拷贝和移动赋值运算符不应为虚函数

拷贝和移动赋值运算符的参数应分别为所属类的左值和右值引用，这类运算符即使是虚函数也不便于被重写。

示例：

class A {
public:
    virtual A& operator = (const A&);  // Non-compliant
};

class B: public A {
public:
    virtual B& operator = (const B&);  // Not override
    virtual A& operator = (const A&);  // Override, but too complex
};