Re: [Boost-users] Syntax of get for Variant and any_cast for Any

From: Doug Gregor (dgregor_at_[hidden])
Date: 2006-03-10 15:50:03

On Mar 10, 2006, at 2:41 PM, Scott Meyers wrote:
> It took me a little while to figure out that get<T> on a variant is  
> really just
> the moral equivalent of dynamic_cast<T> with a funny syntax.  I'd  
> like to
> understand why, given a variant v, this syntax is used
>
>    T* p = get<T>(&v);
>    T& r = get<T>(v);
>
> instead of this:
>
>    T* p = variant_cast<T*>(&v);
>    T& r = variant_cast<T&>(v);
[snip]
> Maybe there are problems to these approaches -- I've hardly  
> investigated them.
> But I would be interested to know if there is a good reason for the  
> syntaxes for
> dynamic_cast, any_cast, and get to vary so much.

I think the answer is more archeological than technical :)

We can't overload dynamic_cast, which is too bad: that's the right  
name for this sort of thing. If only someone would go ahead and write  
the C++0x proposal to add overloading of the cast operators, smart  
pointers, any, variant, python::object, and many other classes would  
get much more intuitive syntax. But I digress...

any_cast() came around first. It probably should have had the syntax  
"T* p = any_cast<T*>(&a)" that you suggest. Kevlin may even have  
thought of that, but any and any_cast came into Boost back in the  
dark days when MSVC 6.0 dominated the C++ world. That means no  
remove_reference, no remove_pointer; I'm speculating here, but it  
wouldn't be the first time that the lack of suitable compilers forced  
Boost libraries into a suboptimal syntax. Only Kevlin would know for  
sure.

get() came along later, when we realized that multiple types,  
including variant, python objects, and other dynamically-typed  
entities, have the same "extract as the static type T" semantics. It  
was built mainly to mimic any_cast, but work with a variety of types,  
not all of which were named "variant" (so, no "variant_cast").

In an ideal world, I think we'd end up using "dynamic_cast" for  
everything. Instead, we use the name "get", and I think the  
"any_cast<T>(&a)" is probably a holdover from the dark days that we  
should change.

        Doug