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: Jeremiah Willcock (jewillco_at_[hidden])
Date: 2010-11-30 11:41:13

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.

-- Jeremiah Willcock