Why does an assignment operator overload in C + + use a reference to return a value?

Class string{

Public

String (const char *str=null);

String (const string& str);  Why is the parameter of the copy constructor a reference? I believe we all know!

string& operator= (const string & str); Assignment function Why is the return value a reference?

~string ();

};

If a value is returned, the copy constructor is called immediately after return *this.

But if the copy constructor is not defined, the default copy constructor is called.
We know that when the default copy constructor is called, there is an error (shallow copy) when there is a pointer in the class.

So if you don't use references to do a return, you have to define a custom copy constructor.

Why does an assignment operator overload in C + + use a reference to return a value?