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Pointers

Pointers. Overview. What are Pointers? How to use Pointers? Use of Pointers. Pointer:. A variable that can only store the address of a memory location (usually the address of other variable). x 500. a 300. 300. pointer. pointee. Syntax:. type * identifier; i.e. int *p;. p 258.

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Pointers

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  1. Pointers

  2. Overview • What are Pointers? • How to use Pointers? • Use of Pointers

  3. Pointer: • A variable that can only store the address of a memory location (usually the address of other variable). x 500 a 300 300 pointer pointee

  4. Syntax: • type * identifier; i.e. int *p; p 258 G pointer

  5. int a=50; • int * p; • p = &a; p 500 ppoints to a a 300 50 300 pointer pointee

  6. * and & operators • Dereferencing *p value at where pointer p is pointing • Address of &a address of variable a

  7. Example: void main(){ int *p; int a=50; p = &a; cout<<a<<endl; cout<<&a<<endl; cout<<p<<endl; cout<<*p<<endl; cout<<&p<<endl; }

  8. Example: void main(){ => int *p; int a=50; p = &a; cout<<a<<endl; cout<<&a<<endl; cout<<p<<endl; cout<<*p<<endl; cout<<&p<<endl; } p 500 pointer

  9. Example: void main(){ int *p; => int a=50; p = &a; cout<<a<<endl; cout<<&a<<endl; cout<<p<<endl; cout<<*p<<endl; cout<<&p<<endl; } p 500 a 300 50 pointer pointee

  10. Example: void main(){ int *p; int a=50; =>p = &a; cout<<a<<endl; cout<<&a<<endl; cout<<p<<endl; cout<<*p<<endl; cout<<&p<<endl; } p 500 a 300 300 50 pointer pointee

  11. Example: void main(){ int *p; int a=50; p = &a; => cout<<a<<endl; // 50 cout<<&a<<endl; cout<<p<<endl; cout<<*p<<endl; cout<<&p<<endl; } p 500 a 300 300 50 pointer pointee

  12. Example: void main(){ int *p; int a=50; p = &a; cout<<a<<endl; // 50 => cout<<&a<<endl; // 300 cout<<p<<endl; cout<<*p<<endl; cout<<&p<<endl; } p 500 a 300 300 50 pointer pointee

  13. Example: void main(){ int *p; int a=50; p = &a; cout<<a<<endl; // 50 cout<<&a<<endl; // 300 => cout<<p<<endl; // 300 cout<<*p<<endl; cout<<&p<<endl; } p 500 a 300 300 50 pointer pointee

  14. Example: void main(){ int *p; int a=50; p = &a; cout<<a<<endl; // 50 cout<<&a<<endl; // 300 cout<<p<<endl; // 300 => cout<<*p<<endl; // 50 cout<<&p<<endl; } p 500 a 300 300 50 pointer pointee

  15. Example: void main(){ int *p; int a=50; p = &a; cout<<a<<endl; // 50 cout<<&a<<endl; // 300 cout<<p<<endl; // 300 cout<<*p<<endl; // 50 => cout<<&p<<endl; // 500 } p 500 a 300 300 50 pointer pointee

  16. Crash!!! Example 2: void main(){ int *p; int a=50; cout<<*p<<endl; } What would be the output? Runtime error!!! p 500 a 300 50 pointer pointee

  17. Dangling Pointer int *p; Note: Dereferencing a dangling pointer is a serious runtime error. p 500 G Dangling pointer pointer

  18. Problem: We can’t differentiate that whether a pointer is dangling or has a valid address. • What’s the solution?

  19. NULL Pointer int *p; p = NULL; // points to // nothing Tip: Always initialize a pointer with NULL if you don’t have a valid address. This can save a lot of your time on debugging. p 500 pointer

  20. Example: void main(){ int *p = NULL; int a=50; p = &a; cout<<a<<endl; cout<<&a<<endl; cout<<p<<endl; cout<<*p<<endl; cout<<&p<<endl; }

  21. Pointer Assignment • An assignment operation b/w two pointers makes them points to the same pointee.

  22. ppoints to a 300 • int a=50; • int * p; • p = &a; • int p2 = p; p 500 p2 200 a 300 300 50 Sharing pointer pointer pointee Note: Pointer assignment only copies the address and not the memory they are pointing to.

