added macros for checking for conflicts between flags values; use it in wxSizer code

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@52310 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/private/flagscheck.h

@@ -0,0 +1,109 @@
+/////////////////////////////////////////////////////////////////////////////
+// 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_