[scope] unique_resource vs unique_ptr

From: Bruno Martínez (bruno.uy_at_[hidden])
Date: 2023-12-04 19:38:31

Hi,

unique_ptr can emulate unique_resource:

    template<auto nullvalue, auto delete_>
    class unique
    {
        using T = decltype(nullvalue);

        struct generic_delete
        {
            class pointer
            {
                T t;
            public:
                pointer(T t) : t(t) {}
                pointer(std::nullptr_t = nullptr) : t(nullvalue) { }
                explicit operator bool() { return t != nullvalue; }
                friend bool operator ==(pointer lhs, pointer rhs) {
return lhs.t == rhs.t; }
                friend bool operator !=(pointer lhs, pointer rhs) {
return lhs.t != rhs.t; }
                operator T() { return t; }
            };

            void operator()(T p)
            {
                delete_(p);
            }
        };
    public:
        using type = std::unique_ptr<struct not_used, generic_delete>;
    };

    int main()
    {
        using unique_fd = unique<-1, close>::type;
        static_assert(sizeof(unique_fd) == sizeof(int), "bloated unique_fd");
        unique_fd fd1(open("fd.txt", O_WRONLY | O_CREAT | O_TRUNC));
        write(fd1.get(), "hello\n", 6);
    }

The std papers say

    While std::unique_ptr can be tweaked by using a custom deleter
type to almost a
perfect handler for resources, it is awkward to use for handle types
that are not pointers.

But what's the awkwardness?

Kind regards,
Bruno