In the case of reference types with data objects (reference variables) that have a
dynamic type as well as the
static type, then compatibility, which is based entirely on technical type attributes, is not sufficient.
Non-Generic Data Types (Except Reference Types)
Two non-generic data types (not
reference types) and data types that contain reference types as components are compatible if all their technical type attributes match.
The layout of structured types does not only refer to the sequence of elementary components in memory, but also to the combination of components with substructures and whether a substructure is a
boxed component. The names of the components and of the semantic attributes defined in ABAP Dictionary, such as
or documentation, however, are not important. In the case of compatible structures, all components are compatible in pairs. This applies recursively down to the level of elementary data types.
If two structures are both constructed identically but different substructures are declared as boxed components, the structures are not compatible.
In the case of table types, the technical type attributes are as follows:
Every enumerated type is unique and only compatible with itself.
The types specified here are not compatible with one another. For example, an elementary data type is never compatible with a structure, even if the structure has only one component.
Statement TYPES cannot be used to define different enumerated types with the same technical type properties. Even data types constructed with
RTTC methods exactly like an existing enumerated type are not compatible with it. An enumerated type defined by direct or indirect reference (including
RTTI) to an existing enumerated type, is compatible with it however.
Generic Data Types
A non-generic data type (not a reference type) is compatible with a
generic data type if its technical attributes are covered by the generic data type.
A reference type is the static type of reference variables and determines to which objects they can
point. At runtime, reference variables also have a dynamic type determined by the type of the object
pointed to by a reference variable. The dynamic type may be more specialized than the static type. For
this reason, the rules for typing checks, assignments, and comparisons cannot be covered by a compatibility
concept based entirely on the technical attributes of the static type. Instead, the following three points show how reference types can be used together:
When typings are checked, the following is possible:
A reference variable can be passed to a formal parameter typed as a reference variable, provided that the type of the formal parameter is more general or equal to the type of the reference variable (known as an
up cast) and the formal parameter cannot be changed within the procedure.
A reference variable can be assigned to a field symbol typed as a reference variable, provided that the reference types are identical.
Assignments between reference variables can be carried out by using an
up cast or a down cast.
Data reference variables can be compared with all data reference variables and object reference variables can be compared with all object reference variables.
As a rule, data reference variables can only be used with data reference variables and object reference variables can be used only with object reference variables. No conversions take place between
They are either passed as unconverted reference variables, assigned to each other, compared with each other, or no action at all takes place.
These rules for the usability of reference variables for assignments are one-way rules. The type
of a target reference variable to which a source reference variable can be assigned does not itself
have to be assignable to the source reference variable. In contrast, the usability of reference variables in the case of comparisons is not a one-way relationship.
If a reference variable is a component of a structure, the usability of the structure is determined
accordingly. In particular, the usability of this structure type with respect to assignments is also a one-way relationship.