Re: [boost] bind/lambda/phoenix and result_of

From: David Abrahams (dave_at_[hidden])
Date: 2006-05-10 06:15:47

"Andy Little" <andy_at_[hidden]> writes:

>>>> I can't ask my library's users to register types.
>>>
>>> Why not?  We all want decltype and auto in the language dont we?
>>> BOOST_TYPEOF is the closest we have. Show your users how to use it
>>> while we wait for language support.
>>
>> Because I want to serve my users, not force them to learn about an
>> interim solution that they won't need when the real feature comes out.
>
> Not long to wait then, assuming upgrading is viable.... ;-)

Should I assume you're just baiting me, or what?  How soon do you
think everyone will be using a C++0x compiler supporting
auto/decltype?

> Meanwhile I have found Boost.Typeof really useful now.

I didn't say it wasn't useful; only that it's inappropriate for my
purposes.

>> Furthermore, it's _my_ library that needs result_of, so basically I
>> would be telling users, "if you get a cryptic error message that
>> looks like ___________, find some of the types in it and register
>> them with this cryptic macro."
>
> The error messages re lack of registration are not that bad IMO. 

Bad for you and bad for a user are two totally distinct things.

> The only problems I had was deciding what types to register when
> using mpl::transform IIRC.
>
>> I'd rather that people enjoy my library than have them vent
>> frustration at me about how unusable Boost is.
>
> The last time I tried to use lambda I seem to remember I had to do a
> lot more than just register my types with one cryptic macro.  I had
> to map my own whacky type deduction scheme to lambdas whacky type
> deduction scheme. See below --^-->

The difference is that IIUC lambda makes some reasonable assumptions
when it doesn't know the answer -- assumptions that are appropriate to
my use case, like that subtracting two values of one type yields the
same type -- whereas typeof does not, and cannot.

And anyway, as I've been saying, I'm *not talking about deducing the
return type of an invoked lambda expression*.  I'm just trying to
capture the function type.  Maybe Boost.Typeof is actually enough for
that job; I don't know.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com