Defining element initialization order when constructing std::tuple

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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

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花落未央

2021-01-25 18:23

There is no requirement in the standard for the order of std::tuple member initialisation, I am afraid.

You can iterate over a tuple in a specific order though, e.g.:

#include <tuple>
#include <iostream>

#include <boost/fusion/adapted/std_tuple.hpp>
#include <boost/fusion/algorithm/iteration/for_each.hpp>

int main()
{
    auto a = std::make_tuple(true, 42, 3.14, "abc");
    boost::fusion::for_each(a, [](auto& value) {
            std::cout << value << '\n';
        });
}

Outputs:

1
42
3.14
abc

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遥遥无期

2021-01-25 18:28

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

#include <tuple>

template<typename... Elems>
    struct construct 
    {
        template<size_t... Ns, typename Head, typename... Rest>
            static constexpr const std::tuple<Rest...> 
                drop_head_impl( const std::index_sequence<Ns...> ns, 
                    const std::tuple<Head, Rest...> tup )
            {
                return std::tuple<Rest...>( std::get<Ns + 1u>( tup )... );
            }

        template<typename Head, typename... Rest>
            static constexpr const std::tuple<Rest...> 
                drop_head( const std::tuple<Head, Rest...> tup )
            {
                return drop_head_impl( std::make_index_sequence<sizeof...(Rest)>(), tup );
            }

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

        template<typename Head, typename Next, typename... Rest>
            static constexpr const std::tuple<Head, Next, Rest...> 
                func_impl( const std::tuple<typename Head::initer_t, typename Next::initer_t, typename Rest::initer_t...> initer )
            {
                std::tuple<Head> head( { std::get<0>( initer ) } ); 
                return std::tuple_cat( head, func_impl<Next, Rest...>( drop_head(initer) ) );
            }

        static constexpr const std::tuple<Elems...> 
            func( const std::tuple<typename Elems::initer_t...> initer )
        {
            return func_impl<Elems...>( 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<typename... Elems>
    struct Widget
    {
        using initer_t = std::tuple<typename Elems::initer_t...>;
        Widget( const initer_t pIniter )
            :elements( construct<Elems...>::func( pIniter ) ) 
        {}
        const std::tuple<Elems...> elements;
    };

int main()
{
    using Button = Widget<Element, Element>;
    using ButtonList = Widget<Button, Button, Element>;

    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.

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