How to Create a Constexpr Zero-Based Array in C 11?

Constexpr Array Generation in C 11

Question:

How can we create a constexpr zero-based array up to a specified size in C 11?

Answer:

In C 14, this task became straightforward thanks to the introduction of constexpr constructors and loops. However, in C 11, we need to adopt a more intricate approach.

Implementation:

1. Step 1: Utilizing a Class Template with a Constexpr Constructor

   We define a class template A that takes the array size N as a template parameter. It includes a constexpr constructor that initializes an array arr of size N.

2. Step 2: Populating the Array

   Within the constexpr constructor, we iterate up to N, assigning each element of arr its corresponding index. This allows us to populate the array with the 0 to n-1 sequence.

3. Step 3: Usage

   We can instantiate the A class template to obtain an instance of the constexpr array. By accessing its arr member, we can interact with its elements.

Here's an example code demonstrating this approach in C 11:

#include <iostream>

template<int N>
struct A {
    constexpr A() : arr() {
        for (auto i = 0; i != N; ++i)
            arr[i] = i; 
    }
    int arr[N];
};

int main() {
    constexpr auto a = A<4>();
    for (auto x : a.arr)
        std::cout << x << '\n';
}

This code will output the expected 0 to 3 sequence, demonstrating the successful creation of a constexpr zero-based array in C 11.

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