問題描述

我想使用 constexpr 填充枚舉數組.數組的內容遵循一定的模式.

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

我有一個將 ASCII 字符集分為四類的枚舉.

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

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

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

我想要一個包含 128 個 Type 的數組.它們可以在一個結構中.數組的索引將對應于 ASCII 字符，值將是每個字符的 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.

所以我可以查詢這個數組來找出一個 ASCII 字符屬于哪個類別.類似的東西

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();

我想知道如果沒有一些冗長的宏技巧，這是否可行.

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

目前，我通過執行以下操作來初始化表.

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 是一些非常冗長的宏 hack 的入口點.類似的東西

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)

我想要一種方法來填充此數組或包含該數組的結構，而無需此宏 hack...

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

也許類似

struct Table {
    Type _[128];
};

constexpr Table table = MagicFunction();

那么，問題是如何編寫這個MagicFunction?

So, the question is how to write this MagicFunction?

注意:我知道 cctype 和喜歡，這個問題更像是一個 這可能嗎? 而不是 這是最好的方法嗎?.

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?.

任何幫助將不勝感激.

謝謝，

推薦答案

忽略所有問題，索引來救援:

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.

這篇關于在編譯時使用 Constexpr 填充數組的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網！