Re: [boost] visit_each

From: Larry Evans (cppljevans_at_[hidden])
Date: 2006-01-25 09:40:50

On 01/24/2006 04:43 PM, David Abrahams wrote:
 > Larry Evans <cppljevans_at_[hidden]> writes:
[snip]
 >>in http://www.boost.org/doc/html/visit_each.html, i.e. the user must
 >>explicitly call v(x) for each subobject, x, of T.
 >
 >
 > Correct.
 >
 >
 >>However, the phrase, 'for each subobject' in visit_each.html
 >>is not really accurate since there is the subobject:
 >>
 >>     minstd_rand& tracked_bridge::rand_gen;
 >>
 >>in tracking_bridge.
 >
 >
 > Technically speaking, that code is wrong and the phrase is accurate.
 >
 >

The original purpose of the fields_visitor library was to visit the
smart pointers in an object (it's used in the current policy_ptr
library in sandbox).  Thus "correct" visit_each would not be a good
substitute since "correct" visit_each, which visited all subobjects
instead of just the smart pointers, could involve mostly useless
visiting (e.g. if the object contained no smart pointers).

Practically, the user, as done in random_signal_system.cpp, could
write technically wrong code to just visit the smart_pointers;
however, as mentioned in my previous post, this might not be easy
since smart pointers could be in superclasses or data members which
the user is not that familiar with.

[snip]
 >>Anyway, the essential difference
 >>is that with visit_each, the user has to know which subobjects to
 >>visit and has to explicitly code the visit.  OTOH, with fields_visitor,
 >>the user can select
[snip]
 >>the object by "declaring" it visitable by wrapping
 >>it in something derived from registrar_heirarchy.
 >
 >
 > I don't see how wrapping the object is going to tell your registrar
 > where the subobjects are.

The fields_visitor.ug.zip uploaded 24.01.2006 04:29 contains
html/participants.html which partly explains this in item 2 in the
<formalpara> titled "prevents thread safety".  Briefly, for each type,
T, there's a singleton, descriptor_builder, which creates an
instance, SomeT, of T after setting a global variable, pointer_T, to
&SomeT. This global variable is a flag to all registrars to calculate
the offset of their registrant (a field in T) with respect to
pointer_T and store that in a singleton fields_descriptor for T.
After SomeT is built, it's destroyed and pointer_T is reset.

 >
 >
 >>The visit_each method is less intrusive (no need to modify the class
 >>to be visited), but more work.
 >>
[snip]
 >
 > A lot of code, almost no comments.  It's not very clear what it's
 > supposed to be doing or illustrating.
 >

OK.  I'll work on it.  Thanks.

 >
 >>The other example program, simple_*.cpp, shows how to do it just
 >>using fields_visitor.
 >
 >
 > where is that?  You must mean simple_record_field_traversal_test.cpp

Yes.  I thought simple_*.cpp would suggest all files starting with
simple_ and ending with .cpp.  Since there's only one in the
directory, I thought it would be clear.  Sorry.

 >
 > Also not very clear to me.  The use of "arg<1>" instead of "_1" is not
 > very idiomatic, which --though it's minor-- doesn't help, IMO.
 >

I'll change it.  Thanks.