C++ Data Structures

C/C++ arrays allow you to define variables that combine several data items of the same kind butstructure is another user defined data type which allows you to combine data items of different kinds.

Structures are used to represent a record, suppose you want to keep track of your books in a library. You might want to track the following attributes about each book:

• Title
• Author
• Subject
• Book ID

Defining a Structure:

To define a structure, you must use the struct statement. The struct statement defines a new data type, with more than one member, for your program. The format of the struct statement is this:

struct [structure tag]
{
   member definition;
   member definition;
   ...
   member definition;
} [one or more structure variables];  

The structure tag is optional and each member definition is a normal variable definition, such as int i; or float f; or any other valid variable definition. At the end of the structure's definition, before the final semicolon, you can specify one or more structure variables but it is optional. Here is the way you would declare the Book structure:

struct Books
{
   char  title[50];
   char  author[50];
   char  subject[100];
   int   book_id;
}book;  

Accessing Structure Members:

To access any member of a structure, we use the member access operator (.). The member access operator is coded as a period between the structure variable name and the structure member that we wish to access. You would use struct keyword to define variables of structure type. Following is the example to explain usage of structure:

#include <iostream>
#include <cstring>
 
using namespace std;
 
struct Books
{
   char  title[50];
   char  author[50];
   char  subject[100];
   int   book_id;
};
 
int main( )
{
   struct Books Book1;        // Declare Book1 of type Book
   struct Books Book2;        // Declare Book2 of type Book
 
   // book 1 specification
   strcpy( Book1.title, "Learn C++ Programming");
   strcpy( Book1.author, "Chand Miyan"); 
   strcpy( Book1.subject, "C++ Programming");
   Book1.book_id = 6495407;

   // book 2 specification
   strcpy( Book2.title, "Telecom Billing");
   strcpy( Book2.author, "Yakit Singha");
   strcpy( Book2.subject, "Telecom");
   Book2.book_id = 6495700;
 
   // Print Book1 info
   cout << "Book 1 title : " << Book1.title <<endl;
   cout << "Book 1 author : " << Book1.author <<endl;
   cout << "Book 1 subject : " << Book1.subject <<endl;
   cout << "Book 1 id : " << Book1.book_id <<endl;

   // Print Book2 info
   cout << "Book 2 title : " << Book2.title <<endl;
   cout << "Book 2 author : " << Book2.author <<endl;
   cout << "Book 2 subject : " << Book2.subject <<endl;
   cout << "Book 2 id : " << Book2.book_id <<endl;

   return 0;
}

When the above code is compiled and executed, it produces the following result:

Book 1 title : Learn C++ Programming
Book 1 author : Chand Miyan
Book 1 subject : C++ Programming
Book 1 id : 6495407
Book 2 title : Telecom Billing
Book 2 author : Yakit Singha
Book 2 subject : Telecom
Book 2 id : 6495700

Structures as Function Arguments:

You can pass a structure as a function argument in very similar way as you pass any other variable or pointer. You would access structure variables in the similar way as you have accessed in the above example:

#include <iostream>
#include <cstring>
 
using namespace std;
void printBook( struct Books book );

struct Books
{
   char  title[50];
   char  author[50];
   char  subject[100];
   int   book_id;
};
 
int main( )
{
   struct Books Book1;        // Declare Book1 of type Book
   struct Books Book2;        // Declare Book2 of type Book
 
   // book 1 specification
   strcpy( Book1.title, "Learn C++ Programming");
   strcpy( Book1.author, "Chand Miyan"); 
   strcpy( Book1.subject, "C++ Programming");
   Book1.book_id = 6495407;

   // book 2 specification
   strcpy( Book2.title, "Telecom Billing");
   strcpy( Book2.author, "Yakit Singha");
   strcpy( Book2.subject, "Telecom");
   Book2.book_id = 6495700;
 
   // Print Book1 info
   printBook( Book1 );

   // Print Book2 info
   printBook( Book2 );

   return 0;
}
void printBook( struct Books book )
{
   cout << "Book title : " << book.title <<endl;
   cout << "Book author : " << book.author <<endl;
   cout << "Book subject : " << book.subject <<endl;
   cout << "Book id : " << book.book_id <<endl;
}

When the above code is compiled and executed, it produces the following result:

Book title : Learn C++ Programming
Book author : Chand Miyan
Book subject : C++ Programming
Book id : 6495407
Book title : Telecom Billing
Book author : Yakit Singha
Book subject : Telecom
Book id : 6495700

Pointers to Structures:

You can define pointers to structures in very similar way as you define pointer to any other variable as follows:

struct Books *struct_pointer;

Now, you can store the address of a structure variable in the above defined pointer variable. To find the address of a structure variable, place the & operator before the structure's name as follows:

struct_pointer = &Book1;

To access the members of a structure using a pointer to that structure, you must use the -> operator as follows:

struct_pointer->title;

Let us re-write above example using structure pointer, hope this will be easy for you to understand the concept:

#include <iostream>
#include <cstring>
 
using namespace std;
void printBook( struct Books *book );

struct Books
{
   char  title[50];
   char  author[50];
   char  subject[100];
   int   book_id;
};
 
int main( )
{
   struct Books Book1;        // Declare Book1 of type Book
   struct Books Book2;        // Declare Book2 of type Book
 
   // Book 1 specification
   strcpy( Book1.title, "Learn C++ Programming");
   strcpy( Book1.author, "Chand Miyan"); 
   strcpy( Book1.subject, "C++ Programming");
   Book1.book_id = 6495407;

