From: MB (mb2act_at_[hidden])
Date: 2005-12-03 21:33:49
Eric Niebler wrote:
> MB wrote:
>>Eric Niebler wrote:
>>>Now, the recommended procedure for customizing
>>>BOOST_FOREACH is to define a function:
>>>inline boost::mpl::true_ *
>>> return 0;
>>>This can go in an associated namespace of your_type /or/ for maximum
>>>portability in the global namespace. (boost::foreach::tag is a typedef
>>>for an enum_at_global_scope_with_a_really_long_ugly_name.) This allows
>>>for a very simple implementation of BOOST_FOREACH that even VC6 seems
>>I tried boost::foreach::tag-version under VC++7.1.
>> warning C4100: 't' : unreferenced formal parameter
> I'll fix that.
>>Anyway I think it is ok for "me".
>>But I think about the case that my ranges which are customized as 'cheap_copy'
>>are derived from anyone else.
>>Won't the same problem as 'iterator_range<>*' come?
> Sorry, I don't understand the question. Are you saying you think it's
> wrong to assume that types derived from iterator_range are cheap to copy?
But I think about the case that my ranges which are customized as 'cheap_copy'
becomes base classes of anyone else.
>But I think about the case that my ranges which are customized as 'cheap_copy'
>are derived from anyone else.
English is as difficult as C++ :-)
>>Now that 'has_cheap_copy' becomes one of the requirements of Range-concepts, doesn't it?
> No. Boost.Range defines the range concept, and FOREACH works correctly
> with any type that satisfies that concept. has_cheap_copy is nothing
> more than an optimization hint to the BOOST_FOREACH macro. It is not
> required and it doesn't change the meaning of anything.
But the hint expands to the derived classes and
the derived classes "must" customize 'has_cheap_copy'.
>>In general, do we have the safe way of telling whether or not a type
>>is cheap to copy without metafunctions?
Is it portability-issue the reason why you avoid metafunctions?
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