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From: Edward Diener (eddielee_at_[hidden])
Date: 2002-11-14 12:49:21

"Anthony Williams" <anthony.williamsNOSPAM_at_[hidden]> wrote in
message news:15827.36666.343000.937688_at_gargle.gargle.HOWL...
> Edward Diener writes:
> > "Anthony Williams" <anthony.williamsNOSPAM_at_[hidden]> wrote in
> > message news:15826.2782.569000.619636_at_gargle.gargle.HOWL...
> > > Edward Diener writes:
> > > > Finally I would like a non-partisan answer from you to my next
> > and
> > > > I am perfectly willing to be wrong and accept the fact that my
lack of
> > > > understanding of "concept" based explanations are at fault. Do you
> > that
> > > > the answers to my 2 questions are readily apparent to most
> > C++
> > > > programmers from reading the property map docs ?
> > >
> > > As Jan Langer pointed out, there are specific pages for each of the
> > property
> > > map categories. If you missed these you will have difficulty piecing
> > > everything together.
> >
> > No, I saw them from the beginning. I just didn't ( and still don't )
> > understand how they fit in with some sort of global get(), put(), and
> > operator[] template functions.
> operator[] has to be a member function if it exists at all, unless the
> built-in operator[] is sufficient.
> get() and put() may or may not be global, and may or may not be
> templates. They must be implemented for each property map so that the
> expressions get(property_map,key_value) and
> put(property_map,key_value,new_value) compile and work OK. This means that
> they may be global, or they may be in an associated namespace of the
> map or key value types, and found by ADL.
> The parameter types may or may not exactly match the types of the property
> etc., provided that the functions can be found, and the supplied arguments
> be converted to the required parameter types --- e.g. if the property map
> actually a pointer to X, then the real parameter for get() could be a
> to Y, if Y is an unambiguous public base of X --- and provided that
> resolution finds the correct function. I would expect in the common case,
> the get() and put() functions are supplied in the same namespace as a
> map type, and the parameter types exactly match the types of the property
> and key value, give or take a const&, but this is not necessarily the
> The presence of a template get() will probably be an impediment if the
> parameters are not an exact match for the supplied types, since deduced
> template parameters have a nasty tendency to exactly match the supplied
> arguments, and may therefore be picked in preference, or cause an

Then the get(), put(), and operator[] values are purely user defined and
there are no common implementations for these which automatically work with
any property map. Everything having to do with property maps are just a
generalized concept of how this should work so that a given property map has
effective get(), put() and operator[] functions which an end user can use to
associate a value with a property ( key ). Whatever implementations exist in
the header files are just specific types of property maps which have already
been developed based on the different categories.

Is this a correct understanding of the property map concept ? If so, it is
something I completely missed from the docs, but which others were able to
understand immediately.

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