  23. Shallow Copy ppoints to a p 500 p2 200 a 300 300 300 50 pointer pointer pointee Sharing

  24. Problem with Shallow Copy What if p delete a? ppoints to a p 500 p2 200 a 300 300 300 50 pointer pointer pointee Wild Pointer

  25. What is the solution to the problem?

  26. Deep Copy ppoints to a p 500 p2 200 a 300 a2 100 300 100 50 50 pointer pointer pointee pointee

  27. Pointer type and Arithmetic • What is type of pointer? int * p; A pointer has not type!!! It’s the type of variable the pointer p will point to.

  28. Why we need to specify the type of variable a pointer points to?

  29. 1. int a=10; int *p = &a; *p = 4; // how many bytes of copy? It would be 1 byte in case a was a char.

  30. 2. Pointer & Arrays 40 42 44 46 48 50 52 int arr[ ]={5,6,7,8,4,3,5}; // size of arr? int *p = arr; // why & isn’t use before arr? cout<<arr<<endl; // 40 cout<<p<<endl; // 40 p=p+3; p 500 40 pointer

  31. 2. Pointer & Arrays 40 42 44 46 48 50 52 int arr[ ]={5,6,7,8,4,3,5}; // size of arr? int *p = arr; // why & isn’t use before arr? cout<<arr<<endl; // 40 cout<<p<<endl; // 40 p=p+3; cout<<p<<endl; // 46 cout<<*p<<endl; // 8 p 500 40 pointer

  32. Example 3: void main( ){ => int arr[ ]={5,6,7,8,4,3,5}; int *p = arr; cout<< arr<<endl; cout<<*arr<<endl; cout<<*(arr+4)<<endl; cout<<p<<endl; cout<<&p<<endl; cout<<*p<<endl; cout<<p[3]<<endl; cout<<p++<<endl; cout<<++*p<<endl; } 40 42 44 46 48 50 52

  33. Example 3: void main( ){ int arr[ ]={5,6,7,8,4,3,5}; => int *p = arr; cout<< arr<<endl; cout<<*arr<<endl; cout<<*(arr+4)<<endl; cout<<p<<endl; cout<<&p<<endl; cout<<*p<<endl; cout<<p[3]<<endl; cout<<p++<<endl; cout<<++*p<<endl; } 40 42 44 46 48 50 52 p 500 40 pointer

  34. Example 3: void main( ){ int arr[ ]={5,6,7,8,4,3,5}; int *p = arr; => cout<< arr<<endl; // 0x40 cout<<*arr<<endl; cout<<*(arr+4)<<endl; cout<<p<<endl; cout<<&p<<endl; cout<<*p<<endl; cout<<p[3]<<endl; cout<<p++<<endl; cout<<++*p<<endl; } 40 42 44 46 48 50 52 p 500 40 pointer

  35. Example 3: void main( ){ int arr[ ]={5,6,7,8,4,3,5}; int *p = arr; cout<< arr<<endl; // 0x40 => cout<<*arr<<endl; // 5 cout<<*(arr+4)<<endl; cout<<p<<endl; cout<<&p<<endl; cout<<*p<<endl; cout<<p[3]<<endl; cout<<p++<<endl; cout<<++*p<<endl; } 40 42 44 46 48 50 52 p 500 40 pointer

  36. Example 3: void main( ){ int arr[ ]={5,6,7,8,4,3,5}; int *p = arr; cout<< arr<<endl; // 0x40 cout<<*arr<<endl; // 5 => cout<<*(arr+4)<<endl; // 4 cout<<p<<endl; cout<<&p<<endl; cout<<*p<<endl; cout<<p[3]<<endl; cout<<p++<<endl; cout<<++*p<<endl; } 40 42 44 46 48 50 52 p 500 40 pointer

