How to Initialize a Non-Default Constructible `std::array` Without Repetition in C ?

Initializing Non-Default Constructible std::array Without Repetition

In C , initializing an std::array of non-default constructible elements can be a challenge. Standard initialization methods becomeumbersome when n, the array size, is large or a template parameter.

Solution:

An elegant solution leverages a combination of iterators and templates. Here's how it works:

We define two helper utilities:

• genseq_t generates a sequence of integers [0, 1, ..., n-1].
• repeat creates an array by repeating a given value the specified number of times.

Using these utilities, we can initialize the std::array in the template function f as follows:

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

Implementation Details:

• genseq_t: It uses variadic templates to create a sequence of integers up to n.
• repeat: This function unpacks the sequence generated by genseq_t and constructs the array by invoking T(N, value) for each integer in the sequence.

Example:

Consider an initialization with value as an integer and n as a template parameter:

<code class="cpp">void f(int value)
{
    std::array<T, 5> items = repeat(value, genseq_t<5>{});
}</code>

Here, items will be initialized with the value repeated five times: {value, value, value, value, value}.

Advantages:

This approach is more concise, especially when n is large. It also eliminates the need for manually repeating the value.

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