User defined Data Types in C++

Data types are means to identify the type of data and associated operations of handling it. There are three types of data types:

1. Pre-defined DataTypes
2. Derived Data Types
3. User-defined DataTypes

In this article, the User-Defined DataType is explained:

User-Defined DataTypes:

The data types that are defined by the user are called the derived datatype or user-defined derived data type.
These types include:

• Class
• Structure
• Union
• Enumeration
• Typedef defined DataType

Below is the detailed description of the following types:

1. Class: The building block of C++ that leads to Object-Oriented programming is a Class. It is a user-defined data type, which holds its own data members and member functions, which can be accessed and used by creating an instance of that class. A class is like a blueprint for an object.

   Syntax:
   

   Example:

   // C++ program to demonstrate
   // Class
   
   #include <bits/stdc++.h>
   using namespace std;
   
   class Geeks {
       // Access specifier
   public:
       // Data Members
       string geekname;
   
       // Member Functions()
       void printname()
       {
           cout << "Geekname is: " << geekname;
       }
   };
   
   int main()
   {
   
       // Declare an object of class geeks
       Geeks obj1;
   
       // accessing data member
       obj1.geekname = "GeeksForGeeks";
   
       // accessing member function
       obj1.printname();
   
       return 0;
   }
   Output:

   Geekname is: GeeksForGeeks
   

2. Structure: A structure is a user defined data type in C/C++. A structure creates a data type that can be used to group items of possibly different types into a single type.

   Syntax:

   struct address {
       char name[50];
       char street[100];
       char city[50];
       char state[20];
       int pin;
   };

   Example:

   // C++ program to demonstrate
   // Structures in C++
   
   #include <iostream>
   using namespace std;
   
   struct Point {
       int x, y;
   };
   
   int main()
   {
       // Create an array of structures
       struct Point arr[10];
   
       // Access array members
       arr[0].x = 10;
       arr[0].y = 20;
   
       cout << arr[0].x << ", " << arr[0].y;
   
       return 0;
   }
   Output:

   10, 20
   

3. Union: Like Structures, union is a user defined data type. In union, all members share the same memory location. For example in the following C program, both x and y share the same location. If we change x, we can see the changes being reflected in y.

   #include <iostream>
   using namespace std;
   
   // Declaration of union is same as the structures
   union test {
       int x, y;
   };
   
   int main()
   {
       // A union variable t
       union test t;
   
       // t.y also gets value 2
       t.x = 2;
   
       cout << "After making x = 2:"
            << endl
            << "x = " << t.x
            << ", y = " << t.y
            << endl;
   
       // t.x is also updated to 10
       t.y = 10;
   
       cout << "After making Y = 10:"
            << endl
            << "x = " << t.x
            << ", y = " << t.y
            << endl;
   
       return 0;
   }
   Output:

   After making x = 2:
   x = 2, y = 2
   After making Y = 10:
   x = 10, y = 10
   

4. Enumeration: Enumeration (or enum) is a user defined data type in C. It is mainly used to assign names to integral constants, the names make a program easy to read and maintain.

   Syntax:

   enum State {Working = 1, Failed = 0}; 

   // Program to demonstrate working
   // of enum in C++
   
   #include <iostream>
   using namespace std;
   
   enum week { Mon,
               Tue,
               Wed,
               Thur,
               Fri,
               Sat,
               Sun };
   
   int main()
   {
       enum week day;
   
       day = Wed;
   
       cout << day;
   
       return 0;
   }
   Output:

   2
   

5. Typedef : C++ allows you to define explicitly new data type names by using the keyword typedef. Using typedef does not actually create a new data class, rather it defines a name for an existing type. This can increase the portability(the ability of a program to be used across different types of machines; i.e., mini, mainframe, micro, etc; without much changes into the code)of a program as only the typedef statements would have to be changed. Using typedef one can also aid in self-documenting code by allowing descriptive names for the standard data types.

   Syntax:

   typedef type name;

   where type is any C++ data type and name is the new name for this data type.
   This defines another name for the standard type of C++.

   Example:

   // C++ program to demonstrate typedef
   #include <iostream>
   using namespace std;
   
   // After this line BYTE can be used
   // in place of unsigned char
   typedef unsigned char BYTE;
   
   int main()
   {
       BYTE b1, b2;
       b1 = 'c';
       cout << " " << b1;
       return 0;
   }
   Output:

   c