Re: Re: [BGL] Property map question

Okay - I think I understand this. Since posting I've re-read the property map documentation online and in the book and am still confused about several points (forgive me ignorance - I'm learning a lot of new concepts (in the canonical sense) at once). Will you help me with these follow-up questions please: Assertions: (please validate) 1. a property map maps some type (an enum or struct tag) to some other type. 2. a property map declaration may contain nested property map declarations but this does not reflect a hierarchy - it's just a template trick. Effectively the property maps are peers. 3. graph containers like adjacency list will internally create a new value_type object upon insertion of a vertex (in the case of a vertex property map) and an edge (in the case of an edge property map). Assuming the above assertions are correct, I'm still confused about several points: 1. a boost::get call to return either a property_map::type or property_map::const_type by value? I think yes. The question is does return by value also copy all the value_type's? I'm guessing not because this would be a huge performance hit. Just the property map itself is copied. Correct? 2. In the case of my algorithm visitor, I am passed a const reference to a graph that will result in a boost::get returning a const_type a property map. I can understand that changing the value_type's of an arbitrary property map could potentially screw up an algorithm (if the algorithm is using the specific property that I am trying to write). Is this why the graph passed to the visitor is const? In the case where I need to write an internal property map that is not being used by the specific algorithm (in the case of my dfs search, dfs is using the vertex color and I want to write another, distinct internal property map that I have registered with my graph class), I should pass a reference to the specific map into the constructor of the visitor? Thank you for your help Jeremy. - Regards Chris "Jeremy Siek" wrote in message news:Pine.GSO.4.44.0209152139410.629-100000@zaphod.osl.iu.edu...

Hi Chris,

On Sun, 15 Sep 2002, Chris Russell wrote: cdr> What determines when boost::get(Graph, vertex_myproperty_t) returns a cdr> boost::property_map::type vs. a cdr> ::const_type?

The constness of the graph object that is passed to get().

cdr> vertex_myproperty_map_t MyMap = get(vertex_myproperty, Graph); cdr> cdr> ^--- this works as I expected. However, if I try to get a copy of my cdr> property map this way inside a dfs visitor method, my compiler tells cdr> me that cdr> there's no suitable conversion from property_map vertex_myproperty_t>::const_type to property_map vertex_myproperty_t>::type (IA32 6.0 Intel compiler).

You've got a const graph object inside the visitor, so you need to use the const property map.

Cheers, Jeremy

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