为什么要用引用传值

片段1

#include <iostream>
#include <string>

class Point {
public:
    Point(int x, int y) {
        this->x = x;
        this->y = y;
    }

    ~Point() {
        std::cout << "point (" << x << ", " << y << ")" << "has been destruct" << std::endl;
    }

    int x;
    int y;
};

class Test {
public:
    Test(Point p): p(p) {
    }

    Point getP() const {
        return p;
    }

private:
    Point p;  // there is a new memory has been locate
};

void run() {
    Test* t;
    {
        Point p(0, 1);  // create a new point
        t = new Test(p);    // copy a new point as parameter && create a new point when locate for t
    }

    Point _p = t->getP();

    std::cout << _p.x << std::endl;
}

int main() {
    run();
}

output

point (0, 1)has been destruct
point (0, 1)has been destruct
point (0, 1)has been destruct

分析

见代码注释

片段2

#include <iostream>
#include <string>

class Point {
public:
    Point(int x, int y) {
        this->x = x;
        this->y = y;
    }

    ~Point() {
        std::cout << "point (" << x << ", " << y << ")" << "has been destruct" << std::endl;
    }

    int x;
    int y;
};

class Test {
public:
    Test(const Point& p): p(p) {   // change to pass by const reference
    }

    Point getP() const {
        return p;
    }

private:
    Point p;  // there is a new memory has been locate
};

void run() {
    Test* t;
    {
        Point p(0, 1);
        t = new Test(p);   // now no new point was created when pass parameter, only locate for t
    }

    Point _p = t->getP();
}

int main() {
    run();
}

output

point (0, 1)has been destruct
point (0, 1)has been destruct