在编译时使用 Constexpr 填充数组

问题描述

我想使用 constexpr 填充枚举数组.数组的内容遵循一定的模式.

I would like to populate an array of enum using constexpr. The content of the array follows a certain pattern.

我有一个将 ASCII 字符集分为四类的枚举.

I have an enum separating ASCII character set into four categories.

enum Type {
    Alphabet,
    Number,
    Symbol,
    Other,
};

constexpr Type table[128] = /* blah blah */;

我想要一个包含 128 个 Type 的数组.它们可以在一个结构中.数组的索引将对应于 ASCII 字符，值将是每个字符的 Type.

I would like to have an array of 128 Type. They can be in a structure. The index of the array will be corresponding to the ASCII characters and the value will be the Type of each character.

所以我可以查询这个数组来找出一个 ASCII 字符属于哪个类别.类似的东西

So I can query this array to find out which category an ASCII character belongs to. Something like

char c = RandomFunction();
if (table[c] == Alphabet) 
    DoSomething();

我想知道如果没有一些冗长的宏技巧，这是否可行.

I would like to know if this is possible without some lengthy macro hacks.

目前，我通过执行以下操作来初始化表.

Currently, I initialize the table by doing the following.

constexpr bool IsAlphabet (char c) {
    return ((c >= 0x41 && c <= 0x5A) ||
            (c >= 0x61 && c <= 0x7A));
}

constexpr bool IsNumber (char c) { /* blah blah */ }

constexpr bool IsSymbol (char c) { /* blah blah */ }

constexpr Type whichCategory (char c) { /* blah blah */ }

constexpr Type table[128] = { INITIALIZE };

其中 INITIALIZE 是一些非常冗长的宏 hack 的入口点.类似的东西

where INITIALIZE is the entry point of some very lengthy macro hacks. Something like

#define INITIALIZE INIT(0)
#define INIT(N) INIT_##N
#define INIT_0 whichCategory(0), INIT_1
#define INIT_1 whichCategory(1), INIT_2
//...
#define INIT_127 whichCategory(127)

我想要一种方法来填充此数组或包含该数组的结构，而无需此宏 hack...

I would like a way to populate this array or a structure containing the array without the need for this macro hack...

也许类似

struct Table {
    Type _[128];
};

constexpr Table table = MagicFunction();

那么，问题是如何编写这个MagicFunction?

So, the question is how to write this MagicFunction?

注意:我知道 cctype 和喜欢，这个问题更像是一个 这可能吗? 而不是 这是最好的方法吗?.

Note: I am aware of cctype and likes, this question is more of a Is this possible? rather than Is this the best way to do it?.

任何帮助将不胜感激.

谢谢，

推荐答案

忽略所有问题，索引来救援:

template<unsigned... Is> struct seq{};
template<unsigned N, unsigned... Is>
struct gen_seq : gen_seq<N-1, N-1, Is...>{};
template<unsigned... Is>
struct gen_seq<0, Is...> : seq<Is...>{};

template<unsigned... Is>
constexpr Table MagicFunction(seq<Is...>){
  return {{ whichCategory(Is)... }};
}

constexpr Table MagicFunction(){
  return MagicFunction(gen_seq<128>{});
}

活生生的例子.

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