I think that std::string does not support conversions as part of the class because those operations do not need direct access to the internal data of the string

I think that std::string does not support conversions as part of the class because those operations do not need direct access to the internal data of the string class. They could be written as helper functions (as opposed to member functions) easily without loss in efficiency. Member functions are used when they need direct access to the private fields. For example, C++11 has a new group of functions, std::to_string(...), and their counterparts std::sto*(...), which are not member functions [1]. [1] http://en.cppreference.com/w/cpp/string/basic_string http://en.cppreference.com/w/cpp/string/basic_string