BaseReaderHandler: allow overriding of Default() implementation

By adding an optional CRTP template parameter, the BaseReaderHandler
can call the "overridden" `Default()` function from the `Derived`
class.

See https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern

include/rapidjson/internal/meta.h

@@ -21,6 +21,11 @@
 #ifndef RAPIDJSON_INTERNAL_META_H_
 #define RAPIDJSON_INTERNAL_META_H_
 
+#ifdef __GNUC__
+RAPIDJSON_DIAG_PUSH
+RAPIDJSON_DIAG_OFF(effc++)
+#endif
+
 //@cond RAPIDJSON_INTERNAL
 namespace rapidjson {
 namespace internal {
@@ -94,4 +99,8 @@ template <typename T> struct RemoveSfinaeFptr<SfinaeResultTag&(*)(T)> { typedef
 } // namespace rapidjson
 //@endcond
 
+#ifdef __GNUC__
+RAPIDJSON_DIAG_POP
+#endif
+
 #endif // RAPIDJSON_INTERNAL_META_H_

include/rapidjson/reader.h

@@ -26,6 +26,7 @@
 
 #include "rapidjson.h"
 #include "encodings.h"
+#include "internal/meta.h"
 #include "internal/pow10.h"
 #include "internal/stack.h"
 
@@ -122,23 +123,23 @@ concept Handler {
 /*! This can be used as base class of any reader handler.
 	\note implements Handler concept
 */
-template<typename Encoding = UTF8<> >
+template<typename Encoding = UTF8<>, typename Derived = void>
 struct BaseReaderHandler {
 	typedef typename Encoding::Ch Ch;
 
 	bool Default() { return true; }
-    bool Null() { return Default(); }
-    bool Bool(bool) { return Default(); }
-    bool Int(int) { return Default(); }
-    bool Uint(unsigned) { return Default(); }
-    bool Int64(int64_t) { return Default(); }
-    bool Uint64(uint64_t) { return Default(); }
-    bool Double(double) { return Default(); }
-    bool String(const Ch*, SizeType, bool) { return Default(); }
-    bool StartObject() { return Default(); }
-    bool EndObject(SizeType) { return Default(); }
-    bool StartArray() { return Default(); }
-    bool EndArray(SizeType) { return Default(); }
+	bool Null() { return static_cast<Override&>(*this).Default(); }
+	bool Bool(bool) { return static_cast<Override&>(*this).Default(); }
+	bool Int(int) { return static_cast<Override&>(*this).Default(); }
+	bool Uint(unsigned) { return static_cast<Override&>(*this).Default(); }
+	bool Int64(int64_t) { return static_cast<Override&>(*this).Default(); }
+	bool Uint64(uint64_t) { return static_cast<Override&>(*this).Default(); }
+	bool Double(double) { return static_cast<Override&>(*this).Default(); }
+	bool String(const Ch*, SizeType, bool) { return static_cast<Override&>(*this).Default(); }
+	bool StartObject() { return static_cast<Override&>(*this).Default(); }
+	bool EndObject(SizeType) { return static_cast<Override&>(*this).Default(); }
+	bool StartArray() { return static_cast<Override&>(*this).Default(); }
+	bool EndArray(SizeType) { return static_cast<Override&>(*this).Default(); }
 };
 
 ///////////////////////////////////////////////////////////////////////////////