不能重载赋值运算符。
构造函数有三种情况，在重载赋值运算符时一定要分清楚。
As the construction function has three kinds of different conditions, assignment operator overriding must be given attention.
重载二元运算符时，也会隐式重载相应的赋值运算符(如果有)。
When a binary operator is overloaded, the corresponding assignment operator, if any, is also implicitly overloaded.
除赋值运算符外，重载运算符可由派生类继承下去。
Outside dividing assignment operation accord with, heavy carry calculates Fu Ke by derive kind successive go down.
除赋值运算符外，重载运算符可由派生类继承下去。
Outside dividing assignment operation accord with, heavy carry calculates Fu Ke by derive kind successive go down.
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