Create STL array of struct with const class member

Question

I have a struct with a class member I'd like to make const because it should never be changed after the constructor:

struct S
{
    S(const double pp) : p(pp){}

    const double p;
};

However, I would like to create an std::array of S, of size 1000 (a size too large to explicitly construct each element).

I am aware STL array elements need a default constructor.

Is it possible to define a function called create_array() which can be used like this:

const double p = from_config("p");
std::array<S, 1000> = create_array(p);

and allow me to keep the class member as const?

Answer 1

Is it possible to define a function called create_array() which can be used like this: std::array<S, 1000> = create_array(p);

Yes, it is possible to do exactly this as shown below:

#include <array>
#include <iostream>

struct S
{
    S(const double pp) : p(pp){}

    const double p;
};

template <typename T, std::size_t ...I>
constexpr auto make_array(std::index_sequence<I...>, double p) {
    return std::array<T, sizeof...(I)>{((I-I), p)...};
}

//version that forwards/passes the args 
template<typename T, std::size_t N>
constexpr auto create_array(double p) {
    return make_array<T>(std::make_index_sequence<N>(), p);
}

int main(void)
{
    const double p = 5.5;
    auto a = create_array<S, 5>(p);
    for(const auto &elem: a)
    {
        std::cout << elem.p << "\n";
    }
    
    return 0;
}

Working demo

The output of the program is:

5.5
5.5
5.5
5.5
5.5