Specialize StrLen for char/wchar_t

Compilers generally provide a much smarter strlen than ours.

include/rapidjson/internal/strfunc.h

@@ -16,6 +16,7 @@
 #define RAPIDJSON_INTERNAL_STRFUNC_H_
 
 #include "../stream.h"
+#include <cwchar>
 
 RAPIDJSON_NAMESPACE_BEGIN
 namespace internal {
@@ -34,6 +35,16 @@ inline SizeType StrLen(const Ch* s) {
     return SizeType(p - s);
 }
 
+template <>
+inline SizeType StrLen(const char* s) {
+    return SizeType(std::strlen(s));
+}
+
+template <>
+inline SizeType StrLen(const wchar_t* s) {
+    return SizeType(std::wcslen(s));
+}
+
 //! Returns number of code points in a encoded string.
 template<typename Encoding>
 bool CountStringCodePoint(const typename Encoding::Ch* s, SizeType length, SizeType* outCount) {