Defining element initialization order when constructing std::tuple

Question

I would like to store the initialization values for elements in a tuple inside a separate tuple, so that I can use the same values as a preset for other tuples of the respective

Answer 1

For anyone interested in a solution, I came up with a way to control the initialization order and retain the constness of elements:

#include 

template
    struct construct 
    {
        template
            static constexpr const std::tuple 
                drop_head_impl( const std::index_sequence ns, 
                    const std::tuple tup )
            {
                return std::tuple( std::get( tup )... );
            }

        template
            static constexpr const std::tuple 
                drop_head( const std::tuple tup )
            {
                return drop_head_impl( std::make_index_sequence(), tup );
            }

        template
            static constexpr const std::tuple 
                func_impl( const std::tuple initer )
            {
                return  std::tuple( { std::get<0>( initer ) } ); 
            }

        template
            static constexpr const std::tuple 
                func_impl( const std::tuple initer )
            {
                std::tuple head( { std::get<0>( initer ) } ); 
                return std::tuple_cat( head, func_impl( drop_head(initer) ) );
            }

        static constexpr const std::tuple 
            func( const std::tuple initer )
        {
            return func_impl( initer );
        }
    };

// Elements are the end points of a Widget hierarchy
struct Element
{
    using initer_t = int;
    Element( const initer_t pIniter )
        :data{ pIniter }
    {
        printf( "Creating %i\n", data );
    }
    const initer_t data;
};

// A Widget class stores any number of Elements and/or other Widget instances
template
    struct Widget
    {
        using initer_t = std::tuple;
        Widget( const initer_t pIniter )
            :elements( construct::func( pIniter ) ) 
        {}
        const std::tuple elements;
    };

int main()
{
    using Button = Widget;
    using ButtonList = Widget;

    Button::initer_t basic_button_initer{ 0, 1 }; // presets for Buttons
    Button::initer_t other_button_initer{ 2, 3 }; 

    ButtonList::initer_t buttonlist_initer{ basic_button_initer, other_button_initer, 4 }; //a preset for a ButtonList

    ButtonList buttonlist{ buttonlist_initer };
    return 0;
}

The construct structure takes the tuple of initer_ts (initer), constructs a tuple containing the first element of Elems... using the first element of initer, then drops the first element of initer and passes the remaining tuple to itself, which causes a tuple with the next element of Elems... to be constructed using the next element in initer. This recursion is stopped by an overload of func_impl for a tuple with one element which simply constructs that element from its initer_t in a tuple and returns it. This single-element tuple gets concatenated to the tuple with the previous element, the result gets returned to the higher level and is concatenated to the single-element tuple there and so on.