© M. Gross, ETH Zürich, 2014 Informatik I für D-MAVT (FS 2014) Exercise 7 – Pointers

© M. Gross, ETH Zürich, 2014 Agenda  Pointers  Defining pointers  Reference operator &  Dereference operator *  Dynamic memory allocation  new and delete  Pointers and arrays  Pointer arithmetic  Dynamic array allocation 2

© M. Gross, ETH Zürich, 2014 Pointers: Overview  Computer memory  Large collection of consecutive memory blocks (1 Byte == 8 bit)  Each memory block has a unique address  Whenever a variable is defined, it is assigned to a memory location  This is where its value is stored  A pointer is a variable that points to another variable  It stores the memory address of the variable it points to instead of the value itself 3

© M. Gross, ETH Zürich, 2014 Pointers: Syntax  Definition  Example:  Attention while defining multiple pointers  * has to be in front of every pointer 4 TypeName *VariableName; int *pointer; int *a, *b, *c;

© M. Gross, ETH Zürich, 2014 Pointers: Syntax  Reference operator &  Returns the address of a variable  Dereference operator *  Accesses the memory location of a pointer 5 float a = 1.5; // float variable float *p; // pointer to float p = &a; // p points to a // (p stores address of a) float b = *p; // assign value at location p to b *p = 3.2; // assign 3.2 to memory location p

© M. Gross, ETH Zürich, 2014 Pointers: Syntax  Don’t confuse * with * 6 int a, b; int *p; p = &a; *p = 5; b = *p; pointer declaration dereference operator

© M. Gross, ETH Zürich, 2014 Pointers: Example 7 Memory address 0x1000 0x1004 0x1008 0x100C 0x1010 int a, b; a b

© M. Gross, ETH Zürich, 2014 Pointers: Example 8 Memory address 0x1000 0x1004 0x1008 0x100C 0x1010 int a, b; a = 12; b = 5; a b 12 5

© M. Gross, ETH Zürich, 2014 Pointers: Example 9 Memory address 0x1000 0x1004 0x1008 0x100C 0x1010 int a, b; a = 12; b = 5; int *p; a b 12 5 p

© M. Gross, ETH Zürich, 2014 Pointers: Example 10 Memory address 0x1000 0x1004 0x1008 0x100C 0x1010 int a, b; a = 12; b = 5; int *p; p = &a; a b 12 5 p 0x1000

© M. Gross, ETH Zürich, 2014 Pointers: Example 11 Memory address 0x1000 0x1004 0x1008 0x100C 0x1010 int a, b; a = 12; b = 5; int *p; p = &a; *p = 36; a b 36 5 p 0x1000

© M. Gross, ETH Zürich, 2014 Pointers: Example 12 Memory address 0x1000 0x1004 0x1008 0x100C 0x1010 int a, b; a = 12; b = 5; int *p; p = &a; *p = 36; b = *p; a b 36 p 0x1000

© M. Gross, ETH Zürich, 2014 Pointers: Example 13 Memory address 0x1000 0x1004 0x1008 0x100C 0x1010 int a, b; a = 12; b = 5; int *p; p = &a; *p = 36; b = *p; a = 23; a b p 0x1000

© M. Gross, ETH Zürich, 2014 Pointers: Example 14 Memory address 0x1000 0x1004 0x1008 0x100C 0x1010 int a, b; a = 12; b = 5; int *p; p = &a; *p = 36; b = *p; a = 23; cout << *p;// prints: 23 cout << p;// prints: 0x1000 a b p 0x1000

© M. Gross, ETH Zürich, 2014 Pointers 15

© M. Gross, ETH Zürich, 2014 Pointers  Pointers are important for  Allocating memory during runtime (instead of compile time) e.g., if you want to declare an array, but decide on the size of the array while the program is running  more flexibility  Passing pointers as function arguments This allows the function to change values outside the scope of the function  The exam! 16

© M. Gross, ETH Zürich, 2014 Dynamic Memory Allocation  new operator  Allocates memory and returns address  Memory needs to be freed manually  delete operator  Frees memory at given address  For arrays, use delete[] instead of delete 17 int *a_p = new int; *a_p = 5; delete a_p;

© M. Gross, ETH Zürich, 2014 Static  Syntax:  Memory valid only as long as variable is valid (scope)  Memory is freed implicitly at the end of the scope Dynamic  Syntax:  Memory location valid until delete is called  Memory is blocked until freed explicitly 18 int x;int *x = new int; Dynamic Memory Allocation

© M. Gross, ETH Zürich, 2014 Pointers and arrays  Arrays are internally represented by pointers  myArray and &myArray[0] are the same, they are the address of the first element of the array 19 int myArray[5] = {1,2,3,4,5};// static array int *pArray;// pointer pArray = &myArray[0]; // pointer to 1st element pArray = myArray;// pointer to 1st element

© M. Gross, ETH Zürich, 2014 Pointer Arithmetic  Increment ++ / Decrement –-  Move pointer one element forward or backward  Add + / Subtract -  Returns new pointer moved by an arbitrary number of elements  Example: 20 int a[5] = {0}; int *a_p = a; *(a_p + 4) = 4; *++a_p = 1; *a_p++ = 2;

© M. Gross, ETH Zürich, 2014 Pointer Arithmetic  [] brackets vs. dereferencing operator * 21 int tacos[5]; tacos[0]  *tacos  the value at address tacos tacos[3]  *(tacos+3)  value at address (tacos + 3)

© M. Gross, ETH Zürich, 2014 Dynamic Array Allocation  With new, the array size can be specified at run time  Example:  For arrays, use delete[] instead of delete 22 int size; cin >> size; // Allocate memory of the needed size int *a_p = new int[size]; a_p[3] = 5;// Use as regular array // Free memory delete[] a_p;

© M. Gross, ETH Zürich, 2014 Dynamic Struct Allocation  Another example with structs: 23 struct Student // Structure delcaration { char vorname[20]; char name[20]; int legi; }; Student* stud1 = new Student;// Dynamic allocation (*stud1).legi=23432;// Assign values strcpy(stud1->vorname,"Hans"); strcpy(stud1->name,"Muster"); // print cout vorname name << endl; cout legi << endl; delete stud1;// Free memory

© M. Gross, ETH Zürich, 2014 Summary  Pointers store the address to a value, not the value itself  To access the value, use the dereference operator *  To get the address of a variable, use the reference operator &  Use new to create arrays of arbitrary size  Always use delete or delete[] if you use new 24