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

From: Edward Diener (eddielee_at_[hidden])
Date: 2004-02-25 06:55:49

Vladimir Prus wrote:
> 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.

This is a programming error, not an error in boost::any.

>
> Anybody has an opinion? Is non-explicit ctor really necessary?

I don't think it should be explicit. After all, the idea is that one can
pass any type to boost::any. Forcing an explicit constructor defeats the
ease of use which boost::any represents among other things. If the
programmer passes a variable of a type which he didn't mean to pass, that is
his problem. I don't think you should force him to telegraph that he is
creating a boost::any variable by forcing an explicit constructor. C++ has
always existed based on the presumption that the programmer knows what he is
doing. While I am not against syntactic sugar which make it easier to use
C++ constructs, I am against making a constructor explicit just to keep
programmers from making errors.