Re: [boost] [ Interest? ] [ out_ptr ] Tiny C++ Abstraction for C-Style Output Pointers

Subject: Re: [boost] [ Interest? ] [ out_ptr ] Tiny C++ Abstraction for C-Style Output Pointers
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2018-07-04 02:17:50

On 4/07/2018 13:48, ThePhD wrote:
>       I am not sure whether this provides much more benefit than just
> passing the smart pointer itself: the `some_function` already sticks
> `SmartPtr` in its signature (so you might as well just pass the SmartPtr
> itself, since you've already nailed the smart pointer to the signature).

The benefit is, as I said, in making it obvious at the call-site that 
the pointer can be modified.  The alternative is:

Caller:
   SmartPtr my_smart_ptr;
   auto r = some_function(foo, bar, /*out*/ my_smart_ptr);

Callee:
   bool some_function(int foo, int bar, SmartPtr& local_smart_ptr);

Where obviously the comment is trying to be an annotation, but as it's 
just a comment it has no real effect and is easily forgotten and then 
there is no evidence at all (at the call site) that my_smart_ptr can be 
modified by the call.

It's not really a pointer-specific issue, it's just a flaw in C++'s 
reference-passing syntax in general.  Some other languages handle this 
better by being more explicit about the purpose of a reference parameter.

So it would be nice if there were a way to have a compiler-enforced 
annotation, as long as it doesn't have negative performance impact.

As I said, this is not really in-scope for what you're trying to do with 
your library, but it's a related problem space and it would be nice if 
one solution could solve both problems uniformly.