为什么非const的std ::数组:: operator []的不constexpr？ [英] Why is non-const std::array::operator[] not constexpr?

问题描述

我试图填补编译时一个二维数组与给定的功能。这里是我的code：

I'm trying to fill a 2D array on compile time with a given function. Here is my code:

template<int H, int W>
struct Table
{
  int data[H][W];
  //std::array<std::array<int, H>, W> data;  // This does not work

  constexpr Table() : data{}
  {
    for (int i = 0; i < H; ++i)
      for (int j = 0; j < W; ++j)
        data[i][j] = i * 10 + j;  // This does not work with std::array
  }
};

constexpr Table<3, 5> table;  // I have table.data properly populated at compile time

它工作得很好， table.data 是正确填充在编译时。

It works just fine, table.data is properly populated at compile time.

但是，如果我改变普通二维数组 INT [H] [W] 与的std ::阵列＆LT;的std ::阵列＆LT; INT， H＆GT ;, W＆GT; ，我在循环体的错误：

However, if I change plain 2D array int[H][W] with std::array<std::array<int, H>, W>, I have an error in the loop body:

error: call to non-constexpr function 'std::array<_Tp, _Nm>::value_type& std::array<_Tp, _Nm>::operator[](std::array<_Tp, _Nm>::size_type) [with _Tp = int; long unsigned int _Nm = 3ul; std::array<_Tp, _Nm>::reference = int&; std::array<_Tp, _Nm>::value_type = int; std::array<_Tp, _Nm>::size_type = long unsigned int]'
data[i][j] = i * 10 + j;
^
Compilation failed

显然，我试图调用的std ::数组::运算符[] ，这是不是 constexpr <非const超载/ code>。现在的问题是，为什么它不是 constexpr ？如果C ++ 14允许我们修改 constexpr 的范围，这是为什么不通过的std ::阵列支持？

Obviously, I'm trying to call non-const overload of std::array::operator[], which is not constexpr. The question is, why it is not constexpr? If C++14 allows us to modify variables declared in constexpr scope, why this is not supported by std::array?

我常想，的std ::阵列就像普通数组，只有更好。但这里有一个例子，在那里我可以使用普通阵列，但不能使用的std ::阵列。

I used to think that std::array is just like plain array, only better. But here is an example, where I can use plain array, but cannot use std::array.

推荐答案

好吧，这的确是标准的监督。甚至有存在解决这个问题的建议：<一href=\"http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0107r0.pdf\">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0107r0.pdf

Ok, it is indeed an oversight in the standard. There even exists a proposal to fix this: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0107r0.pdf

[N3598]去掉constexpr成员函数为const的隐性标记。然而，
  的std ::阵成员函数这一变化后不重新，导致一个令人惊讶的缺乏
  对中的std ::阵列接口constexpr支持。本文通过添加修复了这个疏漏
  constexpr到的std ::阵列的成员函数，可以用最小量的支持它
  的工作。

[N3598] removed the implicit marking of constexpr member functions as const. However, the member functions of std::array were not revisited after this change, leading to a surprising lack of support for constexpr in std::array’s interface. This paper fixes this omission by adding constexpr to the member functions of std::array that can support it with a minimal amount of work.

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