modecomp_opt.cpp

using mode_t = unsigned int;
using size_t = decltype(sizeof(int));

namespace detail
{
    struct invalid_permission_character {};
    struct invalid_permission_string_length {};

    consteval mode_t from_readable_mode_string(const char * const s)
    {
        mode_t result{};

        #define STEP(position, accept)                                   \
        {                                                                \
            if(s[position] == accept)   { result |= 1 << (8-position); } \
            else if(s[position] == '-') { result |= 0; }                 \
            else throw invalid_permission_character{};                   \
        }

        STEP(0, 'r'); STEP(3, 'r'); STEP(6, 'r');
        STEP(1, 'w'); STEP(4, 'w'); STEP(7, 'w');
        STEP(2, 'x'); STEP(5, 'x'); STEP(8, 'x');

        #undef STEP

        return result;
    }}

consteval mode_t from_readable_mode(const char (&permission_string)[10])
{
    return detail::from_readable_mode_string(permission_string);
}

consteval mode_t operator""_mode(const char *s, size_t len)
{
    return len == 9 ? detail::from_readable_mode_string(s)
                    : throw detail::invalid_permission_string_length{};
}

void set_permission(mode_t) { }

int main()
{
    set_permission("rwx---rwx"_mode);
}