Pointers and Const

Pointers and Const

const and pointers combine in three ways. The only question that matters: which part is const - the pointer, or the thing it points to?

Pointer to const

int value = 5;
const int* ptr = &value;    // pointer to a const int

*ptr = 10;   // error: the value is read-only (can't change through ptr)
cout << *ptr;   // fine: reading is allowed

const int* reads "pointer to a const int". The pointed-to value is const, but the pointer itself can be moved to another int.

Const pointer

int* const ptr = &value;    // a const pointer

ptr = &other;   // error: the pointed int can still change
*ptr = 10;      // fine: the int value is not const

int* const reads "a const pointer to int". This time the pointer is locked in place, but the value it points to is mutable.

Const pointer to const

const int* const ptr = &value;   // both const

Neither the pointer nor the value can change through it.

Which const does what

• The const before the type (const int*) protects the pointed value.
• The const after the star (int* const) protects the pointer.

const int*        // pointer to const int  -> value const
int* const        // const pointer to int  -> pointer const
const int* const  // const pointer to const int -> both

TL;DR

• const int* - the pointed value is const.
• int* const - the pointer is const.
• const int* const - both are const.
• The position of the const tells you which part it protects.