Constant Pointers in C++

The const keyword may seem simple at first - it makes any variable constant, meaning it can’t be modified later in your code. However, there are many cases where the use of const isn’t so clear.

The problem

const int *a = &b;

What exactly is constant here? Does this mean you cannot modify the value or the pointer? Or both?

The solution

We can start by defining how we declare pointer variables:

<pointer type> *<variable name> = <value>;

We can intuitively deduct, that in the previous statement, we are clearly creating a pointer of type const int. This means that the resulting variable’s underlying value will be constant, but the pointer will be modifiable.

*a = 6; // not allowed
a = &c; // allowed

We can also choose to write this differently with the same functionality:

int const *a = &b; // same as `const int *a = &b;`

We can achieve the opposite effect by declaring the following variable:

int *const a = &b;

The above variable’s underlying value can be modified, but the pointer cannot be reassigned.

You can also combine these const rules to make a const behemoth that has a constant value and cannot be reassigned:

const int *const a = &b;