Method SetLength

src/include/rapidjson/document.h:1925–1925

        inline static bool Usable(SizeType len) { return                       (MaxSize >= len); }
        inline void     SetLength(SizeType len) { str[LenPos] = static_cast<Ch>(MaxSize -  len); }
        inline SizeType GetLength() const       { return  static_cast<SizeType>(MaxSize -  str[LenPos]); }
    };  // at most as many bytes as "String" above => 12 bytes in 32-bit mode, 16 bytes in 64-bit mode