[boost] [optional] Types without assignment operator

Subject: [boost] [optional] Types without assignment operator
From: Adam Badura (abadura_at_[hidden])
Date: 2012-11-14 06:39:26

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?

If not then how about adding a function like "assign" (possibly named 
otherwise or with additional defaulting argument) which could assign by 
first destroying old object (if any) and then copy/move constructing a new 
one.

This would obviously be bad from exception safety (guarantees) point of 
view, but well... don't call the function if you don't like that. And in 
scenarios like mine it would not be an issue since there would be no object 
to destroy anyway. (Also the function could assert/throw on that 
condition...)

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.

Adam Badura