On 01/24/2006 04:43 PM, David Abrahams wrote:
Larry Evans <cppljevans@cox-internet.com> 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.