  37. Example 3: void main( ){ int arr[ ]={5,6,7,8,4,3,5}; int *p = arr; cout<< arr<<endl; // 0x40 cout<<*arr<<endl; // 5 cout<<*(arr+4)<<endl; // 4 => cout<<p<<endl; //0x40 cout<<&p<<endl; cout<<*p<<endl; cout<<p[3]<<endl; cout<<p++<<endl; cout<<++*p<<endl; } 40 42 44 46 48 50 52 p 500 40 pointer

  38. Example 3: void main( ){ int arr[ ]={5,6,7,8,4,3,5}; int *p = arr; cout<< arr<<endl; // 0x40 cout<<*arr<<endl; // 5 cout<<*(arr+4)<<endl; // 4 cout<<p<<endl; //0x40 => cout<<&p<<endl; //0x500 cout<<*p<<endl; cout<<p[3]<<endl; cout<<p++<<endl; cout<<++*p<<endl; } 40 42 44 46 48 50 52 p 500 40 pointer

  39. Example 3: void main( ){ int arr[ ]={5,6,7,8,4,3,5}; int *p = arr; cout<< arr<<endl; // 0x40 cout<<*arr<<endl; // 5 cout<<*(arr+4)<<endl; // 4 cout<<p<<endl; //0x40 cout<<&p<<endl; //0x500 => cout<<*p<<endl; // 5 cout<<p[3]<<endl; cout<<p++<<endl; cout<<++*p<<endl; } 40 42 44 46 48 50 52 p 500 40 pointer

  40. Example 3: void main( ){ int arr[ ]={5,6,7,8,4,3,5}; int *p = arr; cout<< arr<<endl; // 0x40 cout<<*arr<<endl; // 5 cout<<*(arr+4)<<endl; // 4 cout<<p<<endl; //0x40 cout<<&p<<endl; //0x500 cout<<*p<<endl; // 5 => cout<<p[3]<<endl; // 8 cout<<p++<<endl; cout<<++*p<<endl; } 40 42 44 46 48 50 52 p 500 40 pointer

  41. Example 3: void main( ){ int arr[ ]={5,6,7,8,4,3,5}; int *p = arr; cout<< arr<<endl; // 0x40 cout<<*arr<<endl; // 5 cout<<*(arr+4)<<endl; // 4 cout<<p<<endl; //0x40 cout<<&p<<endl; //0x500 cout<<*p<<endl; // 5 cout<<p[3]<<endl; // 8 => cout<<p++<<endl; // 0x40 cout<<++*p<<endl; } 40 42 44 46 48 50 52 p 500 40 pointer

  42. Example 3: void main( ){ int arr[ ]={5,6,7,8,4,3,5}; int *p = arr; cout<< arr<<endl; // 0x40 cout<<*arr<<endl; // 5 cout<<*(arr+4)<<endl; // 4 cout<<p<<endl; //0x40 cout<<&p<<endl; //0x500 cout<<*p<<endl; // 5 cout<<p[3]<<endl; // 8 cout<<p++<<endl; // 0x40 => cout<<++*p<<endl; // 7 } 40 42 44 46 48 50 52 p 500 40 pointer

  43. Arrays of Pointer data_type *arr[size]; i.e. int *a[7]; char *c[7]; 40 42 44 46 48 50 52 80 82 84 86 88 90 92

  44. Uses: • Sharing & Cost saving • Pass-by-ref • Dynamic Memory Allocation • Making Complex Data Structures • Linked list, Tree etc.

  45. 1. Sharing & cost saving void main(){ int a=5,b=8; swap(&a,&b); } void swap(int *a,int *b){ int t = *a; *a = *b; *b = t; }

  46. 2. DMA void main(){ int size=0; int *p=NULL; cout<<“Enter No. of Students”<<endl; cin>>size; p = (int *)malloc(size*sizeof(int)); for(int i=0;i<size;i++){ cout<<“Enter St “<<i<<“ Marks”<<endl; cin>>p[i]; //= cin>>*p; p++; } }

  47. 3. Data Structures Data Pointer Data Pointer Data Pointer Data Pointer

  48. References: • Pointer And Memory (http://cslibrary.stanford.edu/102/) • Pointer Basics (http://cslibrary.stanford.edu/106/) • http://www.cprogramming.com/tutorial/c/lesson6.html • http://www.cs.cf.ac.uk/Dave/C/node10.html

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