D - Structs

D arrays allow you to define type of variables that can hold several data items of the same kind but structure is another user defined data type available in D programming, 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;
   char [] author;
   char [] subject;
   int   book_id;
};

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:

import std.stdio;

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

void main( )
{
   Books Book1;        /* Declare Book1 of type Book */
   Books Book2;        /* Declare Book2 of type Book */

   /* book 1 specification */
   Book1.title = "D Programming".dup;
   Book1.author = "Raj".dup;
   Book1.subject = "D Programming Tutorial".dup;
   Book1.book_id = 6495407;

   /* book 2 specification */
   Book2.title = "D Programming".dup;
   Book2.author = "Raj".dup;
   Book2.subject = "D Programming Tutorial".dup;
   Book2.book_id = 6495700;

   /* print Book1 info */
   writeln( "Book 1 title : ", Book1.title);
   writeln( "Book 1 author : ", Book1.author);
   writeln( "Book 1 subject : ", Book1.subject);
   writeln( "Book 1 book_id : ", Book1.book_id);

   /* print Book2 info */
   writeln( "Book 2 title : ", Book2.title);
   writeln( "Book 2 author : ", Book2.author);
   writeln( "Book 2 subject : ", Book2.subject);
   writeln( "Book 2 book_id : ", Book2.book_id);
}

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

Book 1 title : D Programming
Book 1 author : Raj
Book 1 subject : D Programming Tutorial
Book 1 book_id : 6495407
Book 2 title : D Programming
Book 2 author : Raj
Book 2 subject : D Programming Tutorial
Book 2 book_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:

import std.stdio;

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

void main( )
{
   Books Book1;        /* Declare Book1 of type Book */
   Books Book2;        /* Declare Book2 of type Book */

   /* book 1 specification */
   Book1.title = "D Programming".dup;
   Book1.author = "Raj".dup;
   Book1.subject = "D Programming Tutorial".dup;
   Book1.book_id = 6495407;

   /* book 2 specification */
   Book2.title = "D Programming".dup;
   Book2.author = "Raj".dup;
   Book2.subject = "D Programming Tutorial".dup;
   Book2.book_id = 6495700;

   /* print Book1 info */
   printBook( Book1 );

   /* Print Book2 info */
   printBook( Book2 );

}

void printBook( Books book )
{
   writeln( "Book title : ", book.title);
   writeln( "Book author : ", book.author);
   writeln( "Book subject : ", book.subject);
   writeln( "Book book_id : ", book.book_id);
}

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

Book title : D Programming
Book author : Raj
Book subject : D Programming Tutorial
Book book_id : 6495407
Book title : D Programming
Book author : Raj
Book subject : D Programming Tutorial
Book book_id : 6495700

Structs initializations

Structs can be initialized in two forms, one using construtor and other using the {} format. An example is shown below.

import std.stdio;

struct Books
{
   char [] title;
   char [] subject = "Empty".dup;
   int   book_id = -1;
   char [] author = "Raj".dup;

};

void main( )
{
   Books Book1 = Books("D Programming".dup, "D Programming Tutorial".dup, 6495407 );
   printBook( Book1 );

   Books Book2 = Books("D Programming".dup, "D Programming Tutorial".dup, 6495407,"Raj".dup );
   printBook( Book2 );

   Books Book3 =  {title:"Obj C programming".dup, book_id : 1001};
   printBook( Book3 );
}

void printBook( Books book )
{
   writeln( "Book title : ", book.title);
   writeln( "Book author : ", book.author);
   writeln( "Book subject : ", book.subject);
   writeln( "Book book_id : ", book.book_id);
}

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

Book title : D Programming
Book author : Raj
Book subject : D Programming Tutorial
Book book_id : 6495407
Book title : D Programming
Book author : Raj
Book subject : D Programming Tutorial
Book book_id : 6495407
Book title : Obj C programming
Book author : Raj
Book subject : Empty
Book book_id : 1001

Static Members

Static variables are initialized only once. For example to have unique ids for the book, we can make the book_id as static and increment the book id. An example is shown below.

import std.stdio;

struct Books
{
   char [] title;
   char [] subject = "Empty".dup;
   int   book_id;
   char [] author = "Raj".dup;
   static int id = 1000;
};

void main( )
{
   Books Book1 = Books("D Programming".dup, "D Programming Tutorial".dup,++Books.id );
   printBook( Book1 );

   Books Book2 = Books("D Programming".dup, "D Programming Tutorial".dup,++Books.id);
   printBook( Book2 );

   Books Book3 =  {title:"Obj C programming".dup, book_id:++Books.id};
   printBook( Book3 );
}

void printBook( Books book )
{
   writeln( "Book title : ", book.title);
   writeln( "Book author : ", book.author);
   writeln( "Book subject : ", book.subject);
   writeln( "Book book_id : ", book.book_id);
}

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

Book title : D Programming
Book author : Raj
Book subject : D Programming Tutorial
Book book_id : 1001
Book title : D Programming
Book author : Raj
Book subject : D Programming Tutorial
Book book_id : 1002
Book title : Obj C programming
Book author : Raj
Book subject : Empty
Book book_id : 1003