[Boost-users] Re: Using MPL to writeis_derived_from_std_basic_ostream

2 Sep 2004

      "David Abrahams"  wrote in message
news:un008whj3.fsf@boost-consulting.com...
...
"Jonathan Turkanis"  writes:
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"Angus Leeming"  wrote in message
news:ch7h3d$466$1@sea.gmane.org...
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I'm trying to ascertain whether a given class is, or is derived from,
std::basic_ostream, but I've ground to a halt.
...
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Note that with the current cvs it should not be necessary to write
mpl::and_< mpl::bool_, ...
You can simply say
mpl::and_< has_char_type<U>, ...
It's hard for me to understand why you would need has_xxx for this
purpose, or how you would use it.
Well, you could look at the attachment .... ;-)

An arbitrary type T is equal to or derived from a specialization of
std::basic_ostream iff (almost):

    (i) it has a member type char_type;
    (ii) it has a member type traits_type; and
    (iii) either:
        (a) it is the same as basic_ostream, OR
        (b) it is derived from basic_ostream

This asumes that T doesn't have different member types char_type or traits_type
which hide those inherited from std::basic_ostream.

There's and easier way to do this (featured in the first attachment), but it
get's in trouble with private derivation.

If you have a simpler or more correct way, please let me know.

Jonathan