Pointer as Function parameter
Pointer in function parameter list is used to hold address of argument passed during function call. This is also known as call by reference. When a function is called by reference any change made to the reference variable will effect the original variable.
Example: Sorting an array using Pointer
#include <stdio.h> #include <conio.h> void sorting(int *x, int y); void main() { int a[5],b,c; clrscr(); printf("enter 5 nos"); for(b=0; b<5; b++) { scanf("%d",&a[b]); } sorting(a, 5); getch(); } void sorting(int *x, int y) { int i,j,temp; for(i=1; i<=y-1; i++) { for(j=0; j*(x+j+1)) { temp=*(x+j); *(x+j)=*(x+j+1); *(x+j+1)=temp; } } } for(i=0; i<5; i++) { printf("\t%d",*(x+i)); } }
Function returning Pointer
A function can also return a pointer to the calling function. In this case you must be careful, because local variables of function doesn't live outside the function, hence if you return a pointer connected to a local variable, that pointer be will pointing to nothing when function ends.
#include <stdio.h> #include <conio.h> int* larger(int*, int*); void main() { int a=15; int b=92; int *p; p=larger(&a, &b); printf("%d is larger",*p); } int* larger(int *x, int *y) { if(*x > *y) return x; else return y; }
Safe ways to return a valid Pointer.
- Either use argument with functions. Because argument passed to the functions are declared inside the calling function, hence they will live outside the function called.
- Or, use static local variables inside the function and return it. As static variables have a lifetime until main() exits, they will be available througout the program.
Pointer to functions
It is possible to declare a pointer pointing to a function which can then be used as an argument in another function. A pointer to a function is declared as follows,
type (*pointer-name)(parameter); Example : int (*sum)(); //legal declaraction of pointer to function int *sum(); //This is not a declaraction of pointer to function
A function pointer can point to a specific function when it is assigned the name of the function.
int sum(int, int); int (*s)(int, int); s = sum;
s is a pointer to a function sum. Now sum can be called using function pointer s with the list of parameter.
s (10, 20);
Example of Pointer to Function
#include <stdio.h> #include <conio.h> int sum(int x, int y) { return x+y; } int main( ) { int (*fp)(int, int); fp = sum; int s = fp(10, 15); printf("Sum is %d",s); getch(); return 0; }
Output : 25
Complicated Function Pointer Example
You will find a lot of complex function pointer examples around, lets see one such example and try to understand it.
void *(*foo) (int*) ;
It appears complex but it is very simple. In this case (*foo) is a pointer to the function, whose return type is void* and argument is of int* type.
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