C++ 中的返回值

C++中大致有三种返回值：值拷贝（副本），值引用和指针，返回什么类型的值要根据当时情况而定。

如果返回的是大型对象的副本，那么在每一次的函数调用后返回，都会调用该对象类型的拷贝构造函数构造一个新的副本，这是一个耗时的过程。

因此在不需要返回对象副本的时候，尽量返回对象的引用或者指针，与此同时，如果不会改变引用或者指针指向的对象的内容，都加上const限定符。

不论是返回拷贝，引用还是指针，返回值都会作为左值来使用，因此也可以将返回值赋值给其他变量（类型可以是值类型，引用类型或者指针类型）

实例：

class Widget
{
public:
	Widget()
	{
		a = 0;
		printf("Widget constructor with default value: this:%p a:%d \n", this, a);
	}
	Widget(int v)
	{
		a = v;
		printf("Widget constructor with parameters: this:%p a:%d \n", this, a);
	}
	Widget(const Widget& widget)
	{
		this->a = widget.a;
		printf("Widget copy constructor with reference: this:%p widget:%p a:%d \n", this, &widget, a);
	}
	Widget(const Widget* widget)
	{
		this->a = widget->a;
		printf("Widget copy constructor with pointer: this:%p widget:%p a:%d \n", this, widget, a);
	}
	Widget& operator = (const Widget& widget)
	{
		this->a = widget.a;
		printf("Widget operator =: this:%p widget:%p a:%d \n", this, &widget, a);

		return *this;
	}
	Widget& operator + (const Widget& widget)
	{
		this->a += widget.a;
		printf("Widget operator +: this:%p widget:%p a:%d \n", this, &widget, a);

		return *this;
	}
	Widget getByCopy()
	{
		printf("Widget get object by copy: this:%p a:%d \n", this, a);

		return *this;
	}
	Widget& getByRef()
	{
		printf("Widget get object by ref: this:%p a:%d \n", this, a);

		return *this;
	}
	Widget* getByPointer()
	{
		printf("Widget get object by pointer: this:%p a:%d \n", this, a);

		return this;
	}

	int a;
};

int _tmain(int argc, _TCHAR* argv[])
{
	Widget w1(1); // 调用构造函数 Widget(int v)
	Widget w2(2);
	Widget w3; // 调用默认构造函数
	w3 = (w1 + w2); // 先调用+操作符，在调用=操作符

	printf("\n");
	printf("w4 -------------------- \n");
	w3.getByCopy(); // 返回副本
	printf("w4_1 -------------------- \n");
	Widget w4_1 = w3.getByCopy(); // 感觉应该会有两次拷贝，实际只有一次Widget(const Widget& widget)拷贝构造函数的调用，估计是编译器的优化
	printf("w4_2 -------------------- \n");
	Widget w4_2;
	w4_2 = w3.getByCopy(); // 先调用Widget(const Widget& widget)拷贝构造函数，再调用=赋值拷贝函数
	printf("w4_3 -------------------- \n");
	Widget& w4_3 = w3.getByCopy(); // 调用一次Widget(const Widget& widget)拷贝构造函数
	printf("-------------------- \n\n");

	printf("w5 -------------------- \n");
	w3.getByRef(); // 返回引用
	printf("w5_1 -------------------- \n");
	Widget w5_1 = w3.getByRef(); // 调用一次Widget(const Widget& widget)拷贝构造函数
	printf("w5_2 -------------------- \n");
	Widget w5_2;
	w5_2 = w3.getByRef(); // 没有调用拷贝构造函数，调用一次=赋值拷贝函数
	printf("w5_3 -------------------- \n");
	Widget& w5_3 = w3.getByRef(); // 直接赋值给引用
	printf("-------------------- \n\n");

	printf("w6 -------------------- \n");
	w3.getByPointer(); // 返回指针
	printf("w6_1 -------------------- \n");
	Widget w6_1 = w3.getByPointer(); // 调用一次Widget(const Widget* widget)拷贝构造函数
	printf("w6_2 -------------------- \n");
	Widget w6_2;
	w6_2 = w3.getByPointer(); // 由于没有针对指针的赋值拷贝函数，先调用拷贝构造函数，创建一个临时变量，再调用=赋值拷贝函数
	printf("w6_3 -------------------- \n");
	Widget* w6_3 = w3.getByPointer(); // 直接赋值给指针
	printf("-------------------- \n");

	getchar();

	return 0;
}

运行结果：

Widget constructor with parameters: this:0035FB70 a:1
Widget constructor with parameters: this:0035FB64 a:2
Widget constructor with default value: this:0035FB58 a:0
Widget operator +: this:0035FB70 widget:0035FB64 a:3
Widget operator =: this:0035FB58 widget:0035FB70 a:3

w4 --------------------
Widget get object by copy: this:0035FB58 a:3
Widget copy constructor with reference: this:0035F9FC widget:0035FB58 a:3
w4_1 --------------------
Widget get object by copy: this:0035FB58 a:3
Widget copy constructor with reference: this:0035FB4C widget:0035FB58 a:3
w4_2 --------------------
Widget constructor with default value: this:0035FB40 a:0
Widget get object by copy: this:0035FB58 a:3
Widget copy constructor with reference: this:0035FA08 widget:0035FB58 a:3
Widget operator =: this:0035FB40 widget:0035FA08 a:3
w4_3 --------------------
Widget get object by copy: this:0035FB58 a:3
Widget copy constructor with reference: this:0035FB28 widget:0035FB58 a:3
--------------------

w5 --------------------
Widget get object by ref: this:0035FB58 a:3
w5_1 --------------------
Widget get object by ref: this:0035FB58 a:3
Widget copy constructor with reference: this:0035FB1C widget:0035FB58 a:3
w5_2 --------------------
Widget constructor with default value: this:0035FB10 a:0
Widget get object by ref: this:0035FB58 a:3
Widget operator =: this:0035FB10 widget:0035FB58 a:3
w5_3 --------------------
Widget get object by ref: this:0035FB58 a:3
--------------------

w6 --------------------
Widget get object by pointer: this:0035FB58 a:3
w6_1 --------------------
Widget get object by pointer: this:0035FB58 a:3
Widget copy constructor with pointer: this:0035FAF8 widget:0035FB58 a:3
w6_2 --------------------
Widget constructor with default value: this:0035FAEC a:0
Widget get object by pointer: this:0035FB58 a:3
Widget copy constructor with pointer: this:0035FA14 widget:0035FB58 a:3
Widget operator =: this:0035FAEC widget:0035FA14 a:3
w6_3 --------------------
Widget get object by pointer: this:0035FB58 a:3
--------------------