C++ Memory Management: new and delete

C++ allows us to allocate the memory of a variable or an array in run time. This is known as dynamic memory allocation.

In other programming languages such as Java and Python, the compiler automatically manages the memories allocated to variables. But this is not the case in C++.

In C++, we need to deallocate the dynamically allocated memory manually after we have no use for the variable.

We can allocate and then deallocate memory dynamically using the new and delete operators respectively.

C++ new Operator

The new operator allocates memory to a variable. For example,

// declare an int pointer
int* pointVar;

// dynamically allocate memory
// using the new keyword 
pointVar = new int;

// assign value to allocated memory
*pointVar = 45;

Here, we have dynamically allocated memory for an int variable using the new operator.

Notice that we have used the pointer pointVar to allocate the memory dynamically. This is because the new operator returns the address of the memory location.

In the case of an array, the new operator returns the address of the first element of the array.

From the example above, we can see that the syntax for using the new operator is

pointerVariable = new dataType;

delete Operator

Once we no longer need to use a variable that we have declared dynamically, we can deallocate the memory occupied by the variable.

For this, the delete operator is used. It returns the memory to the operating system. This is known as memory deallocation.

The syntax for this operator is

delete pointerVariable;

Consider the code:

// declare an int pointer
int* pointVar;

// dynamically allocate memory
// for an int variable 
pointVar = new int;

// assign value to the variable memory
*pointVar = 45;

// print the value stored in memory
cout << *pointVar; // Output: 45

// deallocate the memory
delete pointVar;

Here, we have dynamically allocated memory for an int variable using the pointer pointVar.

After printing the contents of pointVar, we deallocated the memory using delete.

Note: If the program uses a large amount of unwanted memory using new, the system may crash because there will be no memory available for the operating system. In this case, the delete operator can help the system from crash.

Example 1: C++ Dynamic Memory Allocation

#include <iostream>
using namespace std;

int main() {

  // declare an int pointer
  int* pointInt;

  // declare a float pointer
  float* pointFloat;

  // dynamically allocate memory
  pointInt = new int;
  pointFloat = new float;

  // assigning value to the memory
  *pointInt = 45;
  *pointFloat = 45.45f;

  cout << *pointInt << endl;
  cout << *pointFloat << endl;

  // deallocate the memory
  delete pointInt;
  delete pointFloat;

  return 0;
}

Output

45
45.45

In this program, we dynamically allocated memory to two variables of int and float types. After assigning values to them and printing them, we finally deallocate the memories using the code

delete pointInt;
delete pointFloat;

Example 2: C++ new and delete Operator for Arrays

// C++ Program to store GPA of n number of students and display it
// where n is the number of students entered by the user

#include <iostream>
using namespace std;

int main() {

  int num;
  cout << "Enter total number of students: ";
  cin >> num;
  float* ptr;
    
  // memory allocation of num number of floats
  ptr = new float[num];

  cout << "Enter GPA of students." << endl;
  for (int i = 0; i < num; ++i) {
    cout << "Student" << i + 1 << ": ";
    cin >> *(ptr + i);
  }

  cout << "\nDisplaying GPA of students." << endl;
  for (int i = 0; i < num; ++i) {
    cout << "Student" << i + 1 << ": " << *(ptr + i) << endl;
  }

  // ptr memory is released
  delete[] ptr;

  return 0;
}

Output

Enter total number of students: 4
Enter GPA of students.
Student1: 3.6
Student2: 3.1
Student3: 3.9
Student4: 2.9

Displaying GPA of students.
Student1: 3.6
Student2: 3.1
Student3: 3.9
Student4: 2.9

In this program, we have asked the user to enter the number of students and store it in the num variable.

Then, we have allocated the memory dynamically for the float array using new.

We enter data into the array (and later print them) using pointer notation.

After we no longer need the array, we deallocate the array memory using the code delete[] ptr;.

Notice the use of [] after delete. We use the square brackets [] in order to denote that the memory deallocation is that of an array.

Example 3: C++ new and delete Operator for Objects

#include <iostream>
using namespace std;

class Student {
  private:
    int age;

  public:

    // constructor initializes age to 12
    Student() : age(12) {}

    void getAge() {
      cout << "Age = " << age << endl;
    }
};

int main() {

  // dynamically declare Student object
  Student* ptr = new Student();

  // call getAge() function
  ptr->getAge();

  // ptr memory is released
  delete ptr;

  return 0;
}

Output

Age = 12

In this program, we have created a Student class that has a private variable age.

We have initialized age to 12 in the default constructor Student() and print its value with the function getAge().

In main(), we have created a Student object using the new operator and use the pointer ptr to point to its address.

The moment the object is created, the Student() constructor initializes age to 12.

We then call the getAge() function using the code:

ptr->getAge();

Notice the arrow operator ->. This operator is used to access class members using pointers.