子魂.cpp

#include <iostream>
#include <string>
#include <type_traits>

template<unsigned I>
struct tag: tag<I-1>{};

template<>
struct tag<0>{};

struct init
{
    template<typename T>
    operator T();
};

template<typename T>
constexpr auto size_(tag<4>) -> decltype(T{init{}, init{}, init{}, init{}}, 0u)
{
    return 4u;
}

template<typename T>
constexpr auto size_(tag<3>) -> decltype(T{init{}, init{}, init{}}, 0u)
{
    return 3u;
}

template<typename T>
constexpr auto size_(tag<2>) -> decltype(T{init{}, init{}}, 0u)
{
    return 2u;
}

template<typename T>
constexpr auto size_(tag<1>) -> decltype(T{init{}}, 0u)
{
    return 1u;
}

template<typename T>
constexpr auto size_(tag<0>) -> decltype(T{}, 0u)
{
    return 0u;
}

template<typename T>
constexpr size_t size()
{
    static_assert(std::is_aggregate_v<T>);
    return size_<T>(tag<4>{});
}

template<typename T, typename F>
void for_each_member(const T& v, F f)
{
    static_assert(std::is_aggregate_v<T>);
    if constexpr(size<T>() == 4u)
    {
        const auto& [m0, m1, m2, m3] = v;
        f(m0);f(m1);f(m2);f(m3);
    }
    else if constexpr(size<T>() == 3u)
    {
        const auto& [m0, m1, m2] = v;
        f(m0);f(m1);f(m2);
    }
    else if constexpr(size<T>() == 2u)
    {
        const auto& [m0, m1] = v;
        f(m0);f(m1);
    }
    else if constexpr(size<T>() == 1u)
    {
        const auto& [m0] = v;
        f(m0);
    }
}

int main() {
    struct X { std::string s{ " " }; }x;
    struct Y { double a{}, b{}, c{}, d{}; }y;
    std::cout << size<X>() << '\n';
    std::cout << size<Y>() << '\n';

    auto print = [](const auto& member) {
        std::cout << member << ' ';
    };
    for_each_member(x, print);
    for_each_member(y, print);
}