来看一个示例：
#define func(x) __func(x, "func(" #x ")")
void __func(int &x, const char *str) {
cout << str << " is left value" << endl;
return;
}
void __func(int &&x, const char *str) {
cout << str << " is right value" << endl;
return;
}
int main() {
int x = 1234, y = 456;
func(1234);
func(x);
func(x + y);
func(x++);
func(++x);
func(x + 123);
func(x *= 2);
func(y += 3);
func(y * 3);
return 0;
}
程序输出：
func(1234) is right value
func(x) is left value
func(x + y) is right value
func(x++) is right value
func(++x) is left value
func(x + 123) is right value
func(x *= 2) is left value
func(y += 3) is left value
func(y * 3) is right value
一个简单的判断左值还是右值的方法：到了下一行还能不能通过单一变量访问到该值。能便是左值，否则是右值，其中字面量一定是右值。
比如func(x)中x的值在下一行可以访问到，那么x就是左值
func(x + y)中x + y的值在下一行访问不到，那么x + y就是右值
其他同理。
再来看一组示例：
#define func(x) __func(x, "func(" #x ")")
#define func2(x) __func2(x, "func2(" #x ")")
void __func2(int &x, const char *str) {
cout << str << " is left value" << endl;
return;
}
void __func2(int &&x, const char *str) {
cout << str << " is right value" << endl;
return;
}
void __func(int &x, const char *str) {
cout << str << " is left value" << endl;
func2(x);
return;
}
void __func(int &&x, const char *str) {
cout << str << " is right value" << endl;
func2(x);
return;
}
在fun函数中调用func2函数，程序输出：
func(1234) is right value
func2(x) is left value
func(x) is left value
func2(x) is left value
func(x + y) is right value
func2(x) is left value
func(x++) is right value
func2(x) is left value
func(++x) is left value
func2(x) is left value
func(x + 123) is right value
func2(x) is left value
func(x *= 2) is left value
func2(x) is left value
func(y += 3) is left value
func2(x) is left value
func(y * 3) is right value
func2(x) is left value
可以发现，调用func2函数都是以左值的方式调用的，即使原来的值是右值。
比如func(y * 3)调用是右值对应的函数，而在func2中却调用了左值对应的函数。如果我们仍然想以右值的特性向下传递，可以利用move语义，让他以右值的属性向下传递
void __func(int &&x, const char *str) {
cout << str << " is right value" << endl;
//向下传递右值属性
func2(move(x));
return;
}
这时输出：
func(1234) is right value
func2(move(x)) is right value
func(x) is left value
func2(x) is left value
func(x + y) is right value
func2(move(x)) is right value
func(x++) is right value
func2(move(x)) is right value
func(++x) is left value
func2(x) is left value
func(x + 123) is right value
func2(move(x)) is right value
func(x *= 2) is left value
func2(x) is left value
func(y += 3) is left value
func2(x) is left value
func(y * 3) is right value
func2(move(x)) is right value
还可以使用forward向下传递右值属性
void __func(int &&x, const char *str) {
cout << str << " is right value" << endl;
func2(forward<int &&>(x));
return;
}
move和forward可以理解是为了调用到正确的重载函数版本。
再来看一下右值引用在面向对象中的应用
class myvector {
public:
myvector(int n = 10) : __size(n), data(new int[n]) {
cout << this << endl;
cout << "default constructor" << endl;
}
//左值引用
myvector(const myvector &v) : __size(v.size()), data(new int[__size]) {
cout << this << endl;
cout << "deep copy constructor" << endl;
for (int i = 0; i < size(); i++) data[i] = v[i];
return;
}
myvector operator+(const myvector &v) {
myvector ret(v.size() + this->size());
myvector &now = *this;
for (int i = 0; i < size(); i++) {
ret[i] = now[i];
}
for (int i = size(); i < ret.size(); i++) {
ret[i] = v[i - size()];
}
return ret;
}
int &operator[](int ind) const {
return this->data[ind];
}
int size() const { return __size; }
~myvector() {
cout << this << endl;
cout << "destructor" << endl;
if (data) {
delete[] data;
}
data = nullptr;
}
private:
int __size;
int *data;
};
ostream &operator<<(ostream &out, const myvector &v) {
for (int i = 0; i < v.size(); i++) {
out << v[i] << " ";
}
return out;
}
int main() {
myvector v1, v2;
for (int i = 0; i < v1.size(); i++) v1[i] = rand() % 100;
for (int i = 0; i < v2.size(); i++) v2[i] = rand() % 100;
myvector v3(v1 + v2);
cout << v1 << endl;
cout << v2 << endl;
cout << v3 << endl;
return 0;
}
实现一个vector，程序输出：
中间调用了深拷贝，若通过右值引用，实现一个移动拷贝：
//右值引用，移动构造
myvector(myvector &&v) : __size(v.size()), data(v.data) {
cout << this << endl;
cout << "move copy constructor" << endl;
v.data = nullptr;//这里一定要记得
v.__size = 0;
}
移动拷贝不用一个个拷贝数据，而是直接拿取临时对象的值
程序输出：
如果我们想直接抢夺一个对象的资源，而不是拷贝，就可以通过move来调用移动构造函数。比如：
int main() {
myvector v1, v2;
for (int i = 0; i < v1.size(); i++) v1[i] = rand() % 100;
for (int i = 0; i < v2.size(); i++) v2[i] = rand() % 100;
myvector v3(v1 + v2);
cout << "v1: " << v1 << endl;
cout << "v2: " << v2 << endl;
cout << "v3: " << v3 << endl;
myvector v4(move(v1));//v1中的数据给v4，move向下传递右值引用
cout << "v1: " << v1 << endl;
cout << "v4: " << v4 << endl;
return 0;
}
关于引用的绑定顺序：
#include <iostream>
using namespace std;
void func1(int &x) {
cout << __PRETTY_FUNCTION__ << "called" << endl;
return;
}
void func1(const int &x) {
cout << __PRETTY_FUNCTION__ << "called" << endl;
return;
}
void func1(int &&x) {
cout << __PRETTY_FUNCTION__ << "called" << endl;
return;
}
void func1(const int &&x) {
cout << __PRETTY_FUNCTION__ << "called" << endl;
return;
}
int main() {
int n;
const int y = 123;
func1(n);// 优先绑定func1(int &);
func1(y);// func1(const int &);
func1(123 + 456);// func1(int &&);
return 0;
}
程序输出：
值得注意的是，const左值引用可以绑定任何值，比如：
#include <iostream>
using namespace std;
//只有const左值引用时
void func1(const int &x) {
cout << __PRETTY_FUNCTION__ << "called" << endl;
return;
}
int main() {
int n;
const int y = 123;
func1(n);// 优先绑定func1(int &);
func1(y);// func1(const int &);
func1(123 + 456);// func1(int &&);
return 0;
}
程序输出：
