Question

I am writing a program that splits graphs, I got a class
Graph and an Algorithm class. I compute the partitioning in my Algorithm class and split the graph with a method in the Graph class according to the partitioning.

My code looks like this:
In my GraphClass:

void bisectGraph(int *iPartitioning, Graph **Subgraphs, Edge **Separator){
...
    // Store separators in an array
Separator = new Edge*[Separators.size()+1]; //Separators is a vector containing the separating edges
if(Separator == NULL)
    writeErrorMsg("Error assigning memory.", "Graph::bisectGraph");
for(i=0, SepIter = Separators.begin(); SepIter != Separators.end(); i++, SepIter++)
    Separator[i] = *SepIter;
Separator[Separators.size()] = NULL;

}

In my Algorithm clas I call it like this:

Edge** separators;
Graph** subgraphs;
int somePartitioning;

g->bisectGraph(somePartitioning, subgraphs, separators);

Works fine so far, but when I want to work on my separators array like this for instance:

for(int i=0; separators[i]!=NULL, i++){
    ...
}

I always get a segmentation fault. ddd tells me that at the end of bisectGraph separators contains some content. Since I can’t find any other mistake I think i got some concept wrong?

Answer 1

The new value of Separator is not being propagated to the separators variable outside the function call. Even though it has type Edge ** you’re assigning to it inside the function, but that only assigns to the function’s copy of the variable. Remember that C++ is pass-by-value unless otherwise specified.

You could change the signature to Edge **&, but it’d be more sensible to use a vector, and take a parameter of type vector<Edge *> &.