typedef int Array[10]; String name; Array ages;

Transcription

1 Morteza Noferesti

2 The C language provides a facility called typedef for creating synonyms for previously defined data type names. For example, the declaration: typedef int Length; Length a, b, len ; Length numbers[10] ; makes the name Length a synonym (or alias) for the data type int. The data type name Length can now be used in declarations in exactly the same way that the data type int can be used: typedef char String[50]; typedef int Array[10]; String name; Array ages; 2

3 Structures sometimes referred to as aggregates are collections of related variables under one name Structures may contain variables of many different data types in contrast to arrays that contain only elements of the same data type Structures are commonly used to define records Pointers and structures facilitate the formation of more complex data structures such as linked lists, queues, stacks and trees Structures are derived data types they are constructed using objects of other types 3

4 struct employee }; char firstname[ 20 ]; char lastname[ 20 ]; int age; char gender; double hourlysalary; struct employee char firstname[ 20 ]; char lastname[ 20 ]; int age; char gender; double hourlysalary; } Ali, Sara, empdts[20]; The name "employee" is called a structure tag 4

5 Members of the same structure type must have unique names, but two different structure types may contain members of the same name without conflict struct employee char Name[ 20 ]; char Name[ 20 ]; // Error!!! int age; char gender; double hourlysalary; } Ali, Sara, empdts[20]; struct employee Reza, *emp; Each structure definition must end with a semicolon 5 struct Student char Name[ 20 ]; // OK int age; char gender; }; struct Student Ce40153[80];

6 A structure cannot contain an instance of itself For example, a variable of type struct employee cannot be declared in the definition for struct employee A pointer to struct employee, however, may be included struct employee // double hourlysalary; struct employee person; /* ERROR */ struct employee *eptr; /* pointer */ }; A structure containing a member that is a pointer to the same structure type is referred to as a self-referential structure 6

7 The structure tag name is optional struct char firstname[ 20 ]; char lastname[ 20 ]; int age; char gender; double hourlysalary; } Ali; If a structure definition does not contain a structure tag name, variables of the structure type may be declared only in the structure definition not in a separate declaration 7

8 Structure definitions do not reserve any space in memory; rather, each definition creates a new data type that is used to define variables sizeof(struct ) = sum of sizeof(members) + alignment padding (Processor- and compiler-specific) struct employee char firstname[ 20 ]; char lastname[ 20 ]; int age; char gender; double hourlysalary; }; struct employee Ali, emp[10]; printf("%d", sizeof(ali)); printf("%d", sizeof(emp)); printf("%d", sizeof(struct employee)); 8

9 struct char a[3]; short int b; long int c; char d[3]; }; it's much, much easier on the processor if the compiler arranges it like this: most compilers will pad the structure (as if with extra, invisible fields) like this: 9

10 Individual members of a struct variable may be accessed using the structure member operator (the dot, "."): myemp.firstname ; employee. firstname; // Error Or, if a pointer to the struct has been declared and initialized employee *emp = &myemp ; by using the structure pointer operator : emp firstname; // arrow operator which could also be written as: (* emp).firstname; 10

11 struct identity sharifc40153[80] = Omid", "Jafarinezhad", 14, , 20, "Samad", "Shekarestani", 90, , 20} ; strcpy(sharifc40153[2].firstname, "Khaje Nezam"); strcpy(sharifc40153[2].lastname, "Shekarestani"); sharifc40153[2]. age = 100; sharifc40153[2]. person.id = ; sharifc40153[2]. person. gpa = 20; FirstName LastName age id person gpa students[0] omid Jafarinezhad Students[1] Samad Shekarestani students[2] Khaje Nezam Shekarestani students[3] 11

12 #define NFRIENDS 10 struct Date unsigned year; unsigned month; unsigned day; }; struct Friend char FirstName[30]; char LastName[30]; struct Date Birthday; }; bool check_birthday(struct Date today, struct Date myfriend) if ((today.month == myfriend.month) && (today.day == myfriend.day)) return (true); return (false); } int main() struct Friend friends[nfriends]; struct Date today = 2017, 11, 25}; //... for (i = 0; i < NFRIENDS; i++) if(check_birthday(today, friends[i].birthday)) printf ("%s %s\n", friends[i].firstname, oj.lastname) ; } // 12

