3
可能重复:
What are copy elision and return value optimization?为什么我的拷贝构造函数不被调用?
我有以下程序:
#include <iostream>
using namespace std;
class Pointt {
public:
int x;
int y;
Pointt() {
x = 0;
y = 0;
cout << "def constructor called" << endl;
}
Pointt(int x, int y) {
this->x = x;
this->y = y;
cout << "constructor called" << endl;
}
Pointt(const Pointt& p) {
this->x = p.x;
this->y = p.y;
cout << "copy const called" << endl;
}
Pointt& operator=(const Pointt& p) {
this->x = p.x;
this->y = p.y;
cout << "op= called" << endl;
return *this;
}
};
Pointt func() {
cout << "func: 1" << endl;
Pointt p(1,2);
cout << "func: 2" << endl;
return p;
}
int main() {
cout << "main:1" << endl;
Pointt k = func();
cout << "main:2" << endl;
cout << k.x << " " << k.y << endl;
return 0;
}
我期望的输出如下:
main:1
func: 1
constructor called
func: 2
copy const called
op= called
main:2
1 2
但我得到以下几点:
main:1
func: 1
constructor called
func: 2
main:2
1 2
的问题是:为什么不从FUNC返回一个对象,以主打电话给我拷贝构造函数?
我明白你为什么期望复制构造函数被调用,但你不应该期望赋值操作符被调用。当在初始化中使用“'='”时,实际上不是赋值运算符,而是复制初始化(在这种情况下优化了)。如果没有优化,将会有2次调用拷贝构造函数。 –