Use `FL_OVERRIDE` for all overridden virtual methods (#611)

FL_OVERRIDE is defined as `override` for C++11 and higher
FL_OVERRIDE is defined as `override` for VisualC 2015 and newer
Don't interfere with Fl_Widget::override()

1 files changed, 58 insertions, 0 deletions

diff --git a/FL/fl_attr.h b/FL/fl_attr.h
index 32e88a2a4..e2d6961da 100644
--- a/FL/fl_attr.h
+++ b/FL/fl_attr.h
@@ -31,4 +31,62 @@
 #  define __fl_attr(x)
 #endif
 
+
+#ifdef FL_DOXYGEN
+
+/**
+  This macro makes it safe to use the C++11 keyword \c override with
+  older compilers.
+*/
+#define FL_OVERRIDE override
+
+#else
+
+#ifdef __cplusplus
+
+// Visual Studio defines __cplusplus = '199711L' which is not helpful.
+// We assume that Visual Studio 2015 (1900) and later support the
+// 'override' keyword. For VS version number encoding see:
+// https://learn.microsoft.com/en-us/cpp/preprocessor/predefined-macros
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1900)
+
+#define FL_OVERRIDE override
+
+#else // not Visual Studio or an older version
+
+#if (__cplusplus >= 202002L)
+// put here definitions applying to C++20 and above
+#endif
+
+#if (__cplusplus >= 201703L)
+// put here definitions applying to C++17 and above
+#endif
+
+#if (__cplusplus >= 201402L)
+// put here definitions applying to C++14 and above
+#endif
+
+#if (__cplusplus >= 201103L)
+// put here definitions applying to C++11 and above
+#define FL_OVERRIDE override
+#else
+// replace non-existing `override` with no-op
+#define FL_OVERRIDE
+#endif
+
+#if (__cplusplus >= 199711L)
+// put here definitions applying to C++98 and above
+#endif
+
+#endif /* not Visual Studio */
+
+#else
+/* C, not C++ */
+
+#endif /* __cplusplus */
+
+#endif /* FL_DOXYGEN */
+
+
 #endif /* !_FL_fl_attr_h_ */