return_on_error

#include <optional>
#include <fcntl.h>
#include <unistd.h>

namespace {
#define return_on_error value_wrapper() ?: ({ return std::nullopt; 0; })

struct value_wrapper {
    explicit operator bool() noexcept { return val >= 0; }
    operator int() noexcept { return (int)val; }
    operator long() noexcept { return val; }
    long val;
};
value_wrapper operator |(int val, value_wrapper wrapper) noexcept {
    wrapper.val = val;
    return wrapper;
}
value_wrapper operator |(long val, value_wrapper wrapper) noexcept {
    wrapper.val = val;
    return wrapper;
}
}

template <typename Fn>
struct on_scope_exit {
    on_scope_exit(Fn &&fn) noexcept: _fn(std::move(fn)) {}
    ~on_scope_exit() noexcept { this->_fn(); }

private:
    Fn _fn;
};

std::optional<char> demo() {
    int fd = open(__FILE__, O_RDONLY) | return_on_error;
    on_scope_exit closefd([=]() { close(fd); });
    char c;
    read(fd, &c, 1) | return_on_error;
    return c;
}

std::optional<char> control() {
    int fd = open(__FILE__, O_RDONLY);
    if (fd < 0) return std::nullopt;
    on_scope_exit closefd([=]() { close(fd); });
    char c;
    int ret = read(fd, &c, 1);
    if (ret < 0) return std::nullopt;
    return c;
}