Re: [lambda] Segmentation fault in simple lambda program

[Jason Merrill <jason at redhat dot com>]

On 08/04/2009 09:35 AM, John Freeman wrote:

> In my opinion, lambdas are not intended as just a shortcut to writing a
> function object class. This is why our proposal did not require that
> lambdas be implemented as classes; it is simply one implementation.
> (Awaiting word to see if this is still the case in the working draft of
> the standard; it may not be, but that doesn't change my opinion :)

From N2859: "The type of the lambda-expression (which is also the type 
of the closure object) is a unique, unnamed non-union class type—called 
the closure type...."

IMO, it's better to define new functionality in terms of already 
familiar features rather than have to specify everything about how a new 
feature interacts with the rest of the language.

> Thus,
> users should never see the class name. In warnings and errors, a lambda
> should be treated like an anonymous class, swapping its
> compiler-generated name with a placeholder like <lambda function>
> (similar to <anonymous class>).

Yep.

> > > Since templates work so differently from normal functions, I'm a little
> > > uncomfortable with the idea of templates that don't involve any template
> > > syntax, just the use of auto in the parameter list. But I'm open to
> > > giving it a try, at least in the lambda context. Maybe outside of
> > > lambda it could be used with a small template introducer...
>
> In response to Jason's concern here, a template implementation is not
> required, just convenient. Hopefully you won't think of it as "templates
> without template syntax", but "deduction using the existing template
> mechanism".

As above, suggesting that it doesn't have to be a template suggests that 
there's a lot more specification left to do, such as how do you deal 
with expressions whose type you don't know yet?  If you allow non-lambda 
uses, how do they fit into overload resolution?  If you just say it's a 
template, we already know how they work.

Jason