C++ Data Structures

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C/C++ arrays allow you to define variables that combine several data items of the same kind but structure 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:

Example

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:

Example

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:

Example

#include <iostream>
 
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, "Anish Chawla"); 
   strcpy( Book1.subject, "C++ Programming");
   Book1.book_id = 6495407;

   // book 2 specification
   strcpy( Book2.title, "Wings Of Fire");
   strcpy( Book2.author, "Abdul Kalam");
   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, Output Will Be :

Example

Book 1 title : Learn C++ Programming
Book 1 author : Anish Chawla
Book 1 subject : C++ Programming
Book 1 id : 6495407
Book 2 title : Wings Of Fire
Book 2 author : Abdul Kalam
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:

Example

#include <iostream>
 
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, "Anish Chawla"); 
   strcpy( Book1.subject, "C++ Programming");
   Book1.book_id = 6495407;

   // book 2 specification
   strcpy( Book2.title, "Wings Of Fire");
   strcpy( Book2.author, "Abdul Kalam");
   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, Output Will Be :

Example

Book title : Learn C++ Programming
Book author : Anish Chawla
Book subject : C++ Programming
Book id : 6495407
Book title : Wings Of Fire
Book author : Abdul Kalam
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:

Example

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:

Example

struct_pointer = &Book1;
                                        

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

Example

struct_pointer->title;
                                        

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

Example

#include <iostream>
 
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, "Anish Chawla"); 
   strcpy( Book1.subject, "C++ Programming");
   Book1.book_id = 6495407;

   // Book 2 specification
   strcpy( Book2.title, "Wings Of Fire");
   strcpy( Book2.author, "Abdul Kalam");
   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, Output Will Be :

Example

Book title : Learn C++ Programming
Book author : Anish Chawla
Book subject : C++ Programming
Book id : 6495407
Book title : Wings Of Fire
Book author : Abdul Kalam
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:

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:

Example

Books Book1, Book2;

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

Example

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

x, y and z are all pointers to long ints


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