Lecture 5 Notes: Pointers

Introduction to C++ Massachusetts Institute of Technology January 12, 2011 6.096 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: 1. Look up the address that the variable name corresponds to 2. 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: 1. &x evaluates to the address of x in memory. 2. *( &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 2.1 Pointers and their Behavior 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 dotarrow notation indicates that ptr “points to” x – that is, the value stored in ptr is 12314, x’s memory address. ptr ... 2.2 2.2.1 12309 12310 x 12311 12312 12313 12314 ... Pointer Syntax/Usage 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 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: 1 2 3 4 5 6 7 8 9 void squareByPtr ( int * numPtr ) { * numPtr = * numPtr * * numPtr ; } int main () { int x = 5; squareByPtr (& x ) ; cout << x ; // Prints 25 } Note the varied uses of the * operator on line 2. 2.2.3 const Pointers 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. 3 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 2 3 4 int * myFunc () { int phantom = 4; return & phantom ; } 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. 4 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: 1. 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. 2. 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 1. to indicate a reference data type (as in int &x;), or 2. 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 long arr [] = {6 , 0 , 9 , 6}; 2 long * ptr = arr ; 3 ptr ++; 5 4 long * ptr2 = arr + 3; When we add 1 to ptr in line 3, we don’t just want to move to the next byte in memory, since each array element takes up multiple bytes; we want to move to the next element in the array. The C++ compiler automatically takes care of this, using the appropriate step size for adding to and subtracting from pointers. Thus, line 3 moves ptr to point to the second element of the array. Similarly, we can add/subtract two pointers: ptr2 - ptr gives the number of array elements between ptr2 and ptr (2). All addition and subtraction operations on pointers use the appropriate step size. 4.1.2 Array Access Notations Because of the interchangeability of pointers and array names, array-subscript notation (the form myArray[3]) can be used with pointers as well as arrays. When used with pointers, it is referred to as pointer-subscript notation. An alternative is pointer-offset notation, in which you explicitly add your offset to the pointer and dereference the resulting address. For instance, an alternate and functionally identical way to express myArray[3] is *(myArray + 3). 4.2 char * Strings You should now be able to see why the type of a string value is char *: a string is actually an array of characters. When you set a char * to a string, you are really setting a pointer to point to the first character in the array that holds the string. You cannot modify string literals; to do so is either a syntax error or a runtime error, depending on how you try to do it. (String literals are loaded into read-only program memory at program startup.) You can, however, modify the contents of an array of characters. Consider the following example: char courseName1 [] = { ’6 ’ , ’. ’ , ’0 ’ , ’9 ’ , ’6 ’ , ’ \0 ’ }; char * courseName2 = " 6.096 " ; Attempting to modify one of the elements courseName1 is permitted, but attempting to modify one of the characters in courseName2 will generate a runtime error, causing the program to crash. 6