std :: array复制语义 [英] std::array copy semantics

问题描述

#include <array>
#include <iostream>

using namespace std;

struct SimpleDebugger
{
    SimpleDebugger(int val = 0) : x(val) {
        cout << "created" << endl;
    }

    SimpleDebugger(const SimpleDebugger &that) : x(that.x) {
        cout << "copied" << endl;
    }

    ~SimpleDebugger() {
        cout << "killed!" << endl;
    }

    int getX() const {
        return x;
    }

    void setX(int val) {
        x = val;
    }

private:
    int x;
};

array<SimpleDebugger, 3> getInts(int i)
{
    array<SimpleDebugger, 3> a;
    a[0].setX(i);
    a[1].setX(i + 1);
    a[2].setX(i + 2);
    cout << "closing getInts" << endl;
    return a;
}

SimpleDebugger (*getIntsArray(int i)) [3] {
    typedef SimpleDebugger SimpleDebugger3ElemArray [3];
    SimpleDebugger3ElemArray *sd = new SimpleDebugger3ElemArray[1];
    (*sd)[0].setX(i);
    (*sd)[1].setX(i + 1);
    (*sd)[2].setX(i + 2);
    cout << "closing getIntsArray" << endl;
    return sd;
}

ostream& operator << (ostream& os, const SimpleDebugger &sd) {
    return (cout << sd.getX());
}

int main() {
    auto x = getInts(5);
    cout << "std::array = " << x[0] << x[1] << x[2] << endl;
    auto y = getIntsArray(8);
    cout << "Raw array = " << (*y)[0] << (*y)[1] << (*y)[2] << endl;
    delete [] y;
}

输出

created
created
created
closing getInts
std::array = 567
created
created
created
closing getIntsArray
Raw array = 8910
killed!
killed!
killed!
killed!
killed!
killed!

我尝试了上述程序，以了解使用 std有多方便： ：array 而不是原始数组，我知道避免使用老式数组是好的样式，甚至更好的方法是使用 std :: vector 。

I tried this above program to see how convenient it is to use std::array over raw arrays, I know that avoiding old-style arrays is good style and even better is to use std::vector.

我想知道在 std :: array 情况下，当函数 getInts（）返回。对于原始数组，我知道这是一个指针副本，清理起来的责任在于被调用者。在 std :: array 中不会发生这种情况，但是它如何在内部存储数据以及如何进行复制？

I'd like to know what happens under the hood in case of std::array, when the function getInts() returns. In case of the raw arrays, I know that it's a pointer copy and the onus of cleaning it up falls on the callee. In std::array this doesn't happen, but how does it internally store the data and how does the copy happen?

推荐答案

std :: array 是一个聚合，其中包含数组作为其唯一的数据成员。复制或移动一个副本会将数组的每个元素复制或移动到新数组中。

std::array is an aggregate, containing an array as its only data member. Copying or moving one will copy or move each element of the array into the new array.

在您的情况下，从函数返回副本时，该副本将被删除；在内部，该数组在 main 的自动存储中创建，函数填充该数组。

In your case the copy is being elided when it's returned from the function; under the hood, the array is created in automatic storage in main, and the function fills in that array.

这篇关于std :: array复制语义的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！