如果在编译器扩展的帮助下初始化arr，则排序(开始(Arr)，end(arr))不能工作。

Question

下面的代码按预期工作：

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

int main()
{
    int arr[] = {6, 4, 2};
    std::sort(std::begin(arr), std::end(arr));
    for (const auto &i : arr)
        std::cout << i << ' ';
    std::cout << std::endl;
}

现在，考虑一下当我们修改不执行排序和打印的代码时会发生什么：

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

int main()
{
    int n;
    std::cin >> n; // take n = 3
    int arr[n];
    for (int i = 0; i != n; ++i)
        std::cin >> arr[i]; // take (arr[0],arr[1],arr[2])=(6,4,2)

    std::sort(std::begin(arr), std::end(arr));
    for (const auto &i : arr)
        std::cout << i << ' ';
    std::cout << std::endl;
}

对于gcc，这将导致错误(https://godbolt.org/z/W4av94TKj)：

source>: In function 'int main()':
<source>:13:25: error: no matching function for call to 'begin(int [n])'
   13 |     std::sort(std::begin(arr), std::end(arr));
      |               ~~~~~~~~~~^~~~~
In file included from /opt/compiler-explorer/gcc-trunk-20220623/include/c++/13.0.0/bits/range_access.h:36,
                 from /opt/compiler-explorer/gcc-trunk-20220623/include/c++/13.0.0/string:51,
                 from /opt/compiler-explorer/gcc-trunk-20220623/include/c++/13.0.0/bits/locale_classes.h:40,
                 from /opt/compiler-explorer/gcc-trunk-20220623/include/c++/13.0.0/bits/ios_base.h:41,
                 from /opt/compiler-explorer/gcc-trunk-20220623/include/c++/13.0.0/ios:42,
                 from /opt/compiler-explorer/gcc-trunk-20220623/include/c++/13.0.0/ostream:38,
                 from /opt/compiler-explorer/gcc-trunk-20220623/include/c++/13.0.0/iostream:39,
                 from <source>:1:
/opt/compiler-explorer/gcc-trunk-20220623/include/c++/13.0.0/initializer_list:88:5: note: candidate: 'template<class _Tp> constexpr const _Tp* std::begin(initializer_list<_Tp>)'
   88 |     begin(initializer_list<_Tp> __ils) noexcept
      |     ^~~~~
/opt/compiler-explorer/gcc-trunk-20220623/include/c++/13.0.0/initializer_list:88:5: note:   template argument deduction/substitution failed:
<source>:13:25: note:   mismatched types 'std::initializer_list<_Tp>' and 'int*'
   13 |     std::sort(std::begin(arr), std::end(arr));
      |               ~~~~~~~~~~^~~~~
[... and a long list of more overloads ...]

为什么这是个错误？我知道C++标准要求数组大小为constexpr。然而，gcc确实提供了允许非constexpr数组大小的扩展。但它没有回答为什么排列数组的行出现错误的问题，而其他的则没有。

Answer 1

数组的std::end重载是

template< class T, std::size_t N >
constexpr T* end( T (&array)[N] ) noexcept;

但

int n;
std::cin >> n; // take n = 3
int arr[n];

意味着没有与模板参数匹配的编译时间大小。STL功能无法在您的VLA上工作。你必须用

std::sort(arr, arr + n);