13 Notice that bit field members of structures are accessed exactly as any other structure member struct Flags int unsigned int unsigned int } foo; f1:3; f2:1; f3:2; f1 f2 f bit 8 bit 8 bit foo.f1 = -2; foo.f2 = 1; foo.f3 = 2; Padded to be an integral number of words Placement is compiler-specific 13

14 Date today; today = create_date1(9, 4, 2008); create_date2(&today, 9, 4, 2008); Date create_date1(int month, int day, int year) Date d; d.month = month; d.day = day; d.year = year; return (d); } 14 void create_date2(date *d, int month, int day, int year) d->month = month; d->day = day; d->year = year; }

15 typedef struct char firstname[ 20 ]; char lastname[ 20 ]; int age; char gender; double hourlysalary; } employee; /* The "alias" employee Ali; /* Create a struct variable */ 15

16 Structures may not be compared using operators == and!= struct a int a; int b; }; struct a b, c; b.a = 10; b.b = 30; c = b; if(c == b) // Error 16

17 Enumeration is a user-defined data type. It is defined using the keyword enum and the syntax is: enum tag_name name_0,, name_n} ; The tag_name is not used directly. The names in the braces are symbolic constants that take on integer values from zero through n. As an example, the statement: enum colors red, yellow, green } ; creates three constants. red is assigned the value 0, yellow is assigned 1 and green is assigned 2 17

18 #include <stdio.h> int main( ) int March[5][7]=0,0,1,2,3,4,5}, 6,7,8,9,10,11,12}, 13,14,15,16,17,18,19}, 20,21,22,23,24,25,26}, 27,28,29,30,31,0,0}}; enum days Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday}; enum week week_one, week_two, week_three, week_four, week_five}; } printf ("Monday the third week of March is March %d\n", March [week_three] [Monday] ); 18

19 Values in an enum start with 0, unless specified otherwise, and are incremented by 1 The identifiers in an enumeration must be unique Multiple members of an enumeration can have the same constant value Assigning a value to an enumeration constant after it has been defined is a syntax error Use only uppercase letters enumeration constant names. This makes these constants stand out in a program and reminds you that enumeration constants are not variables 19

20 enum months JAN = 1, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC }; enum months month; /* initialize array of pointers */ const char *monthname[] = "", "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", November", "December"}; for (month = JAN; month <= DEC; month++ ) printf( "%2d%11s\n", month, monthname[month] ); } 20

21 A union is a derived data type like a structure with members that share the same storage space For different situations in a program, some variables may not be relevant, but other variables are so a union shares the space instead of wasting storage on variables that are not being used The members of a union can be of any data type The number of bytes used to store a union must be at least enough to hold the largest member Only one member, and thus one data type, can be referenced at a time 21

22 union mydataunion int i; char c; float f; } u1, u2; union mydataunion u3; f i c u1.i = 4; u1.c = a ; u2.i = 0xDEADBEEF; 22

23 In a declaration, a union may be initialized with a value of the same type as the first union member union a int c; // OK char b[4]; }; union a test; test.c = 10; printf("%d", test.c); 23

24 A union value doesn t "know" which case it contains union AnElt int i; char c; } elt1, elt2; elt1.i = 4; elt2.c = a ; elt2.i = 0xDEADBEEF; How should your program keep track whether elt1, elt2 hold an int or a char? Basic answer: Another variable holds that info if (elt1 currently has a char) 24

25 Tag every value with its case enum Union_Tag IS_INT, IS_CHAR}; struct TaggedUnion enum Union_Tag tag; union int i; char c; } data; }; 25

26 26

27 class GradeBook public: void setcoursename( string name ) coursename = name; } string getcoursename() return coursename; } void displaymessage() cout << "Welcome to the grade book for\n" << getcoursename() << "!" << endl; } private: string coursename; }; 27

28 GradeBook book( MyBook ); int number; cin >> number; cout << Book << book.getcoursename()<< has << number << pages << endl; 28