'const' keyword in C++

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Use Cases

• We basically have 3 use cases of const keyword:
  1. Restrict the change in value of a Variable
  2. Restrict the change of Pointer
  3. Restrict the class method to change the variables of that class

Restrict the change in value of a Variable

const int x = 7;
x = 10; // ERROR, cannot change the value of x

const int& p = 9;
p = 10; // ERROR, cannot change the value of p

const int* a = NULL;

int val = 2;
a = &val;  // We can change the Pointer

*a = 10; // ERROR, But we can’t change the contents of that Pointer 

Another way of writing:

• const int x = 7 is identical to int const x = 7
• const int& p = 9 is identical to int const & p = 9
• const int* a = NULL is identical to int const * a = NULL
• Generalization: const can be written on either side of datatype name (e.g. int, float, class_name etc.) in order to restrict the change in value.

Restrict the change of a Pointer

int* const a = NULL;

int val = 2;
a = &val;  // ERROR, We can't change the Pointer

*a = 10; // But we can change the contents of that Pointer 

Restrict the class method to change the variables of that class

class Person
{
    int age=7;

    int modify_age() const
    {
        age = 10; // ERROR, const method cannot modify class variables
    }
};

Combining all three

class Dummy
{
    int* ptr = NULL;

    const int* const get_ptr() const
    {
        return ptr;
    }

};

• get_ptr is a constant method which cannot change the variables of that class and returns a pointer which cannot be changed nor can be its content.
• const int* const get_ptr() const is identical to int const * const get_ptr() const
• Second one seems more uniform as const always appear on the right side of what it is applied on. That is:
  • int const makes value const
  • * cont makes pointer const
  • get_ptr() const makes method const

TODO

void function_name(const int x)

const objects can call const methods only

class Person
{
private:
    int age = 7;

public:
    int get_age() const
    {
        return age;
    }

    void set_age(int val)
    {
        age = val;
    }
};

int main()
{
    const Person p;
    p.get_age(); // Works fine
    p.set_age(10); // ERROR, const object p can only call const methods

    return 0;
}