A structure is a hierarchically ordered set of values that may be of different data types. The immediate components of a structure are called members of the structure. A structure that is itself a member of another structure is called a substructure. A structure that is not a substructure is called a major structure.
The hierarchical organization of a structure is specified by using level-numbers, as shown in the following example:
DECLARE 1 S,
2 A FIXED BINARY,
2 B FLOAT BINARY,
2 T,
3 P POINTER,
3 Q CHARACTER(10);
In this example, S is a major structure, also called a level-one structure. All major structures must have a level-number of 1. The members of S are A, B, and T. T is a substructure with members P and Q.
Members normally have a level-number that is 1 greater than their containing structure; however, they may be given any level-number that is greater than the level-number of their containing structure and less than the level-numbers they contain. For example:
DECLARE 1 S,
20 A FIXED BINARY,
20 B FLOAT BINARY,
20 T,
30 P POINTER,
30 Q CHARACTER(10);
The structure shown in this example is equivalent to the structure shown in the example immediately preceding it.
An entire structure may be transmitted in stream or record I/O, passed as an argument, or assigned to another structure of identical size and shape and having members of corresponding identical data types, but no conversions or calculations can be performed on entire structures.
If a structure contains one or more members with a variable size, place all such members at the end of the structure following all the members with a fixed size. This placement improves the addressing of the fixed size members.
Members of structures can be referenced in any context that permits a reference to a variable. If a member's name is not otherwise declared in the same scope (that is, in the same block or containing block), the member may be referenced by its name alone.
The name of a member must be unique within its immediately containing structure, but may be used as the name of members of other structures or as the name of a nonmember. If a member's name has been used for more than one object, each reference to the member must be sufficiently qualified by the names of its containing structures to uniquely identify the intended member. In a qualified reference, the names used are listed in their hierarchical order, separated by periods. For example:
DECLARE A FIXED BINARY;
DECLARE 1 S,
2 A FLOAT BINARY,
2 T,
3 A POINTER;
In this example, a reference to A is a reference to the fixed-point variable. A reference to S.A is a reference to the floating-point variable. A reference to T.A or S.T.A is a reference to the pointer variable.
Use of the major structure name as a qualifier is recommended. It is also advisable to avoid using the same member name more than once anywhere within the major structure or any contained structures. For more information, see the section Reference Resolution in the chapter References.
A structure may have the UNION attribute applied to it, which causes all immediate members of that structure to occupy the same memory locations. For more information, see the chapter Declarations and Attributes.