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

问题描述

好了，所以我试图通过初始化一堆在编译时constexpr静态常量INT 阵列做一些聪明。虽然运行时的性能完全不通过初始化这些数组管辖，这似乎是一个有趣的小运动。我写了一个测试设置，看看它是否是可能的，而且我最终能够做到这一点：

 结构测试
{
    constexpr静态常量INT数组[10] =阵列＆LT; INT，10，0，增量＆GT; ::阵列;
};constexpr诠释常量的Test ::阵列[10];诠释的main（）
{
    COUT＆LT;＆LT;测试::阵列[3]＆LT;＆LT;的'\\ n';
}
 

在这里，阵列有一个名为阵列静态成员，其中包含10 INT S，从0开始，每个后续元素的值是通过一个名为模板的元编程函子来确定递增（即 { 0,1，...，9} ）。正如预期的那样，该程序打印出号 3 。

真棒，对不对？我可以只写函子现在初始化数组在编译时各种时髦的图案。下一步：通过使非硬code数组大小10 测试 A类模板，像这样：

 模板＆LT;为size_t尺寸＆GT;
结构测试
{
    constexpr静态常量INT数组[尺寸] =阵列＆LT; INT，大小0，增量＆GT; ::阵列;
};模板＆LT;为size_t尺寸＆GT;
constexpr INT常量测试与LT;尺寸＆GT; ::阵列【尺寸】;诠释的main（）
{
    COUT＆LT;＆LT;试验＆所述10 -10; ::阵列[3]＆下;＆下;的'\\ n';
}
 

但是，突然现在不与消息编译：

  test.cc:43:72：错误：数组必须用括号括起来的初始化程序初始化
 

为什么会出现这种情况？是否有一个原因，一旦我把全班学生分成一类模板，或者我在GCC未实现的东西/车跌跌撞撞这种初始化已经失效了？

仅供参考，我可以发表我的code其余要求（阵列例如实施）。现在我认为这应该是足够了。

修改错误可以用不同的，琐碎的，实施阵列的复制来节省一些空间位置：

 模板＆LT;为size_t尺寸＆GT;
结构数组
{
    constexpr静态常量INT数组[尺寸] = {};
};模板＆LT;为size_t尺寸＆GT;
结构测试
{
    constexpr静态常量INT数组[尺寸] =阵列＆LT;尺寸和GT; ::阵列;
};
 

解决方案

以下是非法的;

 静态const int的一个[10] = {};
静态const int的B〔10] = A; //非法
 

所以GCC的错误，其实是对非模板的情况。

您可以使用的std ::阵列而不是C数组。

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';
}

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.

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';
}

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

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

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;
};

解决方案

Following is illegal;

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

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

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

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