一个很好的理由是，其中一个标准中要求在标准容器中可用的类 X 的要求是，表达式 a = b 类型 X& （其中 a 是类型 X 和 b 是类型 X 的右值。

One good reason is that one of the requirements in the standard for a class X to be useable in the standard containers is that the expression a = b must have type X& (where a is an lvalue of type X and b is an rvalue of type X).