Re: [Boost-users] [BGL] Searching for a convenient way to combine predicates for filtered_graph

Subject: Re: [Boost-users] [BGL] Searching for a convenient way to combine predicates for filtered_graph
From: Cedric Laczny (cedric.laczny_at_[hidden])
Date: 2010-11-30 11:57:22

On Tuesday, 30. November 2010 17:41:13 Jeremiah Willcock wrote:
> On Tue, 30 Nov 2010, Cedric Laczny wrote:
> > On Tuesday, 30. November 2010 16:57:17 Jeremiah Willcock wrote:
> >> On Mon, 29 Nov 2010, Cedric Laczny wrote:
> >>> Hi,
> >>> 
> >>> I was wondering if there is a convenient (and to some extent intuitive)
> >>> way to combine several predicates used in filtering a graph?
> >>> The idea is to have some predicates defined and arbitrarily combine
> >>> them so that the filtered_graph will check for compliance of each
> >>> individual predicate and either make this vertex/edge visible or not.
> >>> Of course, the predicates should be specific to vertices or edges,
> >>> respectively, when being combined.
> >>> When using std::vector or such, they would need to be all of the same
> >>> type which does not seem very nice/feasible IMHO.
> >>> Also defining a "big" predicate having a multitude of "smaller"
> >>> predicates as members is not really an option as this is very
> >>> restricted.
> >>> Something like
> >>> big_predicate = predicate1 || predicate2 || predicate3
> >>> (syntax should just illustrate the idea) maybe?
> >> 
> >> If you don't want to use Boost.Lambda, Boost.Bind also has some
> >> operators overloaded
> >> (<URL:http://www.boost.org/doc/libs/1_45_0/libs/bind/bind.html#operators
> >> >) to provide the syntax you wanted.
> > 
> > Thanks for the hint. Unfortunately, I'm still hanging at the point where
> > the type of the specified predicate(s) comes into play.
> > Let's look at an example:
> > 
> > struct VPred1{
> > 
> > 	VPred1() {};
> > 	// etc....
> > 	template< V >
> > 	bool operator( const V& v)
> > 	{return true;}
> > 
> > }
> > struct Vpred2{
> > // s. Vpred1 but operator() returns false
> > }
> > 
> > Vpred1 pred1;
> > Vpred2 pred2;
> > typedef adjacency_list<> Graph;
> > Graph g;
> > // fill graph g
> > filtered_graph< Graph, keep_all, ??? > fg( g, keep_all(), ???);
> > 
> > In the example above I would like to provide both vpred-structs to the
> > filtered_graph like so:
> > filtered_graph< Graph, keep_all, ??? > fg( g, keep_all(), pred1 &&
> > pred2);
> > 
> > This is not supposed to work out-of-the-box, it should just demonstrate
> > the idea a bit more into detail.
> > 
> > Concerning the "???" in the template-specification, I have no idea how
> > this should be specifed. All the examples I have found so far, either
> > for phoenix or bind, are direct applications and do not involve the
> > specification of templates. Therefore I have nowhere to start learning
> > how to do this.
> 
> Yes, that's the hard part.  If you have C++0x (or are willing to use
> compiler extensions), you can use decltype/auto or typeof to infer the
> return type of the operator.  Another technique (that would make your code
> messy) is to split the function after the construction of the predicate
> into a separate function template; that template can then take the
> predicate as a parameter and its type as a template parameter.  You could
> also try asking the bind developers if the return types of the operators
> are documented, or if they would fix and document particular types for
> them.
>

Ok, I will do that then.
But let's say, the return types were known, am I right that a solution to 
combine the predicates would have to look like this?

filtered_graph<> fg(g, keep_all(), bind<bool>(pred1, _1) && bind<bool>(pred2, 
_1));

Thank you so far.

> -- Jeremiah Willcock
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Best,

Cedric