Why is the visitor parameter passed by value rather than reference in the various BGL algorithms?
One of the usage scenarios that we wanted to support with the algorithms was creating visitor objects on the fly, within the argument list of the call to the graph algorithm. In this situation, the visitor object is a temporary object. Now there is a truly unfortunate rule in the C++ standard that says a temporary cannot be bound to a non-const reference parameter. So we had to decide whether we wanted to support this kind of usage and call-by-value, or not and call-by-reference. We chose call-by-value, following in the footsteps of the STL (which passes functors by value). The disadvantage of this decision is that if your visitor contains state and changes that state during the algorithm, the change will be made to a copy of the visitor object, not the visitor object passed in. Therefore you may want the visitor to hold this state by pointer or reference. • Why does the BGL interface use friend functions (or free functions) instead of member functions? For the most part, the differenc
