Understanding Advanced Data Types in C++: Classes, Structures, Unions, and Enumerations

1. Constants, Declarations, and Definitions Advanced Programming Derived Data Types

2. Derived Data Types • Class • Structure • Union • Enumeration • Array • Function • Pointer • Reference CSCI 240: Computing II

3. Class • Collection of Homogeneous Objects • Information Hiding -- Data Members and Member Functions • #include <iostream>using std::cout; using std::endl;class student{ private: char* name; public: void add_name(char *ip1) { name = new char[10]; strcpy(name, ip1);} char* get_name() {return name;} }; main() { student s1; //object or instance s1.add_name("John"); cout << "Name: " << s1.get_name() << endl; } CSCI 240: Computing II

4. Structure • Collection of Objects a Having Meaningful Representation • A Class with ALL PUBLIC MEMBERS • struct date{ int day; char *month; int year; }; • #include <iostream>using std::cout; using std::endl;main() { struct date today; //object today.day = 15; today.month = "May"; today.year = 1995; cout << "Date is: " << today.month << " " << today.day << " " << today.year << endl; } (There is a strong correlation between classes and structures.) CSCI 240: Computing II

5. Union • Objects to Occupy Same Area of Storage • Different Types at Different Times • struct circle{ int radius; }; struct triangle{ int side1; int side2; int angle; }; struct rectangle{ int side1; int side2; }; • union shape{ struct circle s1; struct triangle s2; struct rectangle s3; }; Using the correct name is critical. Sometimes you need to add a tag field to help keep track. It is important that the tag field be in the same location in every variant. CSCI 240: Computing II

6. Enumeration • Assigns Numerical Values to List of Identifiers • enum binary {zero, one}; enum number {one = 1, two, three}; enum boolean {False, True}; • main() { boolean x = False; if (x) {cout << "True!" << endl;} else {cout << "False!" << endl;} } CSCI 240: Computing II

7. Reference • An alias of an object • Must be initialized when defined • No operator acts on reference • Value of a reference cannot be changed after initialization – it always refers to the object it was initialized to. (Compile time, not run time.) • main(){ • int i = 10, &j = i; • j = 5; • cout << i; • } This is the first new C++ capability. You can’t do this in C. CSCI 240: Computing II

8. Object Storage • Persistent – alive after the program termination • Non-persistent – alive during the program execution • C++ allows only non-persistent objects • Automatic variables are allocated and destroyed automatically • Dynamic allocation is achieved by using new and delete operators. CSCI 240: Computing II

9. Constant Declarations • const Keyword Makes the Object Constant • const as a Prefix in a Pointer Declaration MAKES THE OBJECT POINTED TO BE A CONSTANT AND NOT THE POINTER! • Use of *const Makes the POINTER to be a CONSTANT CSCI 240: Computing II

10. Constant Declarations         const int x = 10; /* x is a Constant Object */      const int y[] = {1, 2, 3, 4}; // y is a Array of Constant Objects      const char *ptr = "csci220"; /* ptr: Pointer to a CONST OBJECT      ptr[0] = 'R';  //ERROR!!!     ptr = "Class_Notes";   /* ptr CAN POINT TO ANOTHER CONSTANT OBJECT! */     char *const cptr = "C++";         // cptr is a CONSTANT POINTER cptr[0] = 'c'; //LEGAL cptr = "Assignment";  //ERROR!! cptr CANNOT POINT TO // ANOTHER CONSTANT OBJECT!     const char* const dptr = "Simple_Language"; /* dptr is a CONSTANT POINTER pointing to a CONSTANT OBJECT */     dptr[0] = 's'; //ERROR!!     dptr = "Difficult_Language"; //ERROR!! CSCI 240: Computing II

11. Declaration • Describes the form of an Object • DOES NOT Reserve Any Storage • Initialization is NOT Allowed Definition • Creates an Instance • Creates an Instance • Reserves a Storage • Initialization is Allowed • All Objects MUST BE DEFINED BEFORE THEIR USE CSCI 240: Computing II

12. Declaration != Implementation • Function Without Body • Contains extern Specifier and NO Initializer or Function Body • Static Member in the Class Declaration • Class Name Declaration • typedef Declaration CSCI 240: Computing II

13. Examples •             /* Definitions */            int i, j; //storage is reserved            int k = 10; //initialization            /* Declarations */            int my_function(); //function            extern int x; //external variable            struct S; //structure            typedef int INT; //typedef            /* Implementation */            int my_function()            {                int i = 100;                return(i);            } CSCI 240: Computing II

14. Incomplete Declarations • Dimension is Not Specified • Class/Structure Body is Not Specified • Completed By Subsequent Declaration • struct S; //incomplete S *ps; //Acceptable S s1; //ERROR struct S { int x; char *ptr; }; // Complete Declaration S s2; //FINE int array[]; //incomplete int array[5]; //FINE CSCI 240: Computing II

15. Typedef • Defines an Alias for Previously Defined Data Type • Does NOT Create a New Type • Makes Programs Readable typedef int INT; //INT => int INT x; //x is an integer variable INT y[10]; //Array typedef int *INT_PTR; //Pointer to int INT_PTR ptr; //ptr is a pointer to int /* Unnamed Class or Struct in a "typedef" gets the typedef as its name */ typedef struct { int p; char *q; } S; //struct is called S S my_struct; //instance of S CSCI 240: Computing II

16. Interpretation of Declaration • Order of Evaluation Depends Upon the Precedence and Associativity • int (*fun[])(); • /* Explanation 1. () Alter the Order of Evaluation 2. [] has Highest Precedence => fun is an array of 3. *fun[] => Pointers to 4. () => Functions Returning (required) 5. int => Integers ==> fun is an array of pointers to functions returning integers */ CSCI 240: Computing II

17. Scope Resolution Operator (::) • Can declare local and global variable of same name. • In C, the local variable takes precedence over the global variable throughout its scope. • In C++, the scope resolution operator is used to access the variable of the same name in an outer block. • Example • int i; main() { int i; i = 35; ::i = 34; cout << "Local i = " << i << endl; cout << "Global i = " << ::i << endl; } CSCI 240: Computing II

18. Acknowledgements • These slides were originally produced by Rajeev Raje, modified by Dale Roberts. CSCI 240: Computing II