Re: [boost] Adding polymorphic_value to boost

Subject: Re: [boost] Adding polymorphic_value to boost
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2017-11-20 01:18:54

On 20/11/2017 12:41, Peter Dimov wrote:
> Peter Bartlett wrote:
> 
>> The only nit I had in the paper was that after all the banging on 
>> about it being a value type, we are given operator bool, operator* and 
>> operator->, making it pointer-like again. Could operator T& and 
>> operator T const& work?
> 
> Polymorphic means virtual functions, and when you have a 
> polymorphic_value<T> pv, where T has a virtual function `f`, you can 
> call `pv->f()`. With a conversion to T&, you can't call `pv.f()`.

You could pass it as a parameter to something that accepts a T& or T 
const& and then make polymorphic calls from that, or alternatively 
locally define a separate reference variable:

     Container<Base> c(new Derived);
     Base& r = c;
     r.f();  // calls Derived::f()

But yes, this is significantly messier (and more potentially surprising 
to users) than using operator-> instead.

Additionally since polymorphic_value can be empty, retaining some 
pointer-like syntax seems preferable as again this makes it less 
surprising to consumers.

Although on a peripherally related note, this reminds me of a language 
gripe I have with operator->, in that it is not used to resolve ->* 
expressions.  This means that code such as (p->*f)(x) has to be changed 
to (*p.*f)(x) if p is converted from a raw to a smart pointer, because 
(almost) nobody ever implements operator->* explicitly.  (Although I 
tried it as an experiment and it seems relatively straightforward to do 
so in C++11, albeit at the cost of injecting std::function.)