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:
>>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.
>>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;
> 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.
>>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
>>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.
> 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
> 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.
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