Segmentation fault on vector class implementation-CodePudding

As for many coding tests STL is not allowed so I am trying to implement vector class. To represent the graph I am using an adjacency list. It's giving me segmentation fault at new_allocation method. Also sometimes I get correct output when I run code but when I debug I get SegFault. Following is my code.

#include <iostream>
using namespace std;

template <typename T> class vector{
    T *arr;
    int s;
    int c;
    void new_allocation(){
        T *temp = new T[s   10];       // SEGMENTATION FAULT HERE
        c = s   10;
        for (int i = 0; i < s; i  )
            temp[i] = arr[i];
        arr = temp;
        delete[] temp;
    }
    public:
        vector() { s = c = 0; }
        vector(int userVectorSize){
            s = c = userVectorSize;
            arr = new T[userVectorSize];
        }
        void push_back(T data){
            if (s == c)
                new_allocation();
            arr[s] = data;
            s  ;
        }
        T operator[](int index){ return arr[index]; }
        int size() { return s; }
};

int main(){
    int n, m;
    cin >> n >> m;
    vector<int> graph[n   1];

    for (int i = 0; i < m; i  ){
        int v1, v2;
        cin >> v1 >> v2;
        graph[v1].push_back(v2);
        graph[v2].push_back(v1);
    }

    for (int i = 1; i <= n; i  ){
        cout << i << " -> ";
        for (int k = 0; k < graph[i].size(); k  )
            cout << graph[i][k] << " ";
        cout << endl;
    }
}

CodePudding user response：

this is never going to work

     arr = temp;
    delete[] temp;

you place 'temp' pointer in arr (your working buffer of data elements) and then delete it, so arr points at deleted memory

CodePudding user response：

please initialize the arr in the default ctor:

    vector() {
        s = c = 0;
        arr = nullptr; // (1)
    }

As pm100 mentioned, please pick up the right array to delete[].

    void new_allocation() {
        T *temp = new T[s   10]; // SEGMENTATION FAULT HERE
        c = s   10;
        for (int i = 0; i < s; i  )
            temp[i] = arr[i];

        // (2)
        if (arr != nullptr) {
            delete[] arr;
        }
        arr = temp;
    }

The reason is that your arr is pointed to arbitrary memory by default, so it will still report a Segmentation fault when you try to delete[] arr.

#include <iostream>
using namespace std;

template <typename T> class vector {
    T *arr;
    int s;
    int c;
    void new_allocation() {
        T *temp = new T[s   10]; // SEGMENTATION FAULT HERE
        c = s   10;
        for (int i = 0; i < s; i  )
            temp[i] = arr[i];

        if (arr != nullptr) {
            delete[] arr;
        }
        arr = temp;
    }

  public:
    vector() {
        s = c = 0;
        arr = nullptr;
    }
    vector(int userVectorSize) {
        s = c = userVectorSize;
        arr = new T[userVectorSize];
    }
    void push_back(T data) {
        if (s == c)
            new_allocation();
        arr[s] = data;
        s  ;
    }
    T operator[](int index) { return arr[index]; }
    int size() { return s; }
};

int main() {
    int n, m;
    cin >> n >> m;
    vector<int> graph[n   1];

    for (int i = 0; i < m; i  ) {
        int v1, v2;
        cin >> v1 >> v2;
        graph[v1].push_back(v2);
        graph[v2].push_back(v1);
    }

    for (int i = 1; i <= n; i  ) {
        cout << i << " -> ";
        for (int k = 0; k < graph[i].size(); k  )
            cout << graph[i][k] << " ";
        cout << endl;
    }
}