What is the const Keyword in C++

The const keyword in C++ is a keyword that is used to declare variables and objects as constant, which means the value declared using const cannot be changed or modified later, once they are initialized. This helps them prevent accidental modifications.

For example, in a code, if we are using the value of PI, which has a fixed universal value and doesn't need any change, then we can declare it as a constant.

When you declare the object with the const keyword, then the compiler places that value in ROM (Read-Only Memory), which protects it from being changed in the future. And if we try to reassign that const value, then it will return an error.

Syntax

Here is the following syntax for the const keyword.

const data_type variable_name = value;

Example

#include <iostream>
#include <string>
using namespace std;

class constObject {
public:
    void display() const { // object
        cout << "Here displaying the Const Object" << endl;
    }
};

int main() {
    const int x = 10;                 // const variable
    const string msg = "Hello";       // const string
    const int arr[] = {1, 2, 3};      // const array
    
    int val = 100;
    const int* ptr = &val;            // pointer to const int
    
    const constObject obj;            // const object
    obj.display();                    // only const member functions allowed
    cout<<"const variable: "<<x<<endl;
    cout<<"const string: "<<msg<<endl;
    cout<<"const array calling index 1: "<<arr[1]<<endl;
    cout<<"pointer to const int: "<<*ptr<<endl;
    
    return 0;
}

Output

Here displaying the Const Object
const variable: 10
const string: Hello
const array calling index 1: 2
pointer to const int: 100

Const Member Function

A const member function in C++ is a function that does not change or modify any data members of the class; this makes the function read only and can be called on const objects. This is also declared using the const keyword.

Syntax

Here is the following syntax for a const member function in C++.

class class_name {
  public: return_type function_name() const;
};

Example

#include <iostream>
using namespace std;

class Student {
    int rollNumber;  
public:
    // Constructor initializing roll number
    Student(int r) : rollNumber(r) {}

    // Const member function
    void display() const {
        cout << "Roll Number: " << rollNumber << endl;
    }
};

int main() {
    const Student s(23);  // const object
    s.display();           
    return 0;
}

Output

Roll Number: 23

Const vs. Constexpr

In C++, both const and constexpr create constants, but they are used for different purposes.
const declares a read-only variable, which means the value cannot be changed or modified later once initialized. This is evaluated at both run time and compile time.

Whereas constexpr defines a constant, which evaluates the value only at compile time, therefore making the value a compile time constant. For example, for array sizes, template parameters, etc.

Example

constexpr int x = 5;