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Subject: Re: [boost] [xint] Sixth release, preliminary review again please
From: Chad Nelson (chad.thecomfychair_at_[hidden])
Date: 2010-06-20 12:18:16

Hash: SHA1

> On 06/18/2010 10:22 PM, David Abrahams wrote:

>>> I think you might want to clarify that, since parameter has a
>>> general meaning having nothing to do with the library.
>> The general meaning is the one I had in mind. :-) fixedlength is
>> one of the structs that I use for Boost.Parameter; it takes a
>> (small-p) parameter of a length, in bits. The other structs
>> previously took a Boolean parameter, but now they don't -- their
>> mere existence, or lack thereof, is enough to tell the
>> Boost.Parameter-enabled classes what to do.
> So you're using a deduced parameter interface, I presume.

Yes. I couldn't figure out how to directly pass value, rather than type,
parameters to class templates... I can't tell if I missed something or
if it's just not possible (I suspect the latter, but read on). Since I
had to create a type for everything anyway, and there aren't any
required parameters, the deduced interface made perfect sense.

> It was the first time I'd used Boost.Parameter on a class template.
> I wasn't able to find any examples of how to use it for that, other
> than the sole one in the documentation, so the learning curve was
> pretty steep. That's why the aforementioned "other structs" took a
> parameter even though they really didn't need one, it was my first
> attempt. If you ever feel the urge to update the documentation with
> more examples, please feel free to use XInt's integer_t class, I'm
> sure the next guy who tries to learn it would appreciate it.
>> If you could be more specific about what you found missing from
>> the docs, that might help.

In general, the biggest problem I found was that there was only the one
example of how to use it on class templates. I couldn't tell which of
the design decision you'd made on it were universal, and which you might
have made differently under different circumstances. Using
Boost.Parameter on class templates seems very different from using it on
anything else, so my previous experiences with it didn't help as much as
I expected in figuring out the best way to use it there. With more than
one example, I could have gotten a better idea of what was possible, and
where I could best use those abilities.

I also had some trouble figuring out how to pass values (again, rather
than types) to the class template in a way that MSVC would accept.
Writing a class template that takes the value is easy enough for any
reasonably competent C++ developer to figure out, but finding a way to
actually *use* that value (as, for instance, a parameter to a
non-Boost.Parameter-enabled class template) in MSVC was maddening, until
I stumbled across integral_c. MSVC claims that such parameters aren't
compile-time constants; GCC doesn't have any problem with them. That's a
problem that many people won't run across, but I'd think that it's
common enough to frustrate a good portion of people trying to use

(It did occur to me to try creating a class with a non-explicit
conversion constructor for the type that I wanted to pass in, but I was
put off by the limitations described in type_traits' is_convertible
function -- that it doesn't work with C++ Builder 5 or earlier, or
Metrowerks 7 or earlier. I don't know how commonly used those are, and I
didn't want to limit the people who can use XInt, so I didn't attempt it.)

I'm not criticizing the existing Boost.Parameter documentation. It was
thorough, and sufficient to show me what I needed to do. I just would
have felt more comfortable with it, and understood it more quickly, if
I'd had a second example to compare to the first.
- --
Chad Nelson
Oak Circle Software, Inc.
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