How to Initialize `std::array` for Non-Default Constructible Types Elegantly?

Elegant Initialization of std::array for Non-Default Constructible Types

In this context, initializing an std::array can be a challenge when the contained type (T) lacks a default constructor. A common solution involves manually repeating the value to fill the array, a tedious and error-prone process for large n values.

Fortunately, a more elegant approach exists using a combination of sequence types and a custom repeat function. Consider the following code:

<code class="cpp">template<typename T, int N>
void f(T value)
{
    // Generate a sequence of numbers: 0, 1, ..., N-1
    using genseq_t = genseq_t<N>;
    std::array<T, N> items = repeat(value, genseq_t{});
}</code>

The genseq_t type represents a sequence of numbers from 0 to N-1, and the repeat function takes a value and a sequence, returning an array filled with that value.

The repeat function's implementation involves unpacking the sequence and repeating the value for each element using a concise syntax:

<code class="cpp">template<typename T, int...N>
auto repeat(T value, seq<N...>) -> std::array<T, sizeof...(N)>
{
    // Unpack N, repeating 'value' sizeof...(N) times
    return {(N, value)...};
}</code>

The genseq_t type is defined recursively using a push_back operation to create a sequence:

<code class="cpp">template<int N>
struct genseq : genseq<N - 1>::type::template push_back<N - 1> {};</code>

The custom sequence and repeat function provide a flexible and elegant solution for initializing std::array with non-default constructible types.

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