Pointers and Memory Allocation p. 1. Brooklyn College. Michael Lampis. CISC 3130 Notes. Pointers and Memory Allocation

Transcription

1 Pointers and Memory Allocation CISC 3130 Notes Michael Lamis Brooklyn College Pointers and Memory Allocation. 1

2 int x; Pointers x Pointers and Memory Allocation. 2

3 Pointers int x; int *, *q; Note:Not the same as int *,q; q x Pointers and Memory Allocation. 2

4 int x; int *, *q; = &x; Pointers q x Pointers and Memory Allocation. 2

5 int x; int *, *q; = &x; q = ; Pointers q x Pointers and Memory Allocation. 2

6 int x; int *, *q; = &x; q = ; x = 5; Pointers q x 5 Pointers and Memory Allocation. 2

7 Pointers int x; int *, *q; = &x; q = ; x = 5; Note:Now * == *q == 5 q x 5 Pointers and Memory Allocation. 2

8 int x; int *, *q; = &x; q = ; x = 5; * = 7; Pointers q x 7 Pointers and Memory Allocation. 2

9 Pointers int x; int *, *q; = &x; q = ; x = 5; * = 7; Note:Now x == *q == 7 q x 7 Pointers and Memory Allocation. 2

10 Pointers int x; int *, *q; = &x; q = ; x = 5; * = 7; q What is the difference between = q; and * = *q ;? x 7 Pointers and Memory Allocation. 2

11 Pointers int x; int *, *q; = &x; q = ; x = 5; * = 7; int **; Pointers to ointers are double-cool! q x 7 Pointers and Memory Allocation. 2

12 Pointers int x; int *, *q; = &x; q = ; x = 5; * = 7; int **; = &; Now ** == * == x == 7 q x 7 Pointers and Memory Allocation. 2

13 int *; The new oerator Pointers and Memory Allocation. 3

14 The new oerator int *; = new int; Pointers and Memory Allocation. 3

15 The new oerator int *; = new int[3]; Pointers and Memory Allocation. 3

16 The new oerator int *; = new int[3]; Notice that is ointing to the beginning of the new array Pointers and Memory Allocation. 3

17 The new oerator int *; = new int[3]; *( + 2) = 7; We can access the rest with ointer arithmetic 7 Pointers and Memory Allocation. 3

18 The new oerator int *; = new int[3]; *( + 2) = 7; *( + 3) = 8; But don t go out of bounds!! 7 8 Pointers and Memory Allocation. 3

19 The new oerator int *; = new int[3]; *( + 2) = 7; *( + 3) = 8; [1] = 6; Pointers can be treated like arrays. [i] == *(+i) Pointers and Memory Allocation. 3

20 Pointers as arrays Pointers and arrays are basically the same thing (syntactic sugar). int a[10]; int *b; b = a; //Comiler does not comlain b[2] = 5; //a[2] == 5 now as well *(a+2) = 6; // now both change to 6 Pointers and Memory Allocation. 4

21 Arrays and function calls Array-Pointer equivalence can be confusing when dealing with functions. void myfunc(int * a); is the same as void myfunc(int a[]); The function may change the contents of an array argument (unless const is used). An array argument is not coied The function does not know the array size Pointers and Memory Allocation. 5

22 Array sizes and functions void myfunc(int a[]) { cout << sizeof(a)/sizeof(a[0]) << endl; } int main() { int a[10]; cout << sizeof(a)/sizeof(a[0]) << endl; myfunc(a); } Guess the outut of this rogram... Pointers and Memory Allocation. 6

23 Call-by-Value Normally, when you call a function the argument are coied into new locations before being assed on to the function. Always kee this in mind when assing a comlex object as argument to a function! When giving an array as argument however, a ointer to the first element is coied and given to the function. When a function is given ointers as arguments it may change data values in the main rogram. This is the standard way to do call-by-reference in C. Pointers and Memory Allocation. 7

24 Swa function in C void swa(int *a, int *b) { int tm; tm = *a; *a = *b; *b = tm; } int main() { int x,y; x = 2; y = 3; swa(&x,&y); //Notice the & } Pointers and Memory Allocation. 8

25 Call by reference in C++ Though you can do the same in C++, a more convenient way is offered: define some function arguments as references. Pros The arguments are not coied (can save sace and time and save your from bugs) The function can change their values No need for annoying *s all over the lace Cons As when using ointers, ref arguments must be valid left-hand-side exressions. If f() is a function that takes an int reference, you cannot say f(x3)+. Generally, a left-hand-side exression is anything you are allowed to use to the left of a =. Pointers and Memory Allocation. 9

26 Swa function in C++ void swa(int &a, int &b) { int tm; tm = a; a = b; b = tm; } int main() { int x,y; x = 2; y = 3; swa(x,y); //Notice the absence of & } Pointers and Memory Allocation. 10

27 The delete oerator Dynamic memory allocation gives great ower. With great ower comes great resonsibility! Clean u after yourself! int *x; x = new int;... delete x; //Give memory back when you don t need it x = new int[10]; //Now x can be reused... delete [] x; //Give array back Pointers and Memory Allocation. 11

28 Memory management Memory leaks int *; = new int[10];... = new int[10]; //Now you cannot access the old array! //It is lost until your rogram exits! Pointers and Memory Allocation. 12

29 Memory management Dangling ointers int *,*q; = new int[10]; q = ;... delete [] ; q[2] = 7; //That s a No-No! //This memory is not yours any more delete [] q; //That s a No-No! //You have deleted this already! Pointers and Memory Allocation. 13

30 Guess the outut int *1, *2; 1 = new int; 2 = new int; *1 = 100; *2 = 200; cout << *1 << ", " << *2 << endl; delete 1; 1 = 2; cout << *1 << ", " << *2 << endl; *1 = 300; cout << *1 << ", " << *2 << endl; *2 = 400; cout << *1 << ", " << *2 << endl; delete 1; Pointers and Memory Allocation. 14