[boost] Re: call_traits

From: David Abrahams (dave_at_[hidden])
Date: 2004-07-05 23:30:24

"Edward Diener" <eddielee_at_[hidden]> writes:

> David Abrahams wrote:

>> But not all types, right?  What about pointers and references to
>> functions?
>
> Good question but I would assume it is the same as pointers and references
> to built-in types.

One shouldn't have to assume.

>>  What about POD classes the same size as 2 ints?
>
> It is a user-defined type. I would assume it follows the rule for
> user-defined type since nothing was said in the doc about passing small
> user-defined types by value.

One shouldn't have to assume.

>> Not well enough for me, I'm afraid.  What does "can not be used"
>> mean?  Will it generate an error?
>
> I agree with this criticism of it.

Why is it unacceptable to assume some specifics here, but not in the
two other cases I mentioned?

>> Note also: "If T is a small built in type or a pointer, then
>> param_type is defined as T const, instead of T const&"
>>
>> What is "small"?
>
> I agree with this criticism also. I had assumed it referred to all built-in
> types, and ignored the "small", but this should definitely be explained.
>
>>
>>> It gives the values which call_traits will generate for any user
>>> defined type and various permutations of a basic type.
>>>
>>>> It's even less clear what fallback the library uses when a full
>>>> implementation of call_traits would require partial specialization
>>>> that isn't available.

I didn't just write this (it's from several posts ago); there's no
need to repeat your earlier repetition of what's on the web page ;-)

> "Note that for compilers that do not support either partial specialization
> or member templates, no benefit will occur from using call_traits:
> the

<snip>

>>>> It's also unclear to me what purpose is served by the "reference"
>>>> and "const_reference" members.
>>>
>>> In the beginning of the doc it specifies that both are for returning
>>> values of reference and const reference respectively, while avoiding
>>> the reference to reference problem.
>>
>> boost::add_reference<T>::type
>> boost::add_reference<boost::add_const<T>::type>::type
>
> Too cryptic for me. Please explain.

These constructs do what you just described above and in fact they
work even on compilers without partial specialization.

>>> I am not defending call_traits itself, which I have not used but
>>> which I am thinking about using as part of my calling and returning
>>> style, but rather the doc which seems fairly clear to me about what
>>> it is and does from a user perspective.
>>
>> It's still failing this potential user.
>
> I think the areas you pointed out in the doc should be explained
> better but I still think the idea is a good one.

The basic idea might be OK but I think the execution is now very
out-of-date.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com