12.2 Unions

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A structure is used to define a data type with several members . Each member takes up a separate storage location. For example, the structure:

 struct rectangle {      int width;      int height;  }; 

appears in memory as shown in Figure 12-1.

A union is similar to a structure; however, it defines a single location that can be given many different member names :

 union value {      long int i_value;   // Long integer version of value     float f_value;      // Floating version of value } 

Figure 12-1. Structure and union layout

The members i_value and f_value share the same space. You might think of a structure as a large box divided up into several different compartments, each with its own name . A union is a box, not divided at all, with several different labels placed on the single compartment inside.

In a structure, the members do not interact. Changing one member does not change any others. In a union, all members occupy the same space, so only one may be active at a time. In other words, if you put something in i_value , assigning something to f_value wipes out the old value of i_value .

The following shows how a union may be used:

 /*   * Define a variable to hold an integer or    * a real number (but not both)   */  union value {      long int i_value;   // The real number     float f_value;      // The floating point number } data;  int i;                  // Random integer float f;                // Random floating point number int main(  )  {      data.f_value = 5.0;      data.i_value = 3;   // Data.f_value overwritten     i = data.i_value;   // Legal     f = data.f_value;   // Not legal; will generate unexpected results     data.f_value = 5.5; // Put something in f_value/clobber i_value     i = data.i_value;   // Not legal; will generate unexpected results     return (0); } 

Suppose you want to store the information about a shape. The shape can be any standard shape such as a circle, rectangle, or triangle. The information needed to draw a circle is different from the data needed to draw a rectangle, so you need to define different structures for each shape:

 struct circle {     int radius;           // Radius of the circle in pixels }; struct rectangle {     int height, width;   // Size of the rectangle in pixels }; struct triangle {     int base;            // Length of the triangle's base in pixels     int height;          // Height of the triangle in pixels }; 

Now you define a structure to hold the generic shape. The first member is a code that tells you what type of shape you have. The second is a union that holds the shape information:

 const int SHAPE_CIRCLE    = 0;    // Shape is a circle const int SHAPE_RECTANGLE = 1;    // Shape is a rectangle const int SHAPE_TRIANGLE  = 2;    // Shape is a triangle struct shape {     int kind;               // What kind of shape is stored     union shape_union {     // Union to hold shape information         struct circle    circle_data;     // Data for a circle         struct rectangle rectangle_data;  // Data for a rectangle         struct triangle  triangle_data;   // Data for a triangle     } data; }; 

Graphically you can represent shape as a large box. Inside the box is the single integer kind and our union shape_union . The union is a box with three labels on it. The question is which one is the "real" label. You can't tell from looking at the union, but that's why you defined kind . It tells us which label to read. The layout of the shape structure is illustrated by Figure 12-2.

Figure 12-2. "shape" layout

Now you can store a circle in the generic shape:

 struct shape a_shape; //... a_shape.kind = SHAPE_CIRCLE; a_shape.data.circle_data.radius = 50;  // Define the radius of the circle 

In this example we are define one basic data type (a shape) and adding in specific information for a bunch of different types of shapes . Although we are using a union to organize our data, this sort of data can be better organized using base and derived classes. (See Chapter 21.)

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