Re: [Boost-users] [boost] Create Graph in Function

29 Oct 2009


      On Wed, 28 Oct 2009, James wrote:
...
Jeremiah Willcock wrote:
...
On Wed, 28 Oct 2009, Sandeep Gupta wrote:
...
On Wed, Oct 28, 2009 at 6:12 PM, James Thunes  wrote:
      Perhaps an easy answer, but I am working on adding a boost graph to 
an existing code. I would like to create the graph in
      a function (not main) and then read/modify it in a separate 
function. I am unsure as to what I need to pass between the
      functions to allow this to work.
In other words, I am able to correctly create the graph in a 
function and modify as necessary. This function will be
      called many times as the code runs and it is inefficient to recreate 
the graph every time. I want to create the graph once
      when the code is run and pass the created graph to the function.
Due to the structure of the code (mostly Fortran), I can not define 
the graph in the main function of the code.
My graph is defined as:
      adjacency_list Graph;
Thank you for any help you can give,
      James
Try shared_ptr. Something like.
function(){
   typedef  adjacency_list  Graph;
   shared_ptr< Graph > gptr( new Graph(n) );
   return gptr;
   }
You could also just declare the graph in whichever scope will end up 
storing it and then pass it by reference to the construction function:
void function(Graph& g) {
  // build g, or build a temporary and assign it to g
}
Graph the_graph;
function(the_graph);
Where are you going to be putting the final graph?  Will it be in a 
variable in your main(), even if it is not constructed there?
-- Jeremiah Willcock
Jeremiah,
I'm sorry if I was not clear in my first post.  The majority of the code 
(along with the main routine) is written in Fortran. I am using the graph to 
update a number of arrays which are needed for this Fortran code.  I will 
create the graph in a subroutine (or c++ function) and modify it in another 
c++ function. The graph is not called in the main routine, only in 
subroutines.
I need to know the way the graph is stored so I can pass it back to the main 
function (or perhaps define it as a public variable).
In reference to the code snippet above, am I correct in reading it as the 
creating routine will be defined as:
void function(Graph& g) {
// build g, or build a temporary and assign it to g
}
and then the function to modify it would be defined as:
void modify_the_graph(Graph& g){
  \\ some graph manipulations here
}
Where is the variable holding the graph going to be?  If it doesn't have a 
Fortran type the variable cannot be in Fortran code.  Are you going to 
have a global variable (in C++) that contains the graph?  Otherwise, you 
might need to use a raw (not shared) pointer to the graph, managing the 
memory yourself, and pass it between Fortran and C++ as either some kind 
of interoperability C pointer type (in newer Fortran versions), a large 
enough integer, or maybe a Cray pointer.

-- Jeremiah Willcock