To declare a pointer variable named ptr that points to an integer variable named x:
int *ptr = &x;
int *ptr declares the pointer to an integer value, which we are initializing to the address of x.
We can have pointers to values of any type. The general scheme for declaring pointers is:
data_type *pointer_name; // Add "= initial_value " if applicable
Pointer name is then a variable of type data type * – a "pointer to a data type value."
Once a pointer is declared, we can dereference it with the * operator to access its value:
cout << *ptr;
// Prints the value pointed to by ptr.
// which in the above example would be x’s value
We can use deferenced pointers as l-values:
*ptr = 5; // Sets the value of x
Without the * operator, the identifier x refers to the pointer itself, not the value it points to:
cout << ptr; // Outputs the memory address of x in base 16
Just like any other data type, we can pass pointers as arguments to functions. The same way we’d say void func(int x) {...}, we can say void func(int *x){...}. Here is an example of using pointers to square a number in a similar fashion to pass-by-reference:
#include <iostream>
using namespace std;
void squareByPtr ( int * numPtr ) {
* numPtr = * numPtr * * numPtr ;
}
int main () {
int x = 5;
squareByPtr (& x);
cout << x; // Prints 25
return 0;
}
Note that in line 2 * has different uses.
There are two places the const keyword can be placed within a pointer variable declaration.
This is because there are two different variables whose values you might want to forbid
changing: the pointer itself and the value it points to.
const int *ptr;
declares a changeable pointer to a constant integer. The integer value cannot be changed through this pointer, but the pointer may be changed to point to a different constant integer.
int * const ptr;
declares a constant pointer to changeable integer data. The integer value can be changed through this pointer, but the pointer may not be changed to point to a different constant integer.
const int * const ptr;
forbids changing either the address ptr contains or the value it points to.