C++ Structures (struct)



C++ structures are user-defined data types to group related variables of different types together under a single name. Structures are also known as structs.

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.

Syntax

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.

Example

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.

Example

Following is the example to explain usage of structure −

Open Compiler
#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 −

Example

Open Compiler
#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 −

Syntax

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 −

Syntax

struct_pointer = &Book1;

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

Syntax

struct_pointer->title;

Example

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

Open Compiler
#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.

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.

Advertisements