Initialize member array at compile time

February 4, 2024

Is it possible to initialize the idx value of the array_object at compile time such that member_array[0].idx = 0; member_array[1].idx = 1; … member_array[array_size-1].idx = array_size-1. The value member of every array_object should be initialized with an empty string literal.

struct my_class {
    struct array_object {
        int idx;
        std::string value{};
    };
    static constexpr size_t array_size = /* some compile time constant*/;

    std::array<array_object, array_size> member_array = /* default initialize where idx of each array_object is +1 of the preceeding array_object
                                                        and the first array_object idx is 0 ; array_object value always initialized to empty string*/;
};

>Solution :

You could use a pack of indices to initialize the array:

std::array<array_object, array_size> member_array =
    []<std::size_t... I>(std::index_sequence<I...>) {
        return std::array<array_object, array_size>{{{I, {}}...}};
//                                                    ^  ^^
//                                                   idx value
    }(std::make_index_sequence<array_size>{});

I here holds the indices [0, array_size) which is then expanded to form:

{0, {}}, {1, {}}, {2, {}}, ..., {array_size-1, {}}

Or use a loop:

std::array<array_object, array_size> member_array = []{
    std::array<array_object, array_size> rv;
    for(size_t i = 0; i < array_size; ++i) rv[i].idx = i;
    return rv;
}();