Re: [boost] [pimpl] No documentation for pointer semantics

Subject: Re: [boost] [pimpl] No documentation for pointer semantics
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2014-06-09 21:12:56

On 7/06/2014 12:54, quoth Vladimir Batov:
> Indeed, in my mind having two sides of that "coin" is quite natural... and I
> feel Rob agrees with us on that... he is just "vehemently" against calling
> it "pimpl".

If it helps (I'm not sure that it will), to me "pimpl" implies value 
semantics (typically very thin and implemented via a bare pointer 
internally).  If you wanted to share such an object then you would wrap 
a shared_ptr around the "outside" of the class; it wouldn't be part of 
the internals (and consumers of the class wouldn't need to care whether 
it was implemented with a pimpl or not).

A "handle" is different; it implies pointer semantics and can either be 
directly shared or cheaply copied (where the copied handle points to the 
same underlying object as the original).

Mutability is orthogonal, but I suspect it's a factor in the confusion 
of idioms -- from the sounds of it both Rob and myself are used to 
value-semantics mutable pimpls, while Pete and yourself are used to 
pointer-semantics immutable "handles".  Where an object is immutable the 
differences between value and pointer semantics are very slight (from a 
behavioural perspective, ignoring performance).

To put it another way: other than recompiling, adding or removing a 
pimpl from a class should be invisible to consumers of that class (just 
like any other change to private fields of a class).  I can envisage 
this easily for value semantics; I have a harder time doing so for 
pointer semantics (as this seems like it would require changing consumer 
code more extensively, unless the original class was *already* using 
some kind of handle/body pattern).