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

Subject: Re: [Boost-users] [boost] Create Graph in Function
From: Jeremiah Willcock (jewillco_at_[hidden])
Date: 2009-10-28 22:31:05

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 <j.thunes_at_[hidden]> 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<vecS, vecS, bidirectionalS, no_property, 
>>> EdgeProperty> Graph;
>>>
>>>       Thank you for any help you can give,
>>>       James
>>>
>>>       Try shared_ptr. Something like.
>>> 
>>>
>>>   function(){
>>>    typedef  adjacency_list<vecS, vecS, bidirectionalS, no_property, 
>>> EdgeProperty>  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