mysql-native ResultRange + map

[crimaniak via Digitalmars-d-learn]

On Tuesday, 11 July 2017 at 22:46:00 UTC, Steven Schveighoffer 
wrote:
> On 7/5/17 12:04 PM, crimaniak wrote:
...
> Because of the temporary copy likely inside map closes the 
> connection.
>
> See the bug I reported: 
> https://github.com/mysql-d/mysql-native/issues/117
  Yes, it seems to be the same issue. (BTW, then I tried filter() 
just for curiosity and have the same problem).

> I work around this in my code by moving the temporary copy. But 
> I'm sorry to say you likely will not be able to fix map.
  I have WEB application (no big recordsets to transfer), so, for 
now, I "fix" it inside of my sql wrapper: just load recordset to 
an array and give it instead of ResultRange. I don't like it, but 
it works and all shit is localized in one place.

> I think Nick's comment is interesting, "Seems that using struct 
> dtors without refcounting should in general be regarded as a 
> code smell?"
>
> It's a good question. I've also always added ref counting when 
> wanting to "auto close" a struct's resources. In fact, I can't 
> really think of a good use of destructors for structs that 
> isn't for reference counting itself.

I don't think so. There is architecture problem: non-copyable by 
nature resource represented by copyable struct. We need 
non-copyable class/object semantics here or have to emulate it if 
using the struct to have the correct behavior. It's fully o.k. to 
get some resources required by _this exact copy_ of struct in the 
constructor and then release it in the destructor. For example of 
good use of destructors: position in some game, requires 
non-fixed space to store itself. We want to make copies of 
positions, apply some movies to them and then make copies again 
to search in positions tree. Copy constructor will malloc some 
space and copy source position data, the destructor will free 
this space, and we don't need reference counting here.