Pointer Arithmetic. Lecture 4 Chapter 10. Robb T. Koether. Hampden-Sydney College. Wed, Jan 25, 2017

Transcription

1 Pointer Arithmetic Lecture 4 Chapter 10 Robb T. Koether Hampden-Sydney College Wed, Jan 25, 2017 Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

2 1 Pointer Arithmetic Pointer + int Pointer - Pointer 2 Pointers and Arrays 3 Pointer Compatibility 4 Endianness 5 Assignment Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

3 Outline 1 Pointer Arithmetic Pointer + int Pointer - Pointer 2 Pointers and Arrays 3 Pointer Compatibility 4 Endianness 5 Assignment Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

4 Outline 1 Pointer Arithmetic Pointer + int Pointer - Pointer 2 Pointers and Arrays 3 Pointer Compatibility 4 Endianness 5 Assignment Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

5 Pointer + int Pointer + int int i = 123; int* p = &i; int* q = p + 1; p++; It is permissible to add an integer to a pointer. The integer is multiplied by the size of the object pointed to. That value is added to the address. The result is a pointer of the same type. Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

6 Pointer + int When an integer is added to a pointer, the integer is interpreted as the number of objects of that type from the original address to the new address. The result is a pointer that points to the object at the new address. Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

7 Pointer Arithmetic Pointer Arithmetic int i = 123; // Addr of i is 0x0100 int* p = &i; // p = 0x0100 char* q = (char*)&i; // q = 0x0100 short* r = (short*)&i; // r = 0x0100 0x0100 (i) Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

8 Pointer Arithmetic Pointer Arithmetic int i = 123; // Addr of i is 0x0100 int* p = &i; // p = 0x0100 char* q = (char*)&i; // q = 0x0100 short* r = (short*)&i; // r = 0x0100 p - 1 p p + 1 p + 2 0x00fc 0x0100 (i) 0x0104 0x0108 Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

9 Pointer Arithmetic Pointer Arithmetic int i = 123; // Addr of i is 0x0100 int* p = &i; // p = 0x0100 char* q = (char*)&i; // q = 0x0100 short* r = (short*)&i; // r = 0x0100 q - 1 q q + 1 q + 2 0x00fc 0x0100 (i) 0x0104 0x0108 Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

10 Pointer Arithmetic Pointer Arithmetic int i = 123; // Addr of i is 0x0100 int* p = &i; // p = 0x0100 char* q = (char*)&i; // q = 0x0100 short* r = (short*)&i; // r = 0x0100 r - 1 r r + 1 r + 2 0x00fc 0x0100 (i) 0x0104 0x0108 Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

11 Example Example (Example) PtrPlusInt.cpp Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

12 Outline 1 Pointer Arithmetic Pointer + int Pointer - Pointer 2 Pointers and Arrays 3 Pointer Compatibility 4 Endianness 5 Assignment Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

13 Pointer Subtraction Subtraction of one pointer from another pointer (of the same type) is permitted. Take the difference of the addresses. Divide it by the size of the object pointed to. The result is an int. Interpret the result as the number of objects of that type from the first address to the second address. Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

14 Pointer Subtraction Pointer Subtraction int* p; int* q; int a = q - p; p q Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

15 Pointer Subtraction Pointer Subtraction int* p; int* q; int a = q - p; p q - p = 5 q Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

16 Example Example (Example) PtrMinusPtr.cpp Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

17 Pointer Arithmetic Pointer Arithmetic int a = (ptr2 - ptr1) + 5; int b = ptr2 - (ptr1-5); int c = (ptr2 + 5) - ptr1; These are legal operations. Are they equivalent? Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

18 Pointer Arithmetic Illegal Addition int* mid = (ptr2 + ptr1)/2; Pointer addition is illegal. How can we obtain a pointer to the object halfway between p1 and p2? Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

