::scan

#include <iostream>

#ifdef __cpp_concepts
# define SCAN_REQUIRES(S, T) requires requires(T t) { S >> t; }
#else
# define SCAN_REQUIRES(S, T)
#endif

namespace impl {

    template<typename CharT>
    struct scan_fn;

    template<>
    struct scan_fn<char> {
        template<typename T> SCAN_REQUIRES(std::cin, T)
        operator T() const {
            T t;
            std::cin >> t;
            return t;
        }
    };

    template<>
    struct scan_fn<wchar_t> {
        template<typename T> SCAN_REQUIRES(std::wcin, T)
        operator T() const {
            T t;
            std::wcin >> t;
            return t;
        }
    };

}

inline constexpr impl::scan_fn<char> scan;
inline constexpr impl::scan_fn<wchar_t> wscan;

#undef SCAN_REQUIRES