


How to Elegantly Initialize a `std::array` with Non-Default-Constructible Types?
Elegant Initialization of std::array with Non-Default-Constructible Type
Initializing a std::array with a non-default-constructible element type can be a cumbersome task. Manually repeating the value n times is inefficient and error-prone for large n.
To address this issue, a more elegant approach involves utilizing a sequence-type and a generator. The key idea is to create a sequence of indices from 0 to n-1 and then use a function to repeatedly apply a value to each index.
Here's an implementation:
<code class="cpp">template<typename t int...n> auto repeat(T value, seq<n...>) -> std::array<t sizeof...> { // Unpack N, repeating `value` sizeof...(N) times // Note that (X, value) evaluates to value return {(N, value)...}; }</t></n...></typename></code>
To initialize an std::array using this approach:
<code class="cpp">template<typename t int n> void f(T value) { // genseq_t<n> is seq std::array<t n> items = repeat(value, genseq_t<n>{}); }</n></t></n></typename></code>
Additionally, the following definitions are used:
<code class="cpp">template<int ... n> struct seq { using type = seq<n...>; static const std::size_t size = sizeof ... (N); template<int i> struct push_back : seq<n... i> {}; }; template<int n> struct genseq : genseq<n-1>::type::template push_back<n-1> {}; template struct genseq : seq {}; template<int n> using genseq_t = typename genseq<n>::type;</n></int></n-1></n-1></int></n...></int></n...></int></code>
This solution provides an efficient and elegant way to initialize std::array with non-default-constructible types, regardless of the value of n.
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