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GUIDELINE 5