19 Outline 1 Pointer Arithmetic Pointer + int Pointer - Pointer 2 Pointers and Arrays 3 Pointer Compatibility 4 Endianness 5 Assignment Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

20 Pointers and Constants Pointers and Constants int i = 456; int const j = 123; int const * ptr1 = &j; int * const ptr2 = &i; int const * const ptr3 = &j; A pointer may point to a constant the object pointed to cannot be changed. A pointer itself may be constant the pointer cannot be changed. In fact, a constant pointer may point to a constant object! Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

21 An Array Name as a Pointer Pointers and Arrays int a[3] = {10, 20, 30}; // a is int* const int* ptr = a; // ptr is int* cout << *ptr << endl; cout << *(ptr + 1) << endl; cout << *(ptr + 2) << endl; An array name represents a pointer that points to the first member of the array. The array name is a constant pointer; its value cannot be changed. An array name may be assigned to a pointer of the same type. Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

22 A Pointer as an Array Name Pointers and Arrays int a[3] = {10, 20, 30}; int* ptr = a; cout << ptr[0] << endl; cout << ptr[1] << endl; cout << ptr[2] << endl; A pointer name may be indexed. ptr[i] is equivalent to *(ptr + i). Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

23 Example Example (Example) PointerLoop.cpp Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

24 Outline 1 Pointer Arithmetic Pointer + int Pointer - Pointer 2 Pointers and Arrays 3 Pointer Compatibility 4 Endianness 5 Assignment Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

25 Pointers to void A pointer to void, i.e., a void* object, is a pointer that does not point to any particular kind of object. There is no such thing as a void object. void is treated as a neutral type for situations where the type does not matter or is unknown. A void pointer may hold the address of any type of object. A void pointer may not be dereferenced. Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

26 Assignment of Pointers The assignment ptr = NULL is valid for any type of pointer. The pointer assignment ptr1 = ptr2 is legal if and only if ptr1 and ptr2 point to the same type of object, or ptr1 is a pointer to void. In all other cases, we would have to cast p2 to be compatible with ptr1. Assigning pointers to different types can cause serious problems. Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

27 Pointer Compatibility Pointer Compatibility int i = 123; float f = 4.56; int* ptri = &i; float* ptrf = &f; // Swap ptri and ptrf (and run for cover) void* temp = ptri; ptri = (int*)ptrf; ptrf = (float*)ptri; What are the values of *ptri and *ptrf? Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

28 Pointer Compatibility &i ptri &f ptrf 0x b 0x4091eb85 i = 123 f = 4.56 Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

29 Pointer Compatibility &i ptri &f ptrf 0x b 0x4091eb85 i = 123 f = 4.56 Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

30 Example Example (Example) PtrSwap.cpp Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

31 Outline 1 Pointer Arithmetic Pointer + int Pointer - Pointer 2 Pointers and Arrays 3 Pointer Compatibility 4 Endianness 5 Assignment Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

32 Big Endian and Little Endian Definition (Big Endian, Little Endian) If a processor has Big Endian architecture, then it stores the highest-order byte of the value in the lowest byte address. If a processor has Little Endian architecture, then it stores the lowest-order byte of the value in the lowest byte address. Multi-byte objects are addressed by their lowest byte address. Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

33 Big Endian and Little Endian Suppose we make the assignment int i = ; It so happens that = 3B9ACA If the architecture is Big Endian, then it is stored as "Big" end first 3B 9A CA 00 If the architecture is Little Endian, then it is stored as "Little" end first 00 CA 9A 3B Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

34 Example Example (Example) Endianness.cpp Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

35 Outline 1 Pointer Arithmetic Pointer + int Pointer - Pointer 2 Pointers and Arrays 3 Pointer Compatibility 4 Endianness 5 Assignment Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36

36 Assignment Assignment Read Chapter 10. Robb T. Koether (Hampden-Sydney College) Pointer Arithmetic Wed, Jan 25, / 36