Segfault with vector of pointers to vectors - c++

Question

I have narrowed down the problem to this line:
indg = nets[i]->adjlist[i].size(); // indg is in a method of the Ensemble class
Where the above variables are
vector<DDNetwork*> nets; // this vector is in the Ensemble class
int indg;
class DDNetwork
{
friend class Ensemble;
...
public:
vector< vector<int> > adjlist; // the adjacency list of the network
...
};
I don't understand why indg = nets[i]->adjlist[i].size(); would cause a segfault, is there something I am missing? Also if you need more information I can add it.
EDIT: I just realized what was wrong, I am using the same index for adjlist that I am for nets, the line
indg = nets[i]->adjlist[i].size();
should be:
indg = nets[i]->adjlist[j].size();
EDIT: After stepping through the debugger, I noticed that in the constructor of Ensemble, nets.size() = 10 (expected), but when the method Ensemble::alloc_dev_memory is called, nets.size() = 803384 (unexpected), so I think that JaredPar's second suggestion might explain the problem. Here is the code that adds DDNetwork* instances into the nets variable:
Ensemble::Ensemble(int N, float K, int S, bool seedrand, int ltype, int numNets)
{
this->N = N;
this->K = K;
this->S = S;
this->ltype = ltype;
this->numNets = numNets;
if(seedrand)
srand(time(0));
nets.resize(numNets); // make a vector of pointers to DDNetwork
for(int i=0; i < numNets; ++i)
nets[i] = new DDNetwork(N,K,S,seedrand,ltype);
// pre-compute the S^k for k=0,1,...,Kmax
Spow[0]=1; // S^0 = 1
int k=1;
while(k <= Kmax*2) {
Spow[k] = S*Spow[k-1]; // S^k = S*(S^(k-1))
++k;
}
}
This constructor is called when I instantiate the ensemble variable in my main function:
// instantiate ensemble of networks
Ensemble ens(N, K, S, seed_rand, multiedge, numNets);
// run_the ensemble one time step
ens.run_gpu();
And after that, Ensemble::run_gpu calls Ensemble::alloc_dev_memory, then when nets[i]->adjlist[j].size() is called, that's when I receive the segmentation fault.
How would the nets reference get uninitialized?

Answer 1

The problem is likely one of the following
The DDNetwork* reference in nets[i] is uninitialized causing a segfault when you access the members.
The size of nets and each instance of adjlist is not kept in sync causing one of the offsets to be invalid
Could you post the code which adds DDNetwork* instances into the nets variable?

Answer 2

There are two possibilities. Either there isn't a new DDNetwork at index nets[i] or adjlist[i] hasn't been created.
To have a square vector of vectors you need to resize them properly:
adjlist.resize( MAX );
for (int i = 0; i < MAX; ++i)
adjlist[i].resize( MAX );
...only then can you index them. Alternatively you can push_back proper values.
Note also that you use the same index for the nets array, and the adjlist array, unsure whether that was intended.

Answer 3

I found the source of the segfault by accident, I was doing some crude debugging because GDB didn't have information about my main.cpp file, and in order to print out nets.size(), I had to temporarily make vector<DDNetwork*> nets public, after doing that, I realized that the program didn't segfault anymore. I thought it might have to do with the private/public distinction, but when I moved the line
public:
vector<DDNetwork*> nets;
private:
to
public:
private:
vector<DDNetwork*> nets;
line, the program still didn't segfault, so I tried moving the line vector<DDNetwork*> nets; back to where it used to be, all the way below all of the other method and member declarations, just before the closing brace };, and the program began to segfault as before. What is it about the location of the line vector<DDNetwork*> nets; that was causing the segfault?