GRBCallback for nodes count and LP relaxation of a MILP

Question

I am solving a MILP using Gurobi called from a C++ program. I want to retrieve the number of generated nodes and the value of the objective function of the LP relaxation.

For the nodes count, I used this class:

        class NodeCounter : public GRBCallback {
    public:
       int nodecnt;
    
        void callback() {
            if (where == GRB_CB_MIPNODE) {
                nodecnt =  getDoubleInfo(GRB_CB_MIPNODE_NODCNT);
            }
        }

};

Then I call the the function before the model.optimize() as follows:

         // Create an instance of callback class
        NodeCounter callback;

        // Set the callback function
        model.setCallback(&callback);

but the number of nodes I am getting is not correct!

For the LP relaxation, I called Model.relax() to create the relaxation of the MILP and then I solve it again, my question here is: how to use the callback to get the LP without solving the LP again?

Answer 1

I want to share the solution I have found so far for the count of the number of generated nodes. I changes the previous class to

class NodeCounter : public GRBCallback
{
public:
    double nodecnt;

    void callback()
    {
        if (where == GRB_CB_MIP)
        {
            nodecnt = getDoubleInfo(GRB_CB_MIP_NODCNT);
        }
    }
};

I observed that, in this case, the number of nodes returned is less than 1 the number of nodes declared in the log file. But, if the number of declared nodes in the log file is 0 or 1, the class returns 0 in the two cases. Is there any way to distinguish between these latter two cases?