What is the purpose of the const qualifier in C++?

In C++, the const qualifier is used to declare a variable as read-only (unmodifiable). It can be applied to variables, function parameters, function return values, and member functions.

1. Constant variables are declared using the const keyword and cannot be modified during program execution. This means that once the variable is initialized, its value will remain unchanged.
2. Protecting function parameters: Declaring function parameters as const can prevent the values of the parameters from being modified within the function. This ensures that the values of the parameters will not be accidentally changed within the function.
3. Returning a constant: declaring the return value of a function as const can prevent the value from being modified. This is particularly useful for functions that return constant references or pointers, to avoid altering data within the function through the returned reference or pointer.
4. Protect member functions: Declaring a member function as const indicates that it will not modify the object’s state. This allows the member function to be called on a constant object, as the member function of a constant object cannot change the object’s state.

In general, the purpose of the const qualifier is to ensure that variables, function parameters, function return values, and member functions are not modified during use, thereby enhancing the safety and readability of the code.