Task of const keyword

The const keyword declares that a value cannot be modified after initialisation. Any attempt to change it gives a compile-time error.

Uses:

1. Constant variable - replaces #define and is type safe: const float PI = 3.14159;
2. Pointer to constant - the data cannot change through the pointer: const int *p;
3. Constant pointer - the pointer cannot point elsewhere: int *const p = &x;
4. Constant function argument - protects the argument: void show(const int &x);
5. Constant member function - promises not to change the object's data: int getX() const { return x; }
6. Constant object - can call only const member functions.

const int MAX = 100;
// MAX = 200;   // error: assignment of read-only variable