C++ Dynamic Array Member Variable Assignment

Question

I am having a problem with assigning new values to a dynamic int array that is a data member variable of the class IntersectionFlowRate(). I can initialize and print the values of the array inside the constructor. However, when I exit the constructor to the another class and then later call a function within the IntersectionFlowRate() class passing in variables to overwrite the initial values of the data member it will segmentation fault. I have debugged to find that overwriting the array is causing the seg fault. And that even attempting to access the dynamic array within one of its functions will seg fault.

My question is how can I edit the values of a dynamic int array member variable from within one of its functions i.e setArrayElement(int index, int x).

Here is some of my code. Sorry if I am unclear or missing something ridiculous. I have been stuck on this for hours.

    #ifndef INTERSECTIONFLOWRATE_H
    #define INTERSECTIONFLOWRATE_H

    class IntersectionFlowRate
    {
    public:
        IntersectionFlowRate();
        ~IntersectionFlowRate();
        void setFlowCycle(int index, int flow);

    private:
        int* m_flowRateMotorCycle;

 };
 #endif

in the .h file ^

    #include "IntersectionFlowRate.h"
    #include <cstdlib>
    #include <iostream>
    #include <new>

    using namespace std;
    IntersectionFlowRate::IntersectionFlowRate()
    {
        const int SIZE = 4; //Constant for m_flowRates[] size

        //DYNAMIC MEMORY DELETE LATER
        m_flowRateMotorCycle = new int[SIZE];

        for(int i = 0; i < SIZE; i++){
            m_flowRateMotorCycle[i] = 0;
            cout << m_flowRateMotorCycle[i] << endl;
            cout << "WE GOT HERE" << endl;
        }
    }

    void IntersectionFlowRate::setFlowCycle(int index, int flow){
        cout << "INDEX: " << index << endl;
        cout << "FLOW: " << flow << endl;

        m_flowRateMotorCycle[index] = flow; //seg fault is here
    }

I have another class that creates a pointer to a IntersectionFlowRate() object and then calls its setFlowCycle function passing in two VALID ints. With the debugging I was able pass 0 and 3 to the function setFlowCycle(0, 3) just fine and output those variables within the function.

    #ifndef TRAFFICSIM_H
    #define TRAFFICSIM_H

    #include "IntersectionFlowRate.h"

    using namespace std;

    class TrafficSim
    {
    public:
        TrafficSim(); //Default Constructor
        TrafficSim(const char* file); //Constructor
        ~TrafficSim(); //Destructor


    private:
        IntersectionFlowRate* m_flowRate;
    };
    #endif


    #include "TrafficSim.h"
    #include "IntersectionFlowRate.h"
    #include <iostream>
    #include <string>
    #include <fstream>
    #include <cstdlib>


    using namespace std;

    TrafficSim::TrafficSim()
    {

        IntersectionFlowRate* m_flowRate = new IntersectionFlowRate();
        m_flowRate->setFlowCycle(0, 3);

    }

I replicated the error with this code. If no one else can I am completely unsure of what is possibly wrong anymore.

Answer 1

You are setting a local variable called m_flowRate, not the member variable m_flowRate of your TrafficSim class:

Instead of this:

TrafficSim::TrafficSim()
{
    IntersectionFlowRate* m_flowRate = new IntersectionFlowRate();
    m_flowRate->setFlowCycle(0, 3);
}

It should be this:

 TrafficSim::TrafficSim()
 {
    m_flowRate = new IntersectionFlowRate();
    m_flowRate->setFlowCycle(0, 3);
 }

But overall, it not need be a pointer. It could be an object member within your class. That would cut down on the pointer usage a bit:

class TrafficSim
{
    public:
        TrafficSim(); //Default Constructor
        TrafficSim(const char* file); //Constructor

    private:
        IntersectionFlowRate m_flowRate;
};

Then:

TrafficSim::TrafficSim()
{
    m_flowRate.setFlowCycle(0, 3);
}

As to your question as to how to incorporate usage of std::vector in your class, here is a code sample of the IntersectionFlowRate class, rewritten using vector:

Vector sample

Also, another source of problems is that your classes fail to follow the Rule of 3 when you have pointers to dynamically allocated memory in your class.

Using std::vector takes care of this automatically, but if you insist on using pointers, you need to adhere to the directions at the link posted.

Answer 2

Yes, use a std::vector, it is much simpler, and it is a template so it also pretty fast and works any type (best for primitive types or pointers to objects) , and it also has boundary checking and other useful things.

If you need fast array-like access then you could use std::map which associates a key with a value, like so

    std::map<UINT, YourClass*> m_mapIDs_to_YourClass;

When you first start using stl containers they might seem a little strange, but after a short while you cannot do without them, luckily they have been part of the C++ standard for some time now.

Boundary check for both these containers can be done by comparing your iterator to mapYourMap.end(), if they are equal you have passed the last element and trying to access data through the iterator will cause an exception. Example for std::vector (if vecInt is a vector< int >):

    vector<int>::iterator it = vecInt.begind();
    if (it == vecInt.end()) return; // vector is empty
    do {  // runs through elememts until out of bound, useful for searching
        i++
    while (it != vecInt.end());