const

const is a type qualifier in C/C++ used to indicate a given variable/method is constant and cannot be changed later in the program. For variables, this sets them as immutable:

const int x = 10;
x = 9; // compile-error

When applied to pointers, there are two different ways const is handled:

• For const int *p = x, we can change what it’s pointing to but not change the value inside its address, i.e., *p = 30 may spit out a compiler error.
• For int *const p, we can’t modify what it’s pointing to but we can change the value inside the address. Very confusing!

When we take a function input that doesn’t change, we should use a const for safety — if it doesn’t need to change, then any code we write that changes it will cause a compile-time error (a big hint we’re doing something wrong).

We can also set methods as const:

Fraction Fraction::exampleMethod(Fraction &rhs) const {
	// some code
}

In this case, we cannot change any member data. The this pointer effectively becomes a pointer to a const object.