C Pointers
Creating Pointers
You learned from the previous chapter that we can get the memory address of a variable with the reference operator &:
Example
int myAge = 43; // an int variable
printf("%d", myAge); // Outputs the value of myAge (43)
printf("%p", &myAge); // Outputs the memory address of myAge (0x7ffe5367e044)
Try it Yourself »
A pointeris a variable that stores the memory address of another variable as its value.
A pointer variable points to a data type (like int) of the same type, and is created with the * operator.
The address of the variable you are working with is assigned to the pointer:
Example
int myAge = 43; // An int variable
int* ptr = &myAge; // A pointer variable, with the name ptr, that stores the address of myAge
// Output the value of myAge (43)
printf("%d\n", myAge);
// Output the memory address of myAge (0x7ffe5367e044)
printf("%p\n", &myAge);
// Output the memory address of myAge with the pointer (0x7ffe5367e044)
printf("%p\n", ptr);
Try it Yourself »
Example explained
Create a pointer variable with the name ptr, that points to an int variable (myAge). Note that the type of the pointer has to match the type of the variable you're working with (int in our example).
Use the & operator to store the memory address of the myAge variable, and assign it to the pointer.
Now, ptr holds the value of myAge's memory address.
Dereference
In the example above, we used the pointer variable to get the memory address of a variable (used together with the & reference operator).
You can also get the value of the variable the pointer points to, by using the * operator (the dereference operator):
Example
int myAge = 43; // Variable declaration
int* ptr = &myAge; // Pointer declaration
// Reference: Output the memory address of myAge with the pointer (0x7ffe5367e044)
printf("%p\n", ptr);
// Dereference: Output the value of myAge with the pointer (43)
printf("%d\n", *ptr);
Try it Yourself »
Note that the * sign can be confusing here, as it does two different things in our code:
- When used in declaration (
int* ptr), it creates a pointer variable. - When not used in declaration, it acts as a dereference operator.
Good To Know: There are two ways to declare pointer variables in C:
int* myNum;
int *myNum;
Notes on Pointers
Pointers are one of the things that make C stand out from other programming languages, like Python and Java.
They are important in C, because they allow us to manipulate the data in the computer's memory. This can reduce the code and improve the performance. If you are familiar with data structures like lists, trees and graphs, you should know that pointers are especially useful for implementing those. And sometimes you even have to use pointers, for example, when working with files.
But be careful; pointers must be handled with care, since it is possible to damage data stored in other memory addresses.
