复制也是用的=运算符，那么什么时候是复制构造器，什么适合是调用运算符=重载？

用其他对象定义新对象的时候是复制构造器，如下：

    A a2(a1);
    A a3 = a2;

在已经定义好后，用=进行赋值的是调用运算符=重载

class A {
private:
    int m_elem;

public:
    A() {
        std::cout << "This is construction!" << std::endl;
    }

    A(const A& others) {
        std::cout << "This is copy construction!" << std::endl;
    }

    void operator=(const A& others) {
        std::cout << "This is operator == override!" << std::endl;
    }

};

int main()
{
    A a1; //This is construction!
    A a2(a1); //This is copy construction!
    A a3 = a2; //This is copy construction!
    a1 = a2; //This is operator == override!