A union declaration is a multistatement declaration defining a data area that can be shared intermittently during program execution by one or more fields or groups of fields. A union declaration must be within a structure declaration.

A union declaration is initiated by a UNION statement and terminated by an END UNION statement. Enclosed within these statements are two or more map declarations, initiated and terminated by MAP and END MAP statements. Each unique field or group of fields is defined by a separate map declaration.

A union declaration takes the following form:

UNION
   map-declaration
   map-declaration
   [map-declaration]
    . . .
   [map-declaration]
END UNION

map-declaration

Takes the following form:

MAP
   field-declaration
   [field-declaration]
    . . .
   [field-declaration]
END MAP

field-declaration

Is a structure declaration or RECORD statement contained within a union declaration, a union declaration contained within a union declaration, or the declaration of a data field (having a data type) within a union. See Section 4.17 for a more detailed description of what can be specified in field declarations.

Rules and Behavior

As with normal Fortran type declarations, data can be initialized in field declaration statements in union declarations. However, if fields within multiple map declarations in a single union are initialized, the data declarations are initialized in the order in which the statements appear. As a result, only the final initialization takes effect and all of the preceding initializations are overwritten.

The size of the shared area established for a union declaration is the size of the largest map defined for that union. The size of a map is the sum of the sizes of the fields declared within it.

As the variables or arrays declared in map fields in a union declaration are assigned values during program execution, the values are established in a record in the field shared with other map fields in the union. The fields of only one of the map declarations are defined within a union at any given point in the execution of a program. However, if you overlay one variable with another smaller variable, that portion of the initial variable is retained that is not overlaid. Depending on the application, the retained portion of an overlaid variable may or may not contain meaningful data and can be used at a later point in the program.

Manipulating data by using union declarations is similar to the effect of EQUIVALENCE statements. The difference is that data entities specified within EQUIVALENCE statements are concurrently associated with a common storage location and the data residing there; with union declarations you can use one discrete storage location to alternately contain a variety of fields (arrays or variables).

With union declarations, only one map declaration within a union declaration can be associated at any point in time with the storage location that they share. Whenever a field within another map declaration in the same union declaration is referenced in your program, the fields in the prior map declaration become undefined and are succeeded by the fields in the map declaration containing the newly referenced field.

Example

In the following example, the structure WORDS_LONG is defined. This structure contains a union declaration defining two map fields. The first map field consists of three INTEGER*2 variables (WORD_0, WORD_1, and WORD_2), and the second, an INTEGER*4 variable, LONG:

STRUCTURE /WORDS_LONG/
  UNION
    MAP
      INTEGER*2  WORD_0, WORD_1, WORD_2
    END MAP
    MAP
      INTEGER*4  LONG
    END MAP
  END UNION
END STRUCTURE

The length of any record with the structure WORDS_LONG is 6 bytes. Figure 4-7 shows the memory mapping of any record with the structure WORDS_LONG:

Figure 4-7 Memory Map of Structure WORDS_LONG