Subject: Re: [boost] [optional] Types without assignment operator
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2012-11-14 06:56:54
On Wed, Nov 14, 2012 at 3:39 PM, Adam Badura <abadura_at_[hidden]> wrote:
> I use boost::optional for deferred construction.
>
> Something similar to the idea described in "Bypassing expensive unnecessary
> default construction" in Boost.Optional documentation. Only in my case I do
> this because the mentioned construction requires virtual function calls and
> this cannot be done in owning object constructor and has to be postponed.
>
> So instead of accessing the object directly I always access it by a
> function. That function first checks if the object is already constructed
> (boost::optional is initialized) and if so just returns it. Otherwise it
> first constructs it.
>
> That approach requires only copy construction (or even move construction if
> boost::optional would support moving in the first place). Yet due to
> implementation details the embedded type has to be assignable as well. Even
> thou assignment operator will never be called (in this scenario).
>
> Is there a way to avoid this issue?
>
> Now the Synopsis part of the Boost.Optional documentation does mention
> (Typed)InPlaceFactory assign operators which do look promising for solving
> this issue. But firstly they are not documented in following Detailed
> Semantics part. There is even no link in the Synopsis. (Is this docs bug? In
> which part? Missing docs or extra functions?) And secondly
> (Typed)InPlaceFactory support is "intrusive". I cannot add it to a class
> without changing the class itself. And in my case I cannot change the class
> (boost::program_options::options_description - why cannot it be copy
> assigned, that I don't know...). Maybe adding some extra wrapper would help
> but I don't think it would be a solution. Rather a somewhat ugly workaround.
In-place factories are not intrusive, you can in-place construct value
of any type, as long as it has the necessary constructor. The
description is available here:
http://www.boost.org/doc/libs/release/libs/optional/doc/html/boost_optional/in_place_factories.html
AFAIR, assignment operator is not required by optional if you use
in-place factories in assignment. It is only required if you use
regular assignment with optional.