Pointer. Pointer variable

Transcription

1 Pointer Pointer variable Pointer variable content is a memory address A variable directly contains a specific value. A pointer, on the other hand, contains an address of a variable that contains a specific value. In this sense, a variable name directly references a value, and a pointer indirectly references a value. Referencing a value through a pointer is called indirection Declaring Pointer Variables datatype* identifier Ex: int *p; char *ch; Equivalent statements: int *p; int* p; int * p; 1

2 Declaring Pointer Variables In the statement: int* p, q; Only pis a pointer variable qis an intvariable To avoid confusion, attach the character *to the variable name: int *p, q; int *p, *q; Declaring Pointer Variables Pointee thing that the pointer points to Pointer -drawn as a box Reference i-rawnas an arrow starting in the box and leading to the pointee. Allocating a pointer and allocating a pointeefor it to point to are two separate steps Ex: int*p; p = malloc(sizeof(int)); *p = 10; p pointer reference 10 pointee 2

3 Declaring Pointer Variables Source: Address of Operator (&) Address of operator (&): Ex: A unary operator that returns the address of its operand int x; int *p; p = &x; Assigns the address of xto p 3

4 Dereferencing/Indirection Operator (*) Deferencing/indirection starts at the pointer and follows its arrow over to access its pointee only works if the pointer has a pointee-the pointeemust be allocated and the pointer must be set to point to it Dereferencing operator (or indirection operator): When used as a unary operator, * refers to object to which its operand points Ex: printf( %d\n,*p); Prints the value stored in the memory location pointed to by p Pointer and Array intmy_array[] = {1,23,17,4,-5,100; int*ptr; inti; ptr = &my_array[0]; /* point our pointer to the first element of the array */ printf("\n\n"); for (i= 0; i< 6; i++) { printf("my_array[%d] = %d ",i,my_array[i]); /*<--A */ printf("ptr+ %d = %d\n",i, *(ptr+ i)); /*<--B */ 4

5 2 ways to pass arguments to a function pass-by-value pass-by-reference All arguments in C are passed by value. Many functions require the capability to modify variables in the caller or to pass a pointer to a large data object to avoid the overhead of passing the object by value (which incurs the time and memory overheads of making a copy of the object). In C, you use pointers and the indirection operator to simulate pass-by-reference. When calling a function with arguments that should be modified, the addresses of the arguments are passed. This is normally accomplished by applying the address operator (&) to the variable (in the caller) whose value will be modified. Arrays are not passed using operator & because C automatically passes the starting location in memory of the array (the name of an array is equivalent to &arrayname[0]). When the address of a variable is passed to a function, the indirection operator (*) may be used in the function to modify the value at that location in the caller s memory. 5

6 Pass-By-Value Pass-By-Reference A function receiving an address as an argument must define a pointer parameter to receive the address 6

7 Use pass-by-value Pass arguments to a function for normal function call when no direct change of variable in the caller environment Use pass-by-reference The caller explicitly requires the called function to modify the value of the argument variable in the caller s environment Passing Array during Function Call void reversechar(char letters[], int s) { intj; printf("reverse print:\n"); for (j = s-1; j>=0; j--) { printf("%c", letters[j]); printf("\n"); Function call: char myarray[] = hello ; reversechar(myarray, 10); 7

8 Passing Array during Function Call Issue Passing the array directly into function, any changes apply to the array occur in the function will cause the array content changed in the caller environment Due to passing array to function is actually passing the based address of the array to the function Solution: Set constat the parameter void reversechar(const char letters[]) Compiler will prompt error if any change to the array is attempted in the function Passing Array during Function Call Passing array in using pointer void reversechar(char *pletters) { while(*pletters!= \0 ) { printf( %c, *pletters); ++pletters; 8

9 References Malik, D.S. (2012) C++ Programming: From Problem Analysis to Program Design, Course Technology, 6 th Ed Liang, Y. D Introduction to Programming with C++. Pearson Education, Inc. Chapter 7 Deitel, P C: How to Program. Pearson Education, Inc, Chapter 7 Pointer Basic, 9