[boost] conversions and SNIFAE (was: implicit_cast has been removed, but this is sad)

From: Jens Theisen (jens.theisen_at_[hidden])
Date: 2005-03-21 15:44:55

Thorsten Ottosen wrote:
> ctor() : v( list_of(1)(2).to_container(v) )
> { }

What are you proposing? You don't want to write to v before it's 
construction, do you?

I see that this is somewhat anoying. Even with implicit_cast, because 
then you have to give the containers type in the initialisation again.

I have read somewhere that template default arguments for functions 
might come into the language at some point in the future. In this case, 
one could write:

template< typename Container,
           typename Unused = typename Container::iterator >
operator Container() const { ... }

This would then be a subsitution failure for all the single args a 
std::vector ctor takes untemplated (I may be mistaken), and thus giving 
no candidate for a ctor call.

Unfortunately, this theory could only be tested on a compiler that 
already has this language extension. Does anyone know if there is one?

Any other thoughts?

Jens