Re: [scope] unique_resource vs unique_ptr

From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2023-12-04 21:00:39

On 12/4/23 22:38, Bruno Martínez via Boost wrote:
> 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?

You are hijacking a tool that was not intended to be used this way.
unique_ptr is only supposed to be used with pointers and pointer-like
objects, which is why it accepts nullptr in its interface. The syntax:

  fd1 = nullptr;

is allowed, but it doesn't make sense for the reader who expects fd1 to
be a file descriptor. This would be especially confusing if the resource
type was a pointer type, with the unallocated value being something else
than a null pointer. Case in point, Windows' HANDLE and
INVALID_HANDLE_VALUE.