Define bit mask operators for scoped enums bitfields

Close #3887

include/flatbuffers/flatbuffers.h

@@ -1485,6 +1485,35 @@ volatile __attribute__((weak)) const char *flatbuffer_version_string =
 
 #endif  // !defined(_WIN32) && !defined(__CYGWIN__)
 
+#define DEFINE_BITMASK_OPERATORS(E)\
+    inline E operator | (E lhs, E rhs){\
+        using T = std::underlying_type<E>::type;\
+        return E(T(lhs) | T(rhs));\
+    }\
+    inline E operator & (E lhs, E rhs){\
+        using T = std::underlying_type<E>::type;\
+        return E(T(lhs) & T(rhs));\
+    }\
+    inline E operator ^ (E lhs, E rhs){\
+        using T = std::underlying_type<E>::type;\
+        return E(T(lhs) ^ T(rhs));\
+    }\
+    inline E operator ~ (E lhs){\
+        using T = std::underlying_type<E>::type;\
+        return E(~T(lhs));\
+    }\
+    inline E operator |= (E &lhs, E rhs){\
+        lhs = lhs | rhs;\
+        return lhs;\
+    }\
+    inline E operator &= (E &lhs, E rhs){\
+        lhs = lhs & rhs;\
+        return lhs;\
+    }\
+    inline E operator ^= (E &lhs, E rhs){\
+        lhs = lhs ^ rhs;\
+        return lhs;\
+    }
 /// @endcond
 }  // namespace flatbuffers
 

src/idl_gen_cpp.cpp

@@ -182,7 +182,10 @@ static void GenEnum(const Parser &parser, EnumDef &enum_def,
   code += GenEnumVal(enum_def, minv->name, parser.opts) + ",\n";
   code += "  " + GenEnumVal(enum_def, "MAX", parser.opts) + " = ";
   code += GenEnumVal(enum_def, maxv->name, parser.opts) + "\n";
-  code += "};\n\n";
+  code += "};\n";
+  if (parser.opts.scoped_enums && enum_def.attributes.Lookup("bit_flags"))
+    code += "DEFINE_BITMASK_OPERATORS(" + enum_def.name + ")\n";
+  code += "\n";
 
   // Generate a generate string table for enum values.
   // Problem is, if values are very sparse that could generate really big