元组和总结

Question

如何编写一个函数来检查您是否可以对所有元组元素求和

template <size_t ID, class T>
int check(const T& p)
{
    size_t n = std::tuple_size_v<T>;
    auto sum = std::get<0>(p);
    if constexpr (ID < std::tuple_size_v<T>) {
        if (sum += std::get<ID>(p)) {
            check<ID + 1>(p);
        }
        else
...

Answer 1

您可以使用偏特化和折叠表达式来检查 std::tuple<T1, T2, T3> 是否可以调用 T1{} + T2{} + T3{};

#include <tuple>
#include <iostream>
#include <string>

template <typename Tuple, typename = void>
struct check_impl : public std::false_type {};

template <typename... Ts>
struct check_impl<std::tuple<Ts...>, std::void_t<decltype((std::declval<Ts>() + ...))>> : public std::true_type {};

template <typename Tuple>
constexpr bool check = check_impl<Tuple>::value;

int main() {
    std::cout << check<std::tuple<int, double, char>> << " " << check<std::tuple<int, std::string>>;
}

Answer 2

实现相同且更有效结果的一些替代方法：

#include <tuple>
#include <iostream>
#include <string>

template <typename... Ts>
constexpr auto check_impl(...) noexcept -> bool { return false; }

template <typename... Ts>
constexpr auto check_impl(int) noexcept -> decltype((std::declval<Ts>() + ...), bool{}) {
    return true;
}

template <typename... Ts>
constexpr bool check(std::tuple<Ts...>) noexcept {
    return check_impl<Ts...>(0);
}

int main() {
    std::cout << check(std::tuple<int, double, char>{}) << ' ' << check(std::tuple<int, std::string>{});
}

元组和总结

Tuple and summaring

c++

tuples

metaprogramming