Pointers In 'c' Programming Language

Pointers in 'C'

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Introduction    

Pointers are a special kind of variable To use these new operators are needed Pointers will permit our programs to look 'around corners' Most importantly pointers will permit us to manage memory allocated on the 'heap'

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What is a Pointer?   

Regular variables contain values Pointers are variables that contain the address of another variable As a result we say that a pointer "points to" or "references" another variable

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Declaration of a Pointer 

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To declare a variable a pointer we need a new operator to 'modify' the declaration statement Placing a '*' to the left of a variable name declares that variable to be a pointer

Code Example: int x = 7; int *xp;

// integer x contains value '7' // declares xp as a pointer to // an integer ePublicist.ca © 2007, 2008, 2009

Pointer Assignment  

To assign an address to a pointer we'll need a new operator, the "address of" operator Once assigned, the pointer will contain the "address of" the assigned variable not it's value

Code Example: int *xp; xp = &x; cout < xp;

// declares xp as a pointer to an integer // xp receives the address of 'x' // outputs address of 'x's location

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Dereferencing a Pointer  

To work with the variable the pointer 'references' we'll need yet another operator When used in an executable statement the '*' now means "value of" or "contents of"

Code Example: xp = &x; cout < *xp; *xp = 9;

// xp receives the address of 'x' // outputs value contained by 'x' // changes value contained by 'x' to 9

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Pointer Parameters  

Function parameters can also be pointers The use of pointers as parameters 'extends' the functions ability to affect variables outside it's scope

Code Example: void swap( int *xp, int *yp){ int temp = *xp; // save value of var xp points to *xp = *yp; // assign value of var yp points // to to var xp points to *yp = temp; // assign saved value to var yp } // points to ePublicist.ca © 2007, 2008, 2009

Arrays as Pointers 

Paradoxically, an array variable is essentially a "locked" or const pointer to the first element of an array

Code Example: int iarray[] = { 5,7,9,11,15}; int *iaxp = iarray; // note no need for '&' cout < *iaxp; // outputs 5 cout < iarray[0]; // outputs 5 cout < *iarray; // outputs 5 ePublicist.ca © 2007, 2008, 2009

Pointer Arithmetic  

If the simple expression x + 2 results in a value equal to the contents of the var x plus 2 Then xp + 2 results in the address addressed two integer width after the address contained by xp

Code Example: long x[6]; long *lp = x; *lp = 100; lp++; *lp = 200;

// equivalent to long *lp = &x[0]; // x[0] now contains 100 // lp now points to x[1] // x[1] now contains 100 ePublicist.ca © 2007, 2008, 2009

Array as a Parameter 

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When we pass an array as an argument to a function we are in essence passing the address of the first element of that array, hence the parameter must be a pointer We can also return a pointer as a function's return value See the following demonstration:

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Array as a Parameter // Array as argument example #include <stdio.h> Char *SkipFirst( char string[]){ Return string+1; } Void main() { char word[] = "array"; char str*; str = SkipFirst( word ); Puts( str ); // outputs 'rray' } ePublicist.ca © 2007, 2008, 2009

Dynamic Memory Allocation   

To manage large data structures more effectively we need to store them on the 'heap' The memory allocation functions return the starting address of the memory allocated To calculate the amount of memory we need to request we can use a sizeof operator to tell us the size of a variable or data structure in bytes

Code Example: int int_size = sizeof(int); printf("The size of int is %d\n", int_size); ePublicist.ca © 2007, 2008, 2009

Malloc Function 

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The maaloc() function expects an unsigned integer ( size_t ) as an argument. This indicates the number of bytes of memory to allocate It returns an address devoid of type information, in other words a pointer to void ( void* ) In order to accept the returned address we'll need to cast it to the type of pointer we'll be using to reference

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Malloc Function Demo Code Example: #include <stdio.h> #include void main() { int toal, *ap; int total = 50; ap = (int *)malloc( total * sizeof(int)); if(ap ==NULL) { printf("Failed to allocate memory...\n"); return; } } ePublicist.ca © 2007, 2008, 2009

Pointers to structs 

Pointers to structs permit us to point to the component variables that make up a struct

struct Bike{ float speed; unsigned gears; long lisence; }; struct Bike bk; struct Bike *bkptr = &bk; (*bkptr).speed = 10; bkptr->speed = 10; ePublicist.ca © 2007, 2008, 2009

Summary    

Pointers are powerful extensions to variables We need pointers to be able to work with the heap Pointers also simply the way we work with both arrays and structs For a copy of all demo files included in this presentation write [email protected] with "C Pointers Demo" in the subject line ePublicist.ca © 2007, 2008, 2009