c++ string assign和operator=

已经有operator=，为什么要有assign？
1.两者性能差不多, equally fast
[1]https://stackoverflow.com/questions/34196053/stdstringassign-vs-stdstringoperator
2.assign用法多, operator=意思清晰
[2.1]https://www.learncpp.com/cpp-tutorial/17-5-stdstring-assignment-and-swapping/
[2.2]http://www.cplusplus.com/reference/string/string/assign/