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Easier make_unique with deleter function

From: Phil Endecott <std_proposals_list_at_[hidden]>
Date: Mon, 23 Aug 2021 17:31:10 +0100
Dear Experts,

I have been contemplating what the easiest current or future
way to create a unique_ptr with a custom deleter function is.

For example, libtiff has

TIFF* TIFFOpen(args);
void TIFFClose(TIFF*);

I'd like to be able to write something like:

auto tiffp = make_unique_with_deleter( TIFFOpen(fn), TIFFClose );

Of course it can't be quite that simple. Fundamentally unique_ptr
stores the deleter only in the type, which I think means the
deleter function can only ever be a template parameter:

auto tiffp = make_unique_with_deleter<TIFFClose>( TIFFOpen(fn) );

I haven't quite worked out how to implement that yet; I can't
avoid requiring the pointer type in the template list but I bet
one of you experts knows how to avoid that:

template <typename T, void(*DEL)(T*)>
struct wrap_deleter
  void operator()(T* p) const { DEL(p); }

template <typename T, void(*DEL)(T*)>
auto make_unique_with_deleter(T* raw_ptr)
  return std::unique_ptr<T, wrap_deleter<T,DEL>>(raw_ptr);

auto tiffp = make_unique_with_deleter<TIFF, TIFFClose>( TIFFOpen(fn) );

So anyway:

- Has anything like this been considered before?
- Can anyone suggest how to avoid needing the type template parameter?
- Have I missed an existing feature that can convert a function pointer
value into a functor type, as my wrap_deleter does?

Thanks, Phil.

Received on 2021-08-23 11:31:25