Question

I wrote my first ever C++ template code on expandable array and I am getting a segmentation fault! After an hour of debugging I have realized that I need help. Something is wrong with the constructor or the destructor I think but not sure.

The code is on pastie ready to be compiled.
http://pastie.org/1150617

/* Expandable array in C++ */

#include <iostream>
using namespace std;

template <class T>
class EArray{
private:
    T* arr;
    int size;
public:
    EArray(int l);
    ~EArray();

    void setElement(int i, const T& newval);
    void eraseElement(int i);
    void addElement(int i, const T& newval);
    void push(const T& newval);
    void display();
};

template <class T>
EArray<T>::EArray(int l){
    size = l;
}

template <class T>
EArray<T>::~EArray(){
    delete [] arr;
    arr = NULL;
}

template <class T>
void EArray<T>::setElement(int i, const T& newval){
    if(i < size && i >= 0){
        arr[i] = newval;
    }
}

template <class T>
void EArray<T>::eraseElement(int index){
    size -= 1;
    T* newarr = new T[size];
    for (int i = 0; i < size+1; i++){
        if (i < index){
            newarr[i] = arr[i];
        }
        else if(i > index){
            newarr[i-1] = arr[i];
        }
    }
    delete [] arr;
    arr = newarr;
}

template <class T>
void EArray<T>::addElement(int index, const T& newval){
    size += 1;
    T* newarr = new T[size];
    for(int i = 0; i < size; i++){
        if(i<index){
            newarr[i] = arr[i];
        }
        else if (i == index){
            newarr[i] = newval;
        }
        else{
            newarr[i] = arr[i-1];
        }
    }
    delete [] arr;
    arr = newarr;
}

template <class T>

void EArray<T>::push(const T& newval){
    size += 1;
    T * newarr = new T[size];
    for (int i = 0; i < size-1; i++){
        newarr[i] = arr[i];
    }
    newarr[size-1]=newval;
    delete [] arr;
    arr = newarr;
}

template <class T>
void EArray<T>::display(){
    for(int i = 0; i < size; i++){
        cout << arr[i] << endl;
    }
}

int main(){
    EArray<int> A(6);
    A.setElement(0,34);
    A.setElement(1,544);
    A.setElement(2,32);
    A.setElement(3,324);
    A.setElement(4,24);
    A.display();
    A.addElement(3,12);
    A.display();
    A.eraseElement(4);
    A.display();
    A.push(32456);
    A.display();
}

Answer 1

It has nothing to do with templates. It’s just a problem of memory management. In the constructor of EArray, you have never initialized arr, so by default it contains some invalid pointer.

But then in setElement, you used this invalid pointer arr[i] = newval;, which should cause a SegFault.

It should be fixable by adding

arr = new T[size];

in the constructor (result: before, with segfault — after, running fine).

(BTW, in practice, please use a std::vector.)