std :: begin和std :: end无法使用指针并引用原因？ [英] std::begin and std::end not working with pointers and reference why?

问题描述

为什么 std :: begin（）和 std :: end（）与数组一起使用，但不适用于指针[几乎是数组]和引用数组[原始数组的别名]。

Why std::begin() and std::end() works with array but not pointer[which is almost array] and reference of array [which is alias of original array].

ing了15分钟后，我无法在Google上找到任何东西。

After scratching my head for 15 min i am not able to get anything in google.

只有第一种情况有效，而不是第二种和第三种情况，这可能是什么原因？

Below only first case works, not second and third, what could be the reason for this?

#include <iostream>
#include <vector>
#include <iterator>
#include <algorithm>

int main() 
{
   int first[] = { 5, 10, 15 };  // Fist Case

    if (std::find(std::begin(first), std::end(first), 5) != std::end(first)) {
        std::cout << "found a 5 in array a!\n";
    }

   int *second = new int[3];  // Second Case
    second[0] = 5;
    second[1] = 10;
    second[2] = 15;
    if (std::find(std::begin(second), std::end(second), 5) != std::end(second)) {
        std::cout << "found a 5 in array a!\n";
    }

    int *const&refOfFirst = first;  // Third Case

        if (std::find(std::begin(refOfFirst), std::end(refOfFirst), 5) != std::end(refOfFirst)) {
        std::cout << "found a 5 in array a!\n";
    }
}

错误：

error: no matching function for call to ‘begin(int&)’
  if (std::find(std::begin(*second), std::end(*second), 5) != std::end(*second)) {
                                  ^

推荐答案

只给出一个指向数组开头的指针，无法确定数组的大小。因此开始和 end 无法处理指向动态数组的指针。

Given just a pointer to the start of an array, there's no way to determine the size of the array; so begin and end can't work on pointers to dynamic arrays.

如果要一个知道其大小的动态数组，请使用 std :: vector 。作为奖励，这也将解决您的内存泄漏。

Use std::vector if you want a dynamic array that knows its size. As a bonus, that will also fix your memory leak.

第三种情况失败了，因为再次使用了指针（对指针的引用）。您可以使用对数组本身的引用：

The third case fails because, again, you're using (a reference to) a pointer. You can use a reference to the array itself:

int (&refOfFirst)[3] = first;

或者，以避免指定数组大小：

or, to avoid having to specify the array size:

auto & refOfFirst = first;

和开始和 end 会像在 first 本身上那样工作。

and begin and end will work on this exactly as they would work on first itself.

这篇关于std :: begin和std :: end无法使用指针并引用原因？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！