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Lecture 5 Notes: Pointers
1 Background
1.1 Variables and Memory
When you declare a variable, the computer associates the variable name with a particular location in memory and stores a value there.
When you refer to the variable by name in your code, the computer must take two steps:
- Look up the address that the variable name corresponds to
- Go to that location in memory and retrieve or set the value it contains
C++ allows us to perform either one of these steps independently on a variable with the & and * operators:
- &x evaluates to the address of x in memory.
- *( &x ) takes the address of x and dereferences it – it retrieves the value at that location in memory. *( &x ) thus evaluates to the same thing as x.
1.2 Motivating Pointers
Memory addresses, or pointers, allow us to manipulate data much more flexibly; manipulat ing the memory addresses of data can be more efficient than manipulating the data itself. Just a taste of what we’ll be able to do with pointers:
- More flexible pass-by-reference
- Manipulate complex data structures efficiently, even if their data is scattered in differ ent memory locations
- Use polymorphism – calling functions on data without knowing exactly what kind of data it is (more on this in Lectures 7-8)
2 Pointers and their Behavior
2.1 The Nature of Pointers
Pointers are just variables storing integers – but those integers happen to be memory ad dresses, usually addresses of other variables. A pointer that stores the address of some variable x is said to point to x. We can access the value of x by dereferencing the pointer.
As with arrays, it is often helpful to visualize pointers by using a row of adjacent cells to represent memory locations, as below. Each cell represents 1 block of memory. The dot- arrow notation indicates that ptr “points to” x – that is, the value stored in ptr is 12314, x’s memory address.
ptr x
2.2 Pointer Syntax/Usage
2.2.1 Declaring Pointers
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.”
2.2.2 Using Pointer Values
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
2.3 Null, Uninitialized, and Deallocated Pointers
Some pointers do not point to valid data; dereferencing such a pointer is a runtime error. Any pointer set to 0 is called a null pointer, and since there is no memory location 0, it is an invalid pointer. One should generally check whether a pointer is null before dereferencing it. Pointers are often set to 0 to signal that they are not currently valid. Dereferencing pointers to data that has been erased from memory also usually causes runtime errors. Example:
1 int * myFunc () { 2 int phantom = 4; 3 return & phantom ; 4 }
phantom is deallocated when myFunc exits, so the pointer the function returns is invalid. As with any other variable, the value of a pointer is undefined until it is initialized, so it may be invalid.
3 References
When we write void f(int &x) {...} and call f(y), the reference variable x becomes another name – an alias – for the value of y in memory. We can declare a reference variable locally, as well: int y ; int & x = y ; // Makes x a reference to , or alias of , y
After these declarations, changing x will change y and vice versa, because they are two names for the same thing.
References are just pointers that are dereferenced every time they are used. Just like point ers, you can pass them around, return them, set other references to them, etc. The only differences between using pointers and using references are:
- References are sort of pre-dereferenced – you do not dereference them explicitly.
- You cannot change the location to which a reference points, whereas you can change the location to which a pointer points. Because of this, references must always be initialized when they are declared.
- When writing the value that you want to make a reference to, you do not put an & before it to take its address, whereas you do need to do this for pointers.
3.1 The Many Faces of * and &
The usage of the * and & operators with pointers/references can be confusing. The * operator is used in two different ways:
- When declaring a pointer, * is placed before the variable name to indicate that the variable being declared is a pointer – say, a pointer to an int or char, not an int or char value.
- When using a pointer that has been set to point to some value, * is placed before the pointer name to dereference it – to access or set the value it points to. A similar distinction exists for &, which can be used either
- to indicate a reference data type (as in int &x;), or
- to take the address of a variable (as in int *ptr = &x;).
4 Pointers and Arrays
The name of an array is actually a pointer to the first element in the array. Writing myArray[3] tells the compiler to return the element that is 3 away from the starting el ement of myArray.
This explains why arrays are always passed by reference: passing an array is really passing a pointer.
This also explains why array indices start at 0: the first element of an array is the element that is 0 away from the start of the array.
4.1 Pointer Arithmetic
Pointer arithmetic is a way of using subtraction and addition of pointers to move around between locations in memory, typically between array elements. Adding an integer n to a pointer produces a new pointer pointing to n positions further down in memory.
4.1.1 Pointer Step Size
Take the following code snippet:
1 2 3
long arr [] = {6 , long * ptr = arr ; ptr ++;
