A union is a special data type available in D that enables you to store different data types in the same memory location. You can define a union with many members, but only one member can contain a value at any given time. Unions provide an efficient way of using the same memory location for multiple purposes.

Defining a Union in D

To define a union, you must use the union statement in very similar way as you did while defining structure. The union statement defines a new data type, with more than one member for your program. The format of the union statement is as follows −

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

The union 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 union”s definition, before the final semicolon, you can specify one or more union variables but it is optional. Here is the way you would define a union type named Data which has the three members i, f, and str −

union Data { 
   int i; 
   float f; 
   char str[20]; 
} data; 

A variable of Data type can store an integer, a floating-point number, or a string of characters. This means a single variable (same memory location) can be used to store multiple types of data. You can use any built-in or user defined data types inside a union based on your requirement.

The memory occupied by a union will be large enough to hold the largest member of the union. For example, in the above example, Data type will occupy 20 bytes of memory space because this is the maximum space which can be occupied by character string. The following example displays total memory size occupied by the above union −

import std.stdio; 
  
union Data { 
   int i; 
   float f; 
   char str[20]; 
}; 
  
int main( ) { 
   Data data; 

   writeln( "Memory size occupied by data : ", data.sizeof);

   return 0; 
}

Memory size occupied by data : 20 

import std.stdio;

union Data { 
   int i; 
   float f; 
   char str[13]; 
};  

void main( ) { 
   Data data; 
   
   data.i = 10; 
   data.f = 220.5; 
   
   data.str = "D Programming".dup; 
   writeln( "size of : ", data.sizeof); 
   writeln( "data.i : ", data.i); 
   writeln( "data.f : ", data.f); 
   writeln( "data.str : ", data.str); 
}

size of : 16 
data.i : 1917853764 
data.f : 4.12236e+30 
data.str : D Programming

import std.stdio;

union Data { 
   int i; 
   float f; 
   char str[13]; 
};  
void main( ) { 
   Data data; 
   writeln( "size of : ", data.sizeof);  
   
   data.i = 10; 
   writeln( "data.i : ", data.i); 
   
   data.f = 220.5; 
   writeln( "data.f : ", data.f);  
   
   data.str = "D Programming".dup; 
   writeln( "data.str : ", data.str); 
}

size of : 16 
data.i : 10 
data.f : 220.5 
data.str : D Programming