std::unique_ptr for C functions that need free

Question

Think to a C function that return something that must be freed, for example the POSIX\'s strdup(). I want to use that function in C++11 and avoid a

Answer 1

Assuming it makes sense, it is possible to use a similar pattern with non-pointers? For example for the POSIX's function open that returns an int?

Yes, it can be done. You need a "pointer" type that satisfies the NullablePointer requirements:

struct handle_wrapper {

    handle_wrapper() noexcept : handle(-1) {}
    explicit handle_wrapper(int h) noexcept : handle(h) {}
    handle_wrapper(std::nullptr_t)  noexcept : handle_wrapper() {}

    int operator *() const noexcept { return handle; }
    explicit operator bool() const noexcept { return *this != nullptr; }

    friend bool operator!=(const handle_wrapper& a, const handle_wrapper& b) noexcept {
        return a.handle != b.handle;
    }

    friend bool operator==(const handle_wrapper& a, const handle_wrapper& b) noexcept {
        return a.handle == b.handle;
    }

    int handle;
};

Note that we use -1 as the null handle value here because that's what open() returns on failure. It's also very easy to templatize this code so that it accepts other handle types and/or invalid values.

Then

struct posix_close
{
    using pointer = handle_wrapper;
    void operator()(pointer fd) const
    {
        close(*fd);
    }
};

int
main()
{
    std::unique_ptr p(handle_wrapper(open("testing", O_CREAT)));
    int fd = *p.get();
}

Demo.