RE: [boost] boost::any: explicit constructor?

From: Bjorn.Karlsson_at_[hidden]
Date: 2004-02-25 08:53:45

> From: Vladimir Prus [mailto:ghost_at_[hidden]]
> Hi,
> I just had a couple of bugs caused by the fact that 
> constructor of boost::any 
> is not explicit. For example:
> 
>    void do_something(boost::any& v) { ... }
> 
>    class variable {
>     /////
> 	boost::any& value() {}
>     };
> 
>     variable v;
>     do_something(v);
> 
> The intention was to call do_somethin on 'v.value()' but 
> non-explicit ctor is 
> happy to accept 'variable', after which 'do_something' breaks.
> 
> Anybody has an opinion? Is non-explicit ctor really necessary?

(Note that the example above avoids the issue of implicit conversion (it
won't compile), because it takes the parameter by non-const reference. With
the current implementation, this is a good trick to use.)

I agree that the constructor should be made explicit. I seem to recall that
we discussed this on the list about a year ago, and the main objection
against an explicit constructor was that the behavior across different
compilers wasn't consistent (some compilers didn't like explicit template
constructors at all). But now, a year later, perhaps it's time to go ahead
and make the change? It's unfortunate that it's a change that will break
code; perhaps some legacy mode needs to be provided.

Anyway, there have been several requests for this and thus far no strong
opinions against it.

Bjorn