[boost] Re: class static constants, mpl, and linker problems

From: David Abrahams (dave_at_[hidden])
Date: 2004-01-27 12:06:56

Daniel Krügler <dsp_at_[hidden]> writes:

> Hello David,
>
> David Abrahams schrieb:
>> Matthias Troyer <troyer_at_[hidden]> writes:
>> 
>>>Dear Daniel,
>>>
>>>If you use BOOST_STATIC_CONSTANT you need to additionally provide a
>>>definition on conforming compilers. The correct usage is like in:
>>>
>>>// declaration
>>>class A {
>>>   BOOST_STATIC_CONSTANT(int , x=1);
>>>};
>>>
>>>....
>>>
>>>//definition
>>>#ifndef BOOST_NO_INCLASS_MEMBER_INITIALIZATION
>>>int A::x;
>>>#endif
>>>
>>>
>>>I hope that clarifies this issue,
>> Another approach is to use mpl::int_<n> or integral_c<T,n> which
>> supply the definitions themselves.
>> 
>
> So you mean, if I use integral_c<T,n>::value, the definition of value
> is provided by boost?

Yes.

> According to my assumptions (which might be wrong, as I said) here
> applies the same problem, because integral_c<T,n> also contains a
> BOOST_STATIC_CONSTANT and thus a possibly addressable object, but I am
> willing to be corrected, of course.

You stand corrected.  If you don't believe me: use the source, Luke.

> To make my request more clear: I am **not** speaking of a programming
> style which **should** take advantage of using integral_c<T,n> instead
> of an addressable (constant) object, I am speaking of daily programming
> practice. Since e.g. integral_c<T,n>::value is not a boost-internal
> "you-should-not-use-or-you-are-fired", 

I don't know what any of that means, sorry.

> it seems legal code to write
> something like
>
> const long myvalue = foo(); // run-time determined value
> const long max_value = std::max(myvalue,
>    boost::mpl::integral_c<long,15>::value); // Combine compile-time and
>                                             // run-time information here
>
> without knowing too much about the internals of boost::mpl (besides the
> fact, that it might be necessary to additionally add some further mpl
> source files into my project).

It works.  Try it or use the source.

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