有没有在C ++ 11零大小的std ::阵列的一个原因？ [英] Is there a reason for zero sized std::array in C++11?

问题描述

考虑下面的一块code，这是一个由C ++编译器11完全可以接受的：

Consider the following piece of code, which is perfectly acceptable by a C++11 compiler:

#include <array>
#include <iostream>

auto main() -> int {
  std::array<double, 0> A;

  for(auto i : A) std::cout << i << std::endl;

  return 0;
}

根据标准§23.3.2.8 [零大小的数组的]：

1 阵应为特殊情况下支持ñ== 0 。

1 Array shall provide support for the special case N == 0.

2 在的情况下，N == 0 ，开始（）==结束（）==独特值。结果的返回值
     数据（）是unspeci网络版。

2 In the case that N == 0, begin() == end() == unique value. The return value of
data() is unspecified.

3 电子FF调用的等前（）或回（）的零大小的数组是理解过程网络定义。

3 The effect of calling front() or back() for a zero-sized array is undefined.

4 成员函数掉期（）应具有noexcept-Fi无线特定阳离子相当于
      noexcept（真）。

4 Member function swap() shall have a noexcept-specification which is equivalent to noexcept(true).

正如上面显示的，零大小的的std ::阵列是在C ++ 11完全允许的，符合零大小的数组（例如， INT A [0]; ），在那里它们被明确禁止的，但它们是由未定义行为的成本一些编译器（如GCC）允许的。

As displayed above, zero sized std::arrays are perfectly allowable in C++11, in contrast with zero sized arrays (e.g., int A[0];) where they are explicitly forbidden, yet they are allowed by some compilers (e.g., GCC) in the cost of undefined behaviour.

考虑到这种矛盾，我有以下问题：

Considering this "contradiction", I have the following questions:

• 为什么C ++委员会决定允许零大小的的std ::阵列 S'

是否有任何有价值的用途？

Are there any valuable uses?

推荐答案

如果你有一个泛型函数它是坏如果函数随机打破了特殊参数。例如，假设你可以有一个模板函数，它 N 随机元素组成的向量：

If you have a generic function it is bad if that function randomly breaks for special parameters. For example, lets say you could have a template function that takes N random elements form a vector:

template<typename T, size_t N>
std::array<T, N> choose(const std::vector<T> &v) {
   ...
}

没有的，所以如果这会导致不确定的行为或编译器错误，如果 N 由于某种原因，原来是零。

Nothing is gained if this causes undefined behavior or compiler errors if N for some reason turns out to be zero.

对于原始数组的限制背后的原因是，你不想类型的 sizeof的牛逼== 0 ，这将导致与指针运算组合奇怪的效果。零元素的数组将有大小为零，如果您不添加任何特殊规则吧。

For raw arrays a reason behind the restriction is that you don't want types with sizeof T == 0, this leads to strange effects in combination with pointer arithmetic. An array with zero elements would have size zero, if you don't add any special rules for it.

但的std ::阵列＆LT;＆GT; 是类和类总是有大小＆GT; 0.所以，你不会遇到这些问题的std ::阵列＆LT;＆GT; ，没有模板参数的任意限制了一致的接口为preferable

But std::array<> is a class, and classes always have size > 0. So you don't run into those problems with std::array<>, and a consistent interface without an arbitrary restriction of the template parameter is preferable.

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