Pointers and const in C++

Question

When dealing with pointers and const, I see there are three ways to declare them:

1)

int nValue = 5;
int *const pnPtr = &nValue;

2)

int nValue = 5;
const int *pnPtr = &nValue;

3)

const int 5;
const int *const pnPtr = &nValue;

Example 1 is referred to as "Const pointer to non-const". The address cannot change but the value can change. So we can do something like the following, since nValue in example 1 is a non-const int:

int nValue = 5;
int const *pnPtr = &nValue;

*pnPtr = 6; 

But we cannot do the following in example 1:

int nValue = 5;
int nValue2 = 6;
int const *pnPtr = &nValue;
pnPtr = &nValue2; 

Example 2 is referred to as "Pointer to const". This means that the address can change, but the value cannot. We can do the following:

int nValue = 5;
int nValue2 = 6;

const int *pnPtr = &nValue;
pnPtr = &nValue2;

But we cannot do the following in Example 2:

int nValue = 5;
int nValue2 = 6;

const int *pnPtr = &nValue;
*pnPtr = nValue2;

Example 3 is a "Const pointer to const". This means neither address or value can change:

const int nValue;
const int *const pnPtr = &nValue;  

My question is related to the second example. Why is the second example called a " Pointer to const" when nValue is not a const. It is a regular int declaration. Also, in the second example, what if when we assign it another address, that other address has a different value, can't we just deference that address, and therefore return the different value? Wouldn't that defeat the whole purpose?

Answer 1

The const in the second example applies to the int , ie you have a non- const pointer to a const int . Since types in C and C++ are read right to left, it is actually easiest to put the const always to right. It is also the only place where it can be put consistently:

int i(0);
int      *       p0(&i); // non-const pointer to non-const int
int const*       p1(&i); // non-const pointer to const int
int      * const p2(&i); // const pointer to non-const int
int const* const p3(&i); // const pointer to const int

That is, you can apply const to the pointer ( p0 and p1 can be changed but p2 and p3 cannot) as well as to the entity the pointer points to ( *p0 and *p2 can be changed but *p1 and *p3 cannot be changed). For the second and the forth line, you could swap the int and the const but I'd advise against do so.

Answer 2

In C even integer is not constant but we are declaring our pointer as we are dealing with the constant integer. in this case C implicitly convert that integer to constant when assigning it reference to pointer.

Usually we do this when passing some values to function where we can avoid some unexpected change.