Re: [Boost-users] [BGL] Question on dijkstra-example.cpp

Subject: Re: [Boost-users] [BGL] Question on dijkstra-example.cpp
From: Jeremiah Willcock (jewillco_at_[hidden])
Date: 2010-11-11 08:12:26

On Thu, 11 Nov 2010, Cedric Laczny wrote:

> 
> Hello,
> 
> could you please explain what this code in the dijkstra-example
> 
> (http://www.boost.org/doc/libs/1_44_0/libs/graph/example/dijkstra-example.cpp) exactly does?
> 
> predecessor_map(&p[0]).distance_map(&d[0])
> 
> I understand the non-named-parameter version, but the combination of predecessor_map with distance_map is not so
> obvious to me. What does the "." do there?
> 
> Do I get it right, that the predecessor_map or distance_map, respectively, automatically wrap the vectors p and d
> into iterator_property_maps?

The . there is separating two named parameters; the BGL named parameter 
mechanism uses . to separate parameters instead of , to avoid needing to 
have multiple overloads of each function.  On your second question, there 
is infrastructure in BGL for pointers to be automatically treated as 
property maps, and so &p[0] gets a pointer that will go through this 
mechanism.  I do not recommend writing your own code that way, though; use 
iterator_property_map or shared_array_property_map instead since they 
express your intentions better and work with more graph types.  It is easy 
to get very subtle bugs if you use pointers as property maps in code that 
doesn't know the exact graph type it will be using.

-- Jeremiah Willcock