Const

Definition

The const keyword affects the behavior of a variable. Any variable initialized with the const keyword cannot be modified later on. It is constant.

const int i = 10;
i = 20; // "Error: Cannot modify a constant variable"

Variables or attributes of a class that have been made using const must always be initialized.

Class methods can also be const and they can be invoked by const and non-const instances of the class.

Pointers

Both the pointer and the data being pointed to can be const.

type const* / const type*

This declaration implies that the value being pointed to is const. It should not be altered. However, the pointer itself is not const:

int i = 2011;
int const* ip = &i;
*ip = 2012; // ERROR

int j = 2012;
ip = &j; 

type* const

In this case, the pointer is constant. It cannot point to a different pointer throughout its lifetime.

int i = 2011;
int j = 2012;
int* const p = &i;
p = &j; // ERROR

*p = 2015; 

const type* const

Now, both the pointer and the value are constant.

int i = 100, int j = 200; 
const int* const p = &i;
*p = i; // ERROR 
p = &j; // ERROR

The line, const int* const p, should be read from right to left. p is a constant pointer, * const points to an int that is const. We can modify i directly, but we can’t modify it through p.

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