Re: [Boost-users] [BGL] Default-constructibility of vertices

Subject: Re: [Boost-users] [BGL] Default-constructibility of vertices
From: Jeremiah Willcock (jewillco_at_[hidden])
Date: 2011-12-13 00:04:14

On Mon, 12 Dec 2011, Kelvin Chung wrote:

> Suppose I have a class hierarchy, like so:
>
> struct Foo {
> 	virtual Output doSomething(const Input& in);
> };
>
> class CompositeFoo {
> 	std::vector<Foo*> fooList;	// Use your favorite smart pointer 
> standin for Foo* here
> public:
> 	Output doSomething(const Input& in);
> };
>
> Now, suppose I have a functor:
>
> class Functor {
> 	Foo* foo;
> 	Input input;	// assume Input is copyable
> public:
> 	Output operator()() { foo->doSomething(input); }
> };
>
> Now, the structure of doSomething() calls leads to a DAG structure with 
> Functor as the vertices.  What I want to do is create a graph with BGL that 
> shows that structure, before topologically sorting it and doing other stuff 
> with it.  However, it would appear that the vertices of the graph (which I 
> would assume to be of type boost::adjacency_list<boost::vecS, boost::vecS, 
> boost::directedS, Functor>) must be default-constructible, which Functor is 
> clearly not.  Is there a nice workaround to this without changing the vertex 
> type to Functor* (again, use your favorite smart pointer standin for Functor* 
> here)?

Have you tried using the two-parameter version of add_vertex() (with 
vertex property value as the first parameter)?  I don't know if that would 
work, but it might.  The requirements for property types (at least for 
old-style properties) say that they need to be default constructible, 
though, so you might need to use a pointer.  Is the assumption on Input in 
your example that it isn't default constructible?  Otherwise, Functor 
should be as well.

-- Jeremiah Willcock