1 / 19

Array, Pointer and Reference ( II )

ECE230 Lectures Series. Array, Pointer and Reference ( II ). Ying Wu Electrical & Computer Engineering Northwestern University yingwu@ece.northwestern.edu. What to learn today?. What is a pointer? Relation between array and pointer? Why is pointer an angel/devil? Pointer arithmetic

deron
Télécharger la présentation

Array, Pointer and Reference ( II )

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. ECE230 Lectures Series Array, Pointer and Reference ( II ) Ying Wu Electrical & Computer Engineering Northwestern University yingwu@ece.northwestern.edu

  2. What to learn today? • What is a pointer? • Relation between array and pointer? • Why is pointer an angel/devil? • Pointer arithmetic • References

  3. a int a; Review: variable • Three Properties • Type: • determines how many bytes are allocated for this variable by O/S • Name • is used to address (or retrieve) this variable in system memory stacks • Value • is the content and is used for calculation • What will happen if I declare a variable? • O/S will allocate (reserve) a memory block to hold the value of it • The value can be changed by “assignment” • The value can be retrieved by “addressing” • No one is able to use such a memory block until you let the O/S to recycle it.

  4. Address • “We” and computers have difference languages • “We” store/retrieve information by names • Computers know exactly where the “information” is stored, i.e., the address • Physically, address can be thought as the index of memory units. So, everything in computer memory has an address. • So, how to “bridge” the gap? • Why don’t make the “addresses” accessible? • I.e., to access any content in computers’ memory through manipulating their addresses. • Not all programming language allows this. E.g., Matlab doesn’t have such a concept. • The “pointer” is such a mechanism in C/C++. • It is an angel-devil, as you will see!

  5. count countPtr count 7 7 Core Concept of Pointer • Pointer variables • Contain memory addresses as their values • Normal variables contain a specific value (direct reference) • Pointers contain the address of a variable that has a specific value (indirect reference) • Indirection • Referencing a pointer value

  6. Pointer Declarations and Init. • Pointer declarations • * indicates variable is a pointer int *myPtr; declares a pointer to an int, a pointer of type int * • Multiple pointers require multiple asterisks int *myPtr1, *myPtr2; • Can declare pointers to any data type • Pointers initialization • Initialized to 0, NULL, or an address • 0 or NULL points to nothing

  7. yptr y y 5 500000 600000 600000 5 yPtr address of y is value of yptr Pointer Operators • & (address operator) • Returns the address of its operand • Example int y = 5;int *yPtr;yPtr = &y; // yPtr gets address of y • yPtr “points to” y

  8. int a = 0; int *ptr; a 0 0x100020000 ptr 0x100020004 0xFF2F3FED a = 3; a 0x100020000 ptr 0x100020004 0xFF2F3FED 3 5 a 3 ptr = &a; 0x100020000 ptr 0x100020000 a (*ptr) = 5; 0x100020000 ptr 0x100020000 Examples

  9. Question? • What are the values of pointer variables? • ?? • What are the types of the pointer variables? • A good question! • It is only meaningful to say the type of the variable the pointer pointing to! • Since the value of a ptr is an address, it can point to anything, i.e., *ptr is uncertain UNLESS we know the type! • WHY?

  10. a int a; int *aptr = &a; 0 0x100020000 21493 aptr 0x100020000 double *bptr; bptr = (double*)aprt; 0 0x100020000 21493 bptr 0x100020000 Why uncertain?

