[boost] Re: BOOST_FOREACH and break

From: Eric Niebler (eric_at_[hidden])
Date: 2004-07-23 15:02:53

Rozental, Gennadiy wrote:
> 
> BTW, while we are on topic. Your implementation uses code like this
> 
> struct any_t {...};
> template<typename T> struct static_any_t : any_t {...}
> 
> template<typename T> static_any_t<T> foo(...);
> 
> If( any_t const& ref = foo() ) {
>   // here you are using ref
> }
> 
> Is it indeed allowed in standard and ref is valid reference to temp?
> 
> Gennadiy.

Yup, that's perfectly OK. Binding a temporary to a const reference 
extends the lifetime of the temporary. And if it's a const reference to 
a base, the whole derived object sticks around, not just the base 
sub-object (no slicing). In addition, if the derived object has a 
non-trivial destructor, it will get called when the const reference to 
base goes out of scope /even if base has a non-virtual destructor/. It's 
like magic. But not really. :)

The real magic in BOOST_FOREACH is how to recover the lost type 
information without virtuals, dynamic casts or RTTI. And that's with 
sneaky template tricks and static_cast.

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com