队列创建副本吗? [英] does queue create a copy?

问题描述

如果我将现有对象推入队列:

If I push an existing object into a queue:

struct Node {int x; int y;};
std::vector<Node> vec;
vec.push_back(Node(1, 3));

std::queue<Node> q;
q.push(vec[0]);

在最后一行，q 是否存储 地址(指针或引用，除了对象本身)vec[0]，还是将整个Node对象复制到q中?

At the last line, does q store the address (pointer or reference, whatever except the object itself) of vec[0], or does it copy the whole Node object into q?

推荐答案

它会在您分配右值引用时被复制.如果您分配左值引用，它将被移动.(临时对象).

It will get copied as you assign an rvalue reference. It will get moved if you assign an lvalue reference. (temporary object).

要检查，使用复制构造函数/运算符和移动构造函数/运算符重载:

To check, use copy constructor / operator and move constructor / operator overloading:

#include <iostream>
#include <vector>
#include <queue>

struct Node {
    int x;
    int y;

    Node(int x, int y) : x(x), y(y)
    {
        std::cout << "constructor" << std::endl;
    }

    Node(Node const & original) : x(original.x), y(original.y)
    {
        std::cout << "copy constructor" << std::endl;
    }

    Node(Node const && original) : x(original.x), y(original.y)
    {
        std::cout << "move constructor" << std::endl;
    }

    Node & operator=(Node const & original) {
        std::cout << "assignment operator" << std::endl;
        if(this != &original) {
            x = original.x;
            y = original.y;
        }
        return *this;
    }

    Node & operator=(Node const && original) {
        std::cout << "move operator" << std::endl;
        if(this != &original) {
            x = original.x;
            y = original.y;
        }
        return *this;
    }
};


int main() {

    std::vector<Node> v;

    Node n(1,3);        // constructor
    Node m(3, 4);       // constructor

    m = n;              // assignment operator
    n = Node(2, 3);     // constructor + move operator

    v.push_back({1,2});     // constructor + move constructor
    v.push_back(n);         // copy constructor

    std::queue<Node> q;
    q.push(v[0]);           // copy constructor

    return 0;
}

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