  11. Pointer Operators • *(indirection/dereferencing operator) • Returns the value of what its operand points to • *yPtr returns y (because yPtr points to y). • * can be used to assign a value to a location in memory *yptr = 7; // changes y to 7 • Dereferenced pointer (operand of *) must be an lvalue (no constants) • * and & are inverses • Cancel each other out *&myVar == myVar and &*yPtr == yPtr

  12. 1 // Fig. 5.4: fig05_04.cpp The address of a is the value of aPtr. 2 // Using the & and * operators 3 #include <iostream> 4 The * operator returns an alias to what its operand points to. aPtr points to a, so *aPtr returns a. 5 using std::cout; 6 using std::endl; Notice how * and & are inverses 7 8 int main() 9 { 10 int a; // a is an integer 11 int *aPtr; // aPtr is a pointer to an integer 12 13 a = 7; 14 aPtr = &a; // aPtr set to address of a 15 16 cout << "The address of a is " << &a 17 << "\nThe value of aPtr is " << aPtr; 18 19 cout << "\n\nThe value of a is " << a 20 << "\nThe value of *aPtr is " << *aPtr; 21 22 cout << "\n\nShowing that * and & are inverses of " 23 << "each other.\n&*aPtr = " << &*aPtr 24 << "\n*&aPtr = " << *&aPtr << endl; 25 return 0; 26 } 1. Declare variables 2 Initialize variables 3. Print Program Output The address of a is 006AFDF4 The value of aPtr is 006AFDF4 The value of a is 7 The value of *aPtr is 7 Showing that * and & are inverses of each other. &*aPtr = 006AFDF4 *&aPtr = 006AFDF4

  13. location 3000 3004 3008 3012 3016 b[0] b[1] b[2] b[3] b[4] pointer variable bptr Name of an Array is a Pointer! • Arrays and pointers are closely related • Array name is the pointer, which is pointing to the first elements of the array • Pointers can do array subscripting operations • We have an array b[5] and a pointer bPtr, as shown below • bPtr is equal to b bptr == b • bptr is equal to the address of the first element of b bptr == &b[ 0 ]

  14. location 3000 3004 3008 3012 3016 v[0] v[1] v[2] v[3] v[4] pointer variable vPtr Pointer Arithmetic • Pointer arithmetic • Increment/decrement pointer (++ or --) • Add/subtract an integer to/from a pointer( + or += , -or -=) • Pointers may be subtracted from each other • Pointer arithmetic is meaningless unless performed on an array • 5 element int array on a machine using 4 byte ints • vPtr points to first element v[ 0 ], which is at location 3000 • vPtr = 3000 • vPtr += 2; sets vPtr to 3008 • vPtr points to v[ 2 ]

  15. 0x1FFF2000 + 2  0x1FFF2002 ! • Confusion! • Let a pointer aptr point to an integer variable a, whose address is 0x1FFF2000. • Then I want to know now the value of aptr+2 • Strange! Aptr+2  0x1FFF2002! • WHY? • What is the answer? • 0x1FFF2004? • 0x1FFF2008? • 0x1FFF2010?

  16. Pointers vs. Arrays • Accessing array elements with pointers • Element b[n] can be accessed by *( bPtr + n ) • Called pointer/offset notation • Array itself can use pointer arithmetic. • b[ 3 ] same as *(b + 3) • Pointers can be subscripted (pointer/subscript notation) • bPtr[ 3 ] same as b[ 3 ]

  17. a b Reference is an Alias of a var! • Just keep it in mind: a reference is an alias of a variable, not a copy of the variable. • Declare a reference variable int a; int &b = a; • Important! Whenever b changes, a will change, since a and b are the same thing! int c = 0, a = 3; int &b = a; c = a; b = 4; Question: a = ? c = ?

  18. Alert: wild pointer!!! • All C/C++ programmers have an unpleasant experience, even nightmares: a program runs well sometimes, while the same program may crash without a reason, or have very strange behaviors. • Most likely, the reason is due to a wild pointer • A wild pointer jumps here and there (which you can not predict), and ruin your program. • Why? (we’ll see it when studying functions.) • A lesson to learn: Always initialize a pointer/reference before you use it!

  19. Guess and Think! • Subtracting pointers vPtr2 = &(v[ 2 ]);vPtr1 = &(v[ 0 ]); Question: vPtr2 – vPtr1 = ? Answer: 2 Why: Returns the number of elements between two addresses

More Related