C++ new Keyword

The new keyword in C++ is used to dynamically allocate memory on the heap for variables, arrays, or objects at runtime. It returns a pointer to the allocated memory and ensures that the memory remains allocated until explicitly deallocated using the delete keyword.

Dynamic memory allocation with new is useful when the size or number of elements to be allocated is not known at compile time.

Syntax

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data_type* pointer_name = new data_type;
data_type* pointer_name = new data_type[size]; // For arrays
data_type
The type of the variable or object to allocate memory for.
pointer_name
The name of the pointer that will store the address of the allocated memory.
size
The number of elements to allocate when creating arrays dynamically.

Examples

Example 1: Allocating Memory for a Single Variable

In this example, you will learn how to use new to allocate memory for a single integer.

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#include <iostream>
using namespace std;

int main() {
    int* ptr = new int; // Allocate memory for an integer
    *ptr = 42;          // Assign a value to the allocated memory

    cout << "Value: " << *ptr << endl;

    delete ptr; // Free the allocated memory
    return 0;
}

Output:

Value: 42

Explanation:

  1. The new keyword allocates memory for a single integer and returns its address, which is stored in ptr.
  2. The value 42 is assigned to the allocated memory using *ptr.
  3. The memory is released using the delete keyword to prevent memory leaks.

Example 2: Allocating Memory for an Array

In this example, you will learn how to use new to allocate memory for an array of integers.

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#include <iostream>
using namespace std;
int main() {
    int size = 5;
    int* arr = new int[size]; // Allocate memory for an array of integers
    for (int i = 0; i < size; ++i) {
        arr[i] = i + 1; // Assign values to the array
    }
    cout << "Array elements: ";
    for (int i = 0; i < size; ++i) {
        cout << arr[i] << " ";
    }
    cout << endl;
    delete[] arr; // Free the allocated memory
    return 0;
}

Output:

Array elements: 1 2 3 4 5

Explanation:

  1. The new keyword allocates memory for an array of size 5 and returns a pointer to the first element.
  2. Values are assigned to the array using a loop.
  3. The delete[] keyword is used to release the memory allocated for the array.

Example 3: Allocating Memory for a Custom Object

In this example, you will learn how to use new to allocate memory for an object of a user-defined class.

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#include <iostream>
using namespace std;

class Rectangle {
private:
    int length, width;

public:
    Rectangle(int l, int w) : length(l), width(w) {}
    int area() {
        return length * width;
    }
};

int main() {
    Rectangle* rect = new Rectangle(5, 3); // Allocate memory for a Rectangle object

    cout << "Area: " << rect->area() << endl;

    delete rect; // Free the allocated memory
    return 0;
}

Output:

Area: 15

Explanation:

  1. The new keyword allocates memory for a Rectangle object and calls its constructor.
  2. The pointer rect is used to access the object's methods using the -> operator.
  3. The memory allocated for the object is released using delete.

Key Points about new Keyword

  1. The new keyword dynamically allocates memory on the heap at runtime.
  2. It returns a pointer to the allocated memory, which can be used to access or manipulate the data.
  3. Always pair new with delete (or delete[] for arrays) to avoid memory leaks.
  4. Use new for scenarios where memory requirements are determined at runtime.