Operator overloading operator＋= which appends another String instance into current Stri

利用operator声明且仅声明了==，有什么错误么？

Assignment operator.

Assignment operator.

Operator overloading operator＋= which appends another String instance into current Stri

利用operator声明且仅声明了“==”，有什么错误么？

利用operator声明且仅声明了“==”，有什么错误么？