How to Elegantly Initialize `std::array` with Non-Default Constructible Types in C ?

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

In the realm of C , std::array offers a convenient way to handle fixed-size arrays. However, initializing elements within an array poses a challenge when dealing with types that lack default constructors.

Consider the following code snippet:

<code class="cpp">template<typename T, int N>
void f(T value)
{
    std::array<T, N> items = {value, value, value, value, value};
}</code>

While straightforward, manually repeating the value becomes tedious as N increases. To address this issue, a more elegant approach is to leverage C 17's structured bindings and variadic templates.

Utilizing Sequence Generators and the Repeat Function

The key to this elegant solution lies in generating a sequence-type seq<0,1,2,3,...N-1> using the genseq_t<> generator. With this sequence-type, we can define a repeat function that takes a value and the generated sequence as parameters and returns an std::array filled with the repeated value.

Example Implementation

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

Pattern Details

• genseq_t::type represents the sequence-type that spans from 0 to N-1.
• repeat(value, seq) evaluates to an std::array with each element initialized to value.

This technique allows for concise and elegant initialization of std::array even when T lacks a default constructor.

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