c++ - Populate An Array Using Constexpr at Compile-time

Question

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

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

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

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

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.

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();

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

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 };

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)

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

Maybe something like

struct Table {
    Type _[128];
};

constexpr Table table = MagicFunction();

So, the question is how to write this MagicFunction?

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?.

Any help would be appreciated.

Thanks,

Answer 1

Ignoring ALL the issues, indices to the rescue:

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>{});
}

Live example.