使用constexpr查找表 [英] Lookup table with constexpr

问题描述

我想要创建一个坐标查找表，例如：

I'm looking to create a lookup table of coordinates, something like:

int a[n][2] = {{0,1},{2,3}, ... }

n ，将在编译时创建。我开始寻找 constexpr ，但似乎是一个函数返回一个 constexpr std :: vector< std :: array< int，2> > 不是一个选项，因为我得到：

For a given n, to be created at compile time. I started looking into constexpr, but is seems like a function returning a constexpr std::vector<std::array <int, 2> > isn't an option, as I get:

invalid return type 'std::vector<std::array<int, 2ul> >' of constexpr function

如何创建这样的编译时数组？

How can create such a compile time array?

推荐答案

我将先转储代码，在必要时添加引用和注释。

I'll dump the code first, adding references and comments where necessary/appropriate later. Just leave a comment if the result is somewhat close to what you're looking for.

指示包扩展的技巧（这里需要应用生成器）， Xeo ，来自此答案，修改为使用 std :: size_t 而不是 unsigned 。

Indices trick for pack expansion (required here to apply the generator), by Xeo, from this answer, modified to use std::size_t instead of unsigned.

#include <cstddef>

// by Xeo, from http://stackoverflow.com/a/13294458/420683
template<std::size_t... Is> struct seq{};
template<std::size_t N, std::size_t... Is>
struct gen_seq : gen_seq<N-1, N-1, Is...>{};
template<std::size_t... Is>
struct gen_seq<0, Is...> : seq<Is...>{};

生成器函数：

#include <array>

template<class Generator, std::size_t... Is>
constexpr auto generate_array_helper(Generator g, seq<Is...>)
-> std::array<decltype(g(std::size_t{}, sizeof...(Is))), sizeof...(Is)>
{
    return {{g(Is, sizeof...(Is))...}};
}

template<std::size_t tcount, class Generator>
constexpr auto generate_array(Generator g)
-> decltype( generate_array_helper(g, gen_seq<tcount>{}) )
{
    return generate_array_helper(g, gen_seq<tcount>{});
}

使用示例：

// some literal type
struct point
{
    float x;
    float y;
};
// output support for `std::ostream`
#include <iostream>
std::ostream& operator<<(std::ostream& o, point const& p)
{  return o << p.x << ", " << p.y;  }

// a user-defined generator
constexpr point my_generator(std::size_t curr, std::size_t total)
{
    return {curr*40.0f/(total-1), curr*20.0f/(total-1)};
}

int main()
{
    constexpr auto first_array = generate_array<5>(my_generator);
    constexpr auto second_array = generate_array<10>(my_generator);

    std::cout << "first array: \n";
    for(auto p : first_array)
    {
        std::cout << p << '\n';
    }
    std::cout << "========================\n";

    std::cout << "second array: \n";
    for(auto p : second_array)
    {
        std::cout << p << '\n';
    }
}

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