在编译时初始化静态数组时，g ++（4.7.2）错误或特性？ [英] g++ (4.7.2) bug or feature, when initializing static arrays at compile-time?

问题描述

好吧，所以我试图通过在编译时初始化一堆 constexpr static int const 数组来做一些聪明。即使运行时性能根本不受初始化这些数组的控制，它似乎是一个有趣的小练习。我写了一个测试设置，看看是否可能，我最终能够这样做：

Okay, so I was trying to do something clever by initializing a bunch of constexpr static int const arrays at compile-time. Even though the runtime-performance is not at all governed by initializing these arrays, it seemed like a fun little exercise. I wrote a test-setup to see if it was possible, and I ended up being able to do this:

struct Test
{
    constexpr static int const array[10] = Array<int, 10, 0, Increment>::array;
};

constexpr int const Test::array[10];

int main()
{
    cout << Test::array[3] << '\n';
}

这里， Array 有一个名为 array 的静态成员，其中包含10 int s，从0开始，其中每个后续元素的值为由增量（即 {0,1，...，9} ）的模板元编程函子确定。如预期，程序打印出 3 。

Here, Array has a static member called array which contains 10 ints, starting at 0, where the value of each subsequent element is determined by a Template-Metaprogramming functor called Increment (i.e. {0, 1, ..., 9}). As expected, the program prints out the number 3.

真棒，对不对？我现在可以只写写函数，并在编译时初始化数组将各种有趣的模式。下一步：通过使 Test 类模板来对数组大小为10进行硬编码：

Awesome, right? I can just write functors now and initialize arrays will all kinds of funky patterns at compile-time. Next step: un-hardcode the array-size 10 by making Test a class-template like so:

template <size_t Size>
struct Test
{
    constexpr static int const array[Size] = Array<int, Size, 0, Increment>::array;
};

template <size_t Size>
constexpr int const Test<Size>::array[Size];

int main()
{
    cout << Test<10>::array[3] << '\n';
}

但是，突然它不再编译消息：

However, all of a sudden it doesn't compile anymore with the message:

test.cc:43:72: error: array must be initialized with a brace-enclosed initializer

为什么会发生这种情况？有一个原因，这种初始化已经变得无效，一旦我把类转换成类模板，或者我偶然发现了一些未实现/在GCC中的bug。

Why does this happen? Is there a reason that this kind of initialization has become invalid once I turn the class into a class-template, or have I stumbled upon something unimplemented/buggy in GCC?

FYI，我可以根据请求发布我的代码的剩余部分（例如 Array 的实现）。现在我认为这应该就够了。

FYI, I can post the rest of my code (the implementation of Array for example) on request. For now I think this should be enough.

编辑错误可以通过 Array ，以在此处节省一些空间：

EDIT The error can be reproduced with a different, trivial, implementation of Array to save some space here:

template <size_t Size>
struct Array
{
    constexpr static int const array[Size] = {};
};

template <size_t Size>
struct Test
{
    constexpr static int const array[Size] = Array<Size>::array;
};

推荐答案

以下是非法的：

static const int a[10] = {};
static const int b[10] = a; // Illegal

所以gcc的错误实际上是非模板case。

So the bug of gcc is in fact for the non template case.

您可以使用 std :: array 而不是C-array。

You may use std::array instead of C-array.

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