Unable to use std::unique_ptr in immer::flex_vector

[fabiopakk]

I need to use strictly move-only types in immer like:

immer::flex_vector<std::unique_ptr<unsigned char[]>> v;

But this produces the following error:

util.hpp:150:45: error: call to implicitly-deleted copy constructor of 'value_t' (aka 'std::unique_ptr<unsigned char[]>')
  150 |                 std::addressof(*current)))) value_t(*first);

How can std::unique_ptr be used in immer containers, please?

Fábio Pakk Selmi-Dei (@fabiopakk)

[arximboldi]

You can't. Type has to be copyable. It's meant for types with value semantics. What's your use-case?

[fabiopakk]

Thanks for answering, @arximboldi. My priority is to save memory and not have memory leaks. I was hoping that immer could keep different versions of a container without replicating any element. Is there such a possibility in immer? Perhaps using certain transient containers with selected memory strategies?

[arximboldi]

I'm still not sure what the content of those std::unique_ptr<unsigned char[]> are? could they be replaced by std::string or other value type? Perhaps an immer::box<> of something? In any case you need a value type (copyable). Immer will be efficient at avoiding copies, but it may make some sometimes when updating the container in a way that requires it (you haven't mode it properly or there are multiple references around).

[fabiopakk]

Hi @arximboldi ,

The elements of type unsigned char[] are agnostic and we have no authority over them. We just would like not to copy any of the elements unless it is strictly necessary. For now, using std::shared_ptr instead of std::unique_ptr seems to be a plausible compromise. But this solution has the overhead of reference counting. We would like to avoid that at the same time we don't want memory leaks, of course.

You've mentioned immer::box as an alternative. How would it compare to smart pointers?

Thank you very much,

Fábio

[arximboldi]

immer::box is like a std::shared_ptr but with "value semantics". shared_ptr sounds like a reasonable alternative for your use-case. Immer does reference counting also inside the data-structure.