Re: [boost] Adding any_dynamic_cast?

Subject: Re: [boost] Adding any_dynamic_cast?
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2016-11-12 09:43:19

AMDG

On 11/12/2016 06:48 AM, Peter Dimov wrote:
> I stumbled across
> 
> https://github.com/bytemaster/Boost.DynamicAny
> 
> which implements, in a fully backward compatible manner, dynamic cast
> functionality for boost::any. That is, it can give you X* from an 'any'
> holding a value of type Y if Y derives from X.
> 
> It achieves this by simply changing
> 
>    template<typename ValueType>
>    class holder : public placeholder
>    {
>        ValueType held;
> 
> to
> 
>    template<typename ValueType>
>    class holder : public placeholder, public ValueType
> 
> which then enables dynamic_cast from holder<>* to OtherValueType*.
> 

This can fail if ValueType has virtual bases.

> This is pretty cool, and I don't remember seeing this idea anywhere else.
> 
> Perhaps we should adopt it?
> 

  This idea was proposed a long time ago
by Alexander Nasonov.  He even used the
same name.

There was also another (more correct)
implementation proposed here:
http://lists.boost.org/Archives/boost/2013/11/208957.php

> One might even argue that this is how any_cast should always have
> worked, but I suppose we can't change it now for compatibility reasons.
> 
> Although it's an interesting question whether C++17 'any' shouldn't be
> enhanced in this manner.
> 

In Christ,
Steven Watanabe