12 CREATING NEW TYPES

Transcription

1 Lecture 12 CREATING NEW TYPES of DATA Typedef declaration Enumeration Structure Bit fields Uninon

2 Creating New Types Is difficult to solve complex problems by using programs written with only fundamental types of data. In C++, the programmer can create any type needed, and each of these new types can have all the functionality and power of the built-in types.

3 Typedef The typedef keyword is used to create a new name for an existing data type. In effect, typedef creates a synonym. For example, the statement: typedef int integer; creates integer as a synonym for int.

4 Typedef Other examples: typedef char string[20]; String becomes an alias for an array of 20 chars string name; is the same as: char name[20];

5 Typedef typedef unsigned int uint; uint becomes an alias for unsigned int uint x; is the same as: unsigned int x;

6 Typedef typedef int bool; bool becomes an alias for int. bool OK; is the same as: int OK;

7 Enumerations enum keyword is used to define a set of constants of type int we use the syntax: enum [<enum_name>] {<constant_name> [= <value>],...} [var_list];

8 For example, Enumerations enum bool {false, true}; bool isodd; introduces an user-defined type (i.e. the type bool) for representing boolean values, two enumerators which have integral values starting from 0 (i.e., false is 0 and true is 1)

9 Enumerations enum days {sun=1, mon, tues, wed, thur, fri, sat}; days oneday; establishes an integer type named days, a variable oneday of type days, and a set of enumerators starting from 1 (sun is 1, mon is 2, etc.).

10 Structures provide a way to represent a collection of data of different types Arrays (and also pointers to data lists) are a method of grouping a lot of data together, but all data elements must be of the same type. If you need grouping elements of different types, you must use structures.

11 A structure is a collection of one or more variables grouped under a single name A structure can contain any of C++'s data types, including arrays and other structures.

12 The syntax used to define a new data type with struct (in C++) is: struct <type-name> { <type> <variable-name[, variablename,...]> ;... } <structure-variables> ;

13 The syntax used to define a new data type with struct (in C) is: typedef struct { <type> <variable-name[, variablename,...]> ;... } <type-name>;

14 Though both <type-name> and <structurevariables> are optional, one of the two must appear. Each variable within a structure is called a member of the structure or field of the structure.

15 For example, Structures struct exstru { int m1; char m2; float m3; } var1, var2 ;

16 Other example, struct coord { int x; }; int y; The following statement declares two instances of type coord: coord point1, point2;

17 Individual structure members can be used like other variables of the same type. Members of a structure are accessed using the structure member operator (.), also called the dot operator or record selector. You can assign values to members of var1: var1.m1=22; var1.m2= x ; var1.m3=12.75;

18 Initializing Structures struct mystru { int m1; char m2; float m3; } myvar = { 99, y, } ;

19 Pointers to Structures You can declare and use pointers to structures, just as it can declare pointers to any other data storage type Next statement declares a pointer to type mystru: mystru * pstru;

20 The pointer pstru can be used only if it was initialized. Because a pointer needs a memory address to point to, you must declare an instance of type mystru before anything can point to it. Here's the declaration: exstru myvar; Now you can perform the pointer initialization: pstru = &myvar;

21 There are two methods to use this pointer: First method uses the indirection operator (*). (*pstru).m1 = 100; The second method uses the indirect membership operator (->) pstru -> m1 = 100;

22 Structures That Contain Structures struct coord { int x; int y; }; struct rectangle { coord topleft; coord bottomright; };

23 Allocation of memory for structure rectangle rectangle box; box.topleft.x box.topleft.y

24 Structures That Contain Arrays struct student { char name[10]; int marks[4] float average; };

25 Organization of structure student student stud1;

26 Pointers as Structure Members struct mystru { int *value; int *rate; } myvar; myvar.value = &cost; myvar.rate = &interest;

27 Pointers and Arrays of Structures A C++ array can contain elements of any data types, so the elements of an array can be of type structure. For example, if you define the structure coord: struct coord { int x, y; }; you can declare an array as follows: coord points[100];

28 Organization of array points with elements of type coord points[0] = points[99]; points[0].x = points[99].x;

29 Passing Structures as Arguments to Functions Like any data types, a structure can be passed as an argument to a function. You can use any style of passing arguments to a function: pass-by-value, pass-by-address and pass-by-reference.

30 For example, struct complex {double re,im;}; void print_complex(complex c); void read_complex(complex *c); complex sum_complex(complex &a, complex &b);

31 Bit Fields A bit field is an element of a structure that is defined in terms of bits. Using a special type of struct definition, you can declare a structure element that can range from 1 to 15 bits in length.

32 Bit Fields struct bit_field { unsigned m1 : 1; unsigned m2 : 4; unsigned m3 : 1; unsigned m4 : 10; } myvariable;

33 Bit Fields Representation of Bit Fields in Memory

34 Unions Unions are similar to structures. An union is declared and used in the same ways that a structure is. An union differs from a structure in that only one of its members can be used at a time because all the members of a union occupy the same area of memory. Data members of one union are mapped to the same address within its object.

35 Unions The size of an object of one union is, therefore, the size of its largest data member. union myunion { char c; int i; float f;} var1;