member of
+// another (different) SafelyAddToMask<> instantiation.
+template<int all, int add> struct SafelyAddToMask
+{
+ // This typedefs ensures that no flags in the list conflict. If there's
+ // any overlap between the already constructed part of the mask ('all')
+ // and the value being added to it ('add'), the test that is wxIf<>'s
+ // first parameter will be non-zero and so Added value will be
+ // FlagsHaveConflictingValues<add>. The next statement will try to use
+ // AddedValue::value, but there's no such thing in
+ // FlagsHaveConflictingValues<> and so compilation will fail.
+ typedef typename wxIf<(all & add) == 0,
+ FlagValue<add>,
+ FlagsHaveConflictingValues<add> >::value
+ AddedValue;
+
+ enum { value = all | AddedValue::value };
+};
+
+} // wxPrivate namespace
+
+
+
+// This macro is used to ensure that no two flags that can be combined in
+// the same integer value have overlapping bits. This is sometimes not entirely
+// trivial to ensure, for example in wxWindow styles or flags for wxSizerItem
+// that span several enums, some of them used for multiple purposes.
+//
+// By constructing allowed flags mask using wxADD_FLAG macro and then using
+// this mask to check flags passed as arguments, you can ensure that
+//
+// a) if any of the allowed flags overlap, you will get compilation error
+// b) if invalid flag is used, there will be an assert at runtime
+//
+// Example usage:
+//
+// static const int SIZER_FLAGS_MASK =
+// wxADD_FLAG(wxCENTRE,
+// wxADD_FLAG(wxHORIZONTAL,
+// wxADD_FLAG(wxVERTICAL,
+// ...
+// 0))...);
+//
+// And wherever flags are used:
+//
+// wxASSERT_VALID_FLAG( m_flag, SIZER_FLAGS_MASK );
+
+#define wxADD_FLAG(f, others) \
+ ::wxPrivate::SafelyAddToMask<f, others>::value
+
+// Checks if flags value 'f' is within the mask of allowed values
+#define wxASSERT_VALID_FLAGS(f, mask) \
+ wxASSERT_MSG( (f & mask) == f, \
+ "invalid flag: not within " #mask )
+
+#endif // _WX_PRIVATE_FLAGSCHECK_H_
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