Dynamic memory for object and object array

Objects can be created on the heap with new and destroyed with delete. new allocates memory and calls the constructor; delete calls the destructor and then frees memory.

Single object

Student *p = new Student("Ram", 10);   // calls constructor
p->display();                          // access through pointer
delete p;                              // calls destructor, frees memory

Array of objects

Student *s = new Student[n];           // calls default constructor n times
s[i].display();  // or (s+i)->display()
delete[] s;                            // calls destructor for each element

The class must have a default constructor because no arguments can be passed in new T[n].

Example program

#include <iostream>
using namespace std;

class Student
{
    int roll;
    float marks;
public:
    Student() { roll = 0; marks = 0; }
    Student(int r, float m) { roll = r; marks = m; }
    void read()
    {
        cout << "Enter roll and marks: ";
        cin >> roll >> marks;
    }
    void display() { cout << roll << "  " << marks << endl; }
    ~Student() { cout << "Object " << roll << " destroyed\n"; }
};

int main()
{
    Student *p = new Student(1, 78.5);   // one object
    p->display();
    delete p;

    int n;
    cout << "Number of students: ";
    cin >> n;
    Student *s = new Student[n];         // array of objects
    for (int i = 0; i < n; i++)
        s[i].read();
    for (int i = 0; i < n; i++)
        s[i].display();
    delete[] s;                          // destructors called for all
    return 0;
}

Points

• Use -> with a pointer to an object and . with an array element.
• Use delete[] for arrays; plain delete on an array gives undefined behaviour.
• Dynamic objects live until deleted, so a memory leak occurs if we forget delete.