[boost] Re: [any] ostream inserter

From: Tom Widmer (tom_usenet_at_[hidden])
Date: 2005-01-26 13:18:49

Sérgio Vale e Pace wrote:
>  Jonathan Turkanis wrote:
>>Two comments, though:
>>
>>1. Your code breaks boost::any for use with non-OutputStreamable types.
> 
> 
> that´s the odd part it didn´t. as long I don´t use the insert operator
> i works fine and I don´t understant why it don´t break. see:
> 
> class A {};
> 
> void foo() {
>   A a; int b=0; any x;
>   x = b;
>   cout << x;
>   b = any_cast<int>(x);
> 
>   x = a;
>   // cout << x; // as long as this line is commented everything works fine
>   a = any_cast<A>(x);
> }
> 
> on the other hand if I do invoke the insert operator (<<) on a
> non-OutputStreamable type I get a stack overflow also I don´t
> understand why.

Anything is convertible to boost::any, and since you have an operator<< 
for boost::any, it will catch any object that doesn't have a better 
matched operator<< (i.e. any one that doesn't require a user defined 
conversion). If this didn't happen, you'd just get a compiler error when 
you tried to create an any<A>, since creation of that requires 
instantiation of all virtual functions of holder<A>, including the 
ill-formed print member (e.g. no overload matches the call out<<held).

Dispatching to some kind of default call (which might just set failbit 
on the stream, or you could make it configurable) is probably the best 
way to handle this.

Tom