C++操作符重载

/******** 操作符重载 
***
***   下面三种方式，都支持连续加操作，但是稍有不同，你知道哪里不同吗
***
******/
 
/** Test1
*****/
#if 0
class Calculate
{
	int a;
public:
	Calculate(void) : a(0)
	{
		cout << "Calculate(void)" << endl;
	}
	Calculate(int val) : a(0)
	{
		a = val;
		cout << "Calculate(int val) a = " << val << endl;
	}
	Calculate operator + (const Calculate& obj)
	{
		Calculate sum; // 无参构造
		sum.a = this->a + obj.a;
		cout << "Calculate operator + (const Calculate& obj)" << endl;
		return sum;
	}
	int Getdata(void)
	{
		return a;
	}
};
int main()
{
    Calculate obj0(4);
	Calculate obj1(3);
	Calculate obj2(4);

	Calculate obj3;
	obj3 = obj1 + obj2;
	cout << "obj3.Getdata() = " << obj3.Getdata() << endl << endl;

    cout << "--分割线--下面是第二个打印块" << endl;

    obj3 = obj0 + obj1 + obj2;
    cout << "obj3.Getdata() = " << obj3.Getdata() << endl;
	while (1);  
	return 0;
}
#endif

/****  Test2 
 这个实验相比上面Test1，只改一处地方  ****/
/******   请写出打印结果   *****/
#if 1
class Calculate
{
    int a;
public:
    Calculate(void) : a(0)
    {
        cout << "Calculate(void)" << endl;
    }
    Calculate(int val) : a(0)
    {
        a = val;
        cout << "Calculate(int val) a = " << val << endl;
    }
    Calculate operator + (const Calculate& obj)
    {
        Calculate sum; // 无参构造
        sum.a = this->a + obj.a;
        cout << "Calculate operator + (const Calculate& obj)" << endl;
        return sum.a; /*  相比上面那个实验相，只改这里的返回值  */
    }
    int Getdata(void)
    {
        return a;
    }
};
int main()
{
    Calculate obj0(4);
    Calculate obj1(3);
    Calculate obj2(4);

    Calculate obj3;
    obj3 = obj1 + obj2;
    cout << "obj3.Getdata() = " << obj3.Getdata() << endl << endl;

    cout << "--分割线--下面是第二个打印块" << endl;

    obj3 = obj0 + obj1 + obj2;
    cout << "obj3.Getdata() = " << obj3.Getdata() << endl;
    while (1);
    return 0;
}

#endif

/**
实测结论:  第二个打印块内： Test2 会比Test1的多执行两个构造函数，即Calculate(int val)。

原因：   Test2： obj3 = obj0 + obj1 + obj2 这句代码，先执行obj0 + obj1，会生成一个临时对象(假设称为X)，其类型为返回值Calculate类型，
           同时实际返回的是int类型，这就触发了只带一个int参数的构造函数的执行。
           之后继续执行X + obj2 ，
           同理，会生成一个临时对象Y，先执行完Y的重载+的重载函数后，后调用Y的只带一个int参数的构造函数的执行，然后将Y赋值给obj3.
**/


#if 0
/****  Test3
 *****/
class Calculate
{
    int a;
public:
    Calculate(void) : a(0)
    {
        cout << "Calculate(void)" << endl;
    }
    Calculate(int val) : a(0)
    {
        a = val;
        cout << "Calculate(int val)" << endl;
    }
    Calculate operator + (const Calculate& obj)
    {
        cout << "obj.a = " << obj.a << endl;
        this->a += obj.a;
        cout << "Calculate operator + (const Calculate& obj)" << endl;
        return *this;
    }
    int Getdata(void)
    {
        return a;
    }
};
int main()
{
    Calculate obj0(4);
    Calculate obj1(3);
    Calculate obj2(4);

    Calculate obj3;
    obj3 = obj0 + obj1 + obj2;
    cout << "obj3.Getdata() = " << obj3.Getdata() << endl;
    while (1);
    return 0;
}

#endif