Re: [Boost-Users] Const Graph Property Maps

On Friday 21 March 2003 09:55 pm, dmatt001 wrote:

Hi Guys,

I have three interesting questions in regard to graph properties. Please assume Boost 1_29_0. I'm a little slow in upgrading ... ;-)

1. If I define:

MyGraph_t const myGraph;

Why cannot I get a property map to access my property:

typedef boost::property_map::type MyPropMap_t; MyPropMap_t myPropMap = get(MyPropTag_t(), myGraph);

You want: typedef boost::property_map::const_type MyPropMap_t;

2. Why not use a copy constructor if it exists when copying graph elements? Is the existence of a copy constructor a hard-yard to verfiy with MPL?

I don't know the rationale behind the decision, but I do know that it is _very_ hard to determine if there is a copy constructor in C++ because they are implicitly generated. The question isn't "is there a copy constructor" but "is there an _accessible_ copy constructor" and, unfortunately, we can't test accessibility in a metaprogram :(.

3. I noticed that remove_edge(...) complains in regard to a missing default constructor of a property object. Don't get me wrong; this happened during an editing mishap and I know that we need regular types here. Why is this required if you are removing an element?

The answer is probably linked to the answer to #2: if copy constructors aren't used, then edges (along with their internal properties) are copied by defalut-constructing the target then calling a function to perform the copy. Doug