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Accepted
Denver Toha
Asked:2023-03-09 21:07:46 +0800 CST2023-03-09 21:07:46 +0800 CST

如何处理参数包？

  • 772

我想从参数包中填充结构对象，怎么办？也许你可以为此使用 boost::mpl

struct Operation {
    int (*ops1)();
    int (*ops2)(int);
};

template<auto... Ops>
class Example {
public:
    constexpr Example() {
        /* 
         * 
         * Тут нужно отсортировать Ops и заполнить m_ops
         * если в аргументе шаблона есть указатель на функцию неподходящего типа - ошибка компиляции
         *
         * */
    }
private:
    Operation m_ops;
};
c++

1 个回答

  1. Best Answer

    我之前的回答没有考虑重新排序的需要。在这里，顺序被考虑在内。

    提升 - 未使用。boost::mpl 可以用来实现 is_tuple_of_unique_types，但是这个类型已经很紧凑了。

    
struct Operation {
    int (*ops1)();
    int (*ops2)(int);
};

// метаданные своими руками
template<typename T>
constexpr auto memeber_ptrs_tuple = nullptr;
template<>
constexpr auto memeber_ptrs_tuple<Operation> = std::make_tuple(
            & Operation::ops1,
            & Operation::ops2
            );


// Библиотека
namespace ditails
{
    template< typename ... T>
    constexpr bool is_unique_types=false;

    template< typename T0 >
    constexpr bool is_unique_types<T0> = true;
    template< typename T0, typename T1, typename... Tx >
    constexpr bool is_unique_types<T0,T1,Tx...>
        = is_unique_types<T0,Tx...> && is_unique_types<T0,Tx...>
        && ! std::is_same<T0,T1>::value;


    template< typename T>
    constexpr bool tuple_of_unique_types=false;
    template< typename...T>
    constexpr bool tuple_of_unique_types<std::tuple<T...>> = is_unique_types<T...>;



    template< typename T>
    struct impl_reorder_tuple_cast;

    template< typename... T>
    struct impl_reorder_tuple_cast<std::tuple<T...>>
    {
        template<typename Tuple>
        constexpr static std::tuple<T...> call(Tuple tuple)
        {
            return { std::get<T>(tuple) ... }; // именно эта строчка занимается переупорядочисванием.
        }
    };
    template<typename Result, typename Input>
    constexpr Result reorder_tuple_cast( Input input )
    {
        static_assert( tuple_of_unique_types<Result>  );
        static_assert( tuple_of_unique_types<Input>  );
        static_assert( std::tuple_size_v<Result> == std::tuple_size_v<Input> );
        return impl_reorder_tuple_cast<Result>::call(input);
    }

    template<typename Result, typename ... T>
    constexpr Result make_from_tuple( std::tuple<T...> tuple )
    {
        return Result{ std::get<T>(tuple)... };
    }


    template< typename T, typename... MemPtrs >
    constexpr auto to_tuple( T& obj, const std::tuple<MemPtrs...>& mem_ptrs  )
    {
        return std::make_tuple( (obj.*std::get<MemPtrs>(mem_ptrs))... );
    }
    template< typename T >
    constexpr auto to_tuple( T& obj )
    {
        return to_tuple(obj, memeber_ptrs_tuple< std::remove_cv_t<T> > );
    }
    template< typename T, typename... MemPtrs >
    constexpr auto to_tie( T& obj, const std::tuple<MemPtrs...>& mem_ptrs  )
    {
        return std::tie( (obj.*std::get<MemPtrs>(mem_ptrs))... );
    }
    template< typename T >
    constexpr auto to_tie( T& obj )
    {
        return to_tie(obj, memeber_ptrs_tuple< std::remove_cv_t<T> > );
    }

    template<typename T>
    using memebers_tuple_t = decltype( to_tuple( std::declval<T&>() ) );

    template<typename Result, typename ... T>
    constexpr Result make_from_reordered_args(T...args )
    {
        static_assert( is_unique_types<T...> );
        return make_from_tuple<Result>( reorder_tuple_cast< memebers_tuple_t<Result> >(std::make_tuple(args...) ) );
    }
}

using ditails::make_from_reordered_args;


// использование

template<auto... Ops>
class Example {
public:
private:
    static constexpr Operation m_ops = make_from_reordered_args<Operation>( Ops... );
};

int o0() { return 42; }
int o1(int i) { return i+2; }
int o2(int i, int j) { return i+j; }
int o1d(double i) { return (int)i+2; }

Example<&o1, &o0> ex; // ok
Example<&o0, &o1> ex2;// ok

Example<&o1, &o1> ex3;// cause error
Example<&o1>      ex4;// cause error
Example<&o1, &o2> ex5;// cause error
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