“&”之间的区别，和std :: reference_wrapper？ [英] Difference between "&" and std::reference_wrapper?

问题描述

我有以下代码

#include <iostream>
#include <vector>
#include <functional>

int main() {
    std::vector<double> v(5, 10.3);
    std::vector<double>& r = v;
    //std::vector<std::reference_wrapper<double>> r(v.begin(),v.end());
    for (auto& i : v)
        i *= 3;
    for (auto i : r)
       std::cout << i << " ";
    std::cout << std::endl;
}

我使用向量'r'的引用使用'&在第二行我注释掉我使用std :: reference_wrapper的C ++。两个都做同样的工作？但我认为必须有一个目的，使std :: reference_wrapper，即使我们有'&'来完成这项工作。

I am using reference of the vector 'r' using '&' operator and in the second line which I commented out I am using std::reference_wrapper of C++. Both do pretty much the same job? But I think there must be a purpose of making std::reference_wrapper even if we had '&' to do the job. can anyone explain please?

推荐答案

首先，两行

std::vector<double>& r = v;
std::vector<std::reference_wrapper<double>> r(v.begin(),v.end());

不是指同样的事情。

其次，std :: reference_wrapper在通用编程中非常有用（一个是双引号向量的引用，另一个是双引号向量的引用） ie：templates）其中函数可以通过复制获取其参数，但是您仍然希望通过引用传递参数。或者，如果你想要一个容器有引用，但不能使用引用，因为它们不可复制或移动，那么std :: reference_wrapper可以做这项工作。

Second, std::reference_wrapper is useful in generic programming (ie: templates) where either a function might take its arguments by copy, but you still want to pass in an argument by reference. Or, if you want a container to have references, but can't use references because they aren't copyable or movable, then std::reference_wrapper can do the job.

本质上，std :: reference_wrapper的行为就像一个&除非它是可复制和可重新分配的

Essentially, std::reference_wrapper acts like a "&" reference, except that it's copyable and reassignable

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