How to Initialize Non-Default Constructible std::array Elegantly with Template Metaprogramming?

Initializing Non-Default Constructible std::array Elegantly

When working with non-default constructible types in C , initializing an std::array can be challenging, especially when the array size is determined by a template parameter.

To address this, we introduce a sophisticated approach that leverages sequence-type and generator concepts:

Sequence-Type seq

We define a sequence-type called seq using a template metafunction, which represents a sequence of sequential integers.

<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> {};
};</code>

Generator genseq

Using this sequence, we construct a generator genseq that generates sequences of increasing length:

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

template<>
struct genseq<0> : seq<> {};

template<int N>
using genseq_t = typename genseq<N>::type;</code>

repeat() Function

With the sequence in place, we define a repeat() function to create arrays initialized with the same value:

<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
   //note that (X, value) evaluates to value
   return {(N, value)...}; 
}</code>

Usage

To initialize an std::array of non-default constructible type, we simply call repeat() with the desired value and the sequence generated by genseq_t:

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

This solution ensures elegant and efficient initialization of std::array, effectively eliminating the need for cumbersome manual repetition when n is a large template parameter.

The above is the detailed content of How to Initialize Non-Default Constructible std::array Elegantly with Template Metaprogramming?. For more information, please follow other related articles on the PHP Chinese website!