   // Book 2 specification
   strcpy( Book2.title, "Telecom Billing");
   strcpy( Book2.author, "Yakit Singha");
   strcpy( Book2.subject, "Telecom");
   Book2.book_id = 6495700;
 
   // Print Book1 info, passing address of structure
   printBook( &Book1 );

   // Print Book1 info, passing address of structure
   printBook( &Book2 );

   return 0;
}
// This function accept pointer to structure as parameter.
void printBook( struct Books *book )
{
   cout << "Book title : " << book->title <<endl;
   cout << "Book author : " << book->author <<endl;
   cout << "Book subject : " << book->subject <<endl;
   cout << "Book id : " << book->book_id <<endl;
}

When the above code is compiled and executed, it produces the following result:

Book title : Learn C++ Programming
Book author : Chand Miyan
Book subject : C++ Programming
Book id : 6495407
Book title : Telecom Billing
Book author : Yakit Singha
Book subject : Telecom
Book id : 6495700

The typedef Keyword

There is an easier way to define structs or you could "alias" types you create. For example:

typedef struct
{
   char  title[50];
   char  author[50];
   char  subject[100];
   int   book_id;
}Books;

Now, you can use Books directly to define variables of Books type without using struct keyword. Following is the example:

Books Book1, Book2;

You can use typedef keyword for non-structs as well as follows:

typedef long int *pint32;
 
pint32 x, y, z;

x, y and z are all pointers to long ints

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C++ Basic Input/Output

The C++ standard libraries provide an extensive set of input/output capabilities which we will see in subsequent chapters. This chapter will discuss very basic and most common I/O operations required for C++ programming.

C++ I/O occurs in streams, which are sequences of bytes. If bytes flow from a device like a keyboard, a disk drive, or a network connection etc. to main memory, this is called input operation and if bytes flow from main memory to a device like a display screen, a printer, a disk drive, or a network connection, etc, this is called output operation.

I/O Library Header Files:

There are following header files important to C++ programs:

Header File	Function and Description
<iostream>	This file defines the cin, cout, cerr and clog objects, which correspond to the standard input stream, the standard output stream, the un-buffered standard error stream and the buffered standard error stream, respectively.
<iomanip>	This file declares services useful for performing formatted I/O with so-called parameterized stream manipulators, such as setw and setprecision.
<fstream>	This file declares services for user-controlled file processing. We will discuss about it in detail in File and Stream related chapter.

The standard output stream (cout):

The predefined object cout is an instance of ostream class. The cout object is said to be "connected to" the standard output device, which usually is the display screen. The cout is used in conjunction with the stream insertion operator, which is written as << which are two less than signs as shown in the following example.

#include <iostream>
 
using namespace std;
 
int main( )
{
   char str[] = "Hello C++";
 
   cout << "Value of str is : " << str << endl;
}

When the above code is compiled and executed, it produces the following result:

Value of str is : Hello C++

The C++ compiler also determines the data type of variable to be output and selects the appropriate stream insertion operator to display the value. The << operator is overloaded to output data items of built-in types integer, float, double, strings and pointer values.

The insertion operator << may be used more than once in a single statement as shown above and endlis used to add a new-line at the end of the line.

The standard input stream (cin):

The predefined object cin is an instance of istream class. The cin object is said to be attached to the standard input device, which usually is the keyboard. The cin is used in conjunction with the stream extraction operator, which is written as >> which are two greater than signs as shown in the following example.

#include <iostream>
 
using namespace std;
 
int main( )
{
   char name[50];
 
   cout << "Please enter your name: ";
   cin >> name;
   cout << "Your name is: " << name << endl;
 
}

When the above code is compiled and executed, it will prompt you to enter a name. You enter a value and then hit enter to see the result something as follows:

Please enter your name: cplusplus
Your name is: cplusplus

The C++ compiler also determines the data type of the entered value and selects the appropriate stream extraction operator to extract the value and store it in the given variables.

The stream extraction operator >> may be used more than once in a single statement. To request more than one datum you can use the following:

cin >> name >> age;

This will be equivalent to the following two statements:

cin >> name;
cin >> age;

The standard error stream (cerr):

The predefined object cerr is an instance of ostream class. The cerr object is said to be attached to the standard error device, which is also a display screen but the object cerr is un-buffered and each stream insertion to cerr causes its output to appear immediately.

The cerr is also used in conjunction with the stream insertion operator as shown in the following example.

#include <iostream>
 
using namespace std;
 
int main( )
{
   char str[] = "Unable to read....";
 
   cerr << "Error message : " << str << endl;
}

When the above code is compiled and executed, it produces the following result:

Error message : Unable to read....

The standard log stream (clog):

The predefined object clog is an instance of ostream class. The clog object is said to be attached to the standard error device, which is also a display screen but the object clog is buffered. This means that each insertion to clog could cause its output to be held in a buffer until the buffer is filled or until the buffer is flushed.

The clog is also used in conjunction with the stream insertion operator as shown in the following example.

#include <iostream>
 
using namespace std;
 
int main( )
{
   char str[] = "Unable to read....";
 
   clog << "Error message : " << str << endl;
}

When the above code is compiled and executed, it produces the following result:

Error message : Unable to read....

You would not be able to see any difference in cout, cerr and clog with these small examples, but while writing and executing big programs then difference becomes obvious. So this is good practice to display error messages using cerr stream and while displaying other log messages then clog should be used.

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