6.096-5.Pointers in C++

6.096 Introduction to C++ January 12, 2011 Massachusetts Institute of Technology 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; manipulatiin 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 differeen memory locations • Use polymorphism – calling functions on data without knowing exactly what kind of dataitis(more onthisinLectures7-8) 2 Pointers and their Behavior 2.1 The Nature of Pointers Pointersarejustvariablesstoringintegers – butthoseintegershappentobememory addressses 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 thepointer. 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 “pointsto” x – that is, the value stored in ptr is12314, x’s memory address. 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 addressof 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 valueWe can use deferenced pointers as l-values: *ptr = 5; //Sets the value of x2 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 16Just 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 void squareByPtr ( int * numPtr ) { 2 * numPtr = * numPtr * * numPtr ; 3 } 4 5 int main () { 6 int x = 5; 7 squareByPtr (& x); 8 cout << x; //Prints 25 9 } Note the varied uses of the * operator on line 2. 2.2.3 const Pointers Therearetwoplacesthe const keyword canbeplaced withinapointervariabledeclaration. 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 thispointer,butthepointermaybechanged topointtoadifferent constantinteger. 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. Anypointersetto0iscalled a nullpointer, and since there is no memory location 0, it is an invalidpointer. Oneshouldgenerally check whetherapointerisnullbeforedereferencingit. Pointers are often set to 0 to signal that they are not currently valid. Dereferencingpointerstodatathathasbeenerasedfrommemoryalsousually causesruntime 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 beinvalid. 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, yAfterthesedeclarations,changing x will change y and viceversa,becausethey aretwonames for the same thing. References arejustpointersthat aredereferenced every timethey are used. Justlikepointerrs 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 & Theusageofthe* and & operatorswithpointers/referencescanbeconfusing. 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, * isplacedbeforethe 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. toindicateareferencedatatype(asin int &x;), or 2. totaketheaddressof avariable(asin 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 elemmen of myArray. This explains why arrays are always passed by reference: passing an array is really passing apointer. 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 ++; 0, 9, 6}; 5 4 long *ptr2 = arr + 3;When we add 1 to ptr inline3, wedon’tjust want to move to the nextbytein 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,wecanadd/subtracttwopointers: ptr2 -ptr givesthenumberofarray elements between ptr2 and ptr (2). All addition and subtraction operations on pointers use the appropriate step size. 4.1.2 Array Access Notations Becauseof theinterchangeability ofpointersand 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. Analternativeis pointer-offset notation,inwhichyouexplicitly addyouroffsettothepointer 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, dependingonhowyoutrytodoit. (Stringliteralsareloadedintoread-onlyprogrammemory 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 MIT OpenCourseWare http://ocw.mit.edu 6.096 Introduction to C++ January (IAP) 2011 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.