Pointer arithmetic 20/11/2018. Pointer arithmetic. Pointer arithmetic

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1 ECE 150 Fundamentals of Programming Outline 2 In this lesson, we will: Review that pointers store addresses of specific types See that we can add integers to addresses The result depends on the type See that you can also take differences of pointers Again, the result depends on the type Prof. Hiren Patel, Ph.D. Douglas Wilhelm Harder, M.Math. LEL hdpatel@uwaterloo.ca dwharder@uwaterloo.ca 201 by Douglas Wilhelm Harder and Hiren Patel. Some rights reserved. 3 4 Recall that: Pointers are addresses Addresses are positive integers It makes no sense to add two addresses int *p_value_1{new int{42; int *p_value_2{new int{91; std::cout << (p_value_1 + p_value_2) << std::endl; You can, however, ask What is the next address? However, the next address isn t always the next chronologically What address follows 24 Sussex Dr.? The French Embassy at 42 Sussex Dr. What address comes two before 24 Sussex Dr.? The South African High Commission at 15 Sussex Dr. It s like asking What address is 111 Wellington St + 24 Sussex Dr? 1

2 5 6 You can, however, ask What is the next address? However, the next address isn t always the next chronologically What address follows 24 Sussex Dr? The French Embassy at 42 Sussex Dr 25 Sussex Dr does not exist Suppose you have an array: int *a_data{new int[20]{; std::cout << a_data << std::endl; Suppose that 'a_data' is assigned 0x93a3d0 Question: What is the value of a_data + 1? What address comes two before 24 Sussex Dr? The South African High Commission at 15 Sussex Dr 7 Next address Next address Let s try it out: Let s try it with an array of double: int *a_ray{new int[10]; double *a_ray{new double[10]; std::cout << "a_ray == " << a_ray << std::endl; std::cout << "a_ray + 1 == " << (a_ray + 1) << std::endl; std::cout << "a_ray == " << a_ray << std::endl; std::cout << "a_ray + 1 == " << (a_ray + 1) << std::endl; a_ray == 0x1fb1010 a_ray + 1 == 0x1fb1014 a_ray == 0x1af7010 a_ray + 1 == 0x1af701 2

3 9 10 Next address The k th address Recall that an int occupies 4 bytes, while a double occupies bytes The compiler says: a_ray is the address of a double, so the next double is bytes after the current address We can even walk though an array: double *a_ray{new double[6]{0, 1, 2, 3, 4, 42; for ( std::size_t k{0; k < 6; ++k ) { std::cout << (a_ray + k) << " stores the value " << *(a_ray + k) << std::endl; 0x1af0010 stores the value 0 0x1af001 stores the value 1 0x1af0020 stores the value 2 0x1af002 stores the value 3 0x1af0030 stores the value 4 0x1af003 stores the value Array entries Array entries The following two statements are equivalent: a_ray[k] *(a_ray + k) The first says: Access the k th entry of the array a_ray The following two statements are also equivalent: &( a_ray[k] ) a_ray + k The first says: Find the address of the k th entry of the array a_ray The second says: Find the address of k entries beyond a_ray and access that address The second says: Find the address of k entries beyond a_ray In general, the first is much easier to read In this case, the second is easier to read! 3

4 13 14 Array entries Walking through arrays Now you understand why all arrays in C++ start at index 0 a_ray[0] == *(a_ray + 0) and *(a_ray + 0) == *a_ray The decision to start array indices at 0 was entirely practical We can walk through arrays: int *a_ray{new int[6]{0, 1, 2, 3, 4, 42; for ( int *p_ray{a_ray; p_ray < a_ray + 6; ++p_ray ) { std::cout << p_ray << " stores the value " << *p_ray << std::endl; Increment 'p_ray' to the next valid address of an int 0x13a010 stores the value 0 0x13a014 stores the value 1 0x13a01 stores the value 2 0x13a01c stores the value 3 0x13a020 stores the value 4 0x13a024 stores the value 42 Walking through arrays Question: Should you, or should you not delete p_ray? Why? int *a_ray{new int[6]{0, 1, 2, 3, 4, 42; Remember, it makes no sense to ask What is 111 Wellington St + 24 Sussex Dr? On the other hand 1100 S Ocean Blvd Pennsylvania Avenue NW = &hell for ( int *p_ray{a_ray; p_ray < a_ray + 6; ++p_ray ) { std::cout << p_ray << " stores the value " << *p_ray << std::endl; You might ask, however, How many buildings away is Sir Winston Churchill Secondary School from Denis Morris Catholic High School? Given the addresses 101 Glen Morris Dr and 40 Glen Morris Dr: The wrong answer is 61 The right answer is 3 4

5 17 1 Similarly, we may ask about a difference of addresses: double *a_ray{new double[6]{0, 1, 2, 3, 4, 42; double *p_1{&(a_ray[2]); double *p_2{&(a_ray[5]); std::cout << p_1 << " " << p_2 << std::endl; std::cout << (p_1 - p_2) << std::endl; std::cout << (p_2 - p_1) << std::endl; 0xd xd Question: What is the type of a pointer difference? Remember that pointers are addresses The type std::size_t depends on the address size This type is unsigned addresses are unsigned A reasonable approximation would be a signed equivalent of std::size_t Such a type is defined: std::ptrdiff_t Literally, a pointer-difference type This is unique for operators: the return type is different from the types of the operands Not a problem, a function can have a return type that is different from its arguments, too Question: What is the type of a pointer difference? double *a_ray{new double[6]{0, 1, 2, 3, 4, 42; double *p_1{&(a_ray[2]); double *p_2{&(a_ray[5]); std::ptrdiff_t diff = p_1 - p_2; std::cout << diff << std::endl; -3 The memory occupied by a pointer, std::size_t and std::ptrdiff_t are all the same: std::cout << sizeof( int * ) << std::endl; std::cout << sizeof( std::size_t ) << std::endl; std::cout << sizeof( std::ptrdiff_t ) << std::endl; on ecelinux: 5

6 Summary 21 References 22 Following this lesson, you now Know that integers can be added to addresses and the difference depends on the type Understand that array[k] and *(array + k) are equivalent The first, however, is clearer to read Know that we can calculate the difference between addresses [1] No references? Colophon 23 Disclaimer 24 These slides were prepared using the Georgia typeface. Mathematical equations use Times New Roman, and source code is presented using Consolas. The photographs of lilacs in bloom appearing on the title slide and accenting the top of each other slide were taken at the Royal Botanical Gardens on May 27, 201 by Douglas Wilhelm Harder. Please see for more information. These slides are provided for the ECE 150 Fundamentals of Programming course taught at the University of Waterloo. The material in it reflects the authors best judgment in light of the information available to them at the time of preparation. Any reliance on these course slides by any party for any other purpose are the responsibility of such parties. The authors accept no responsibility for damages, if any, suffered by any party as a result of decisions made or actions based on these course slides for any other purpose than that for which it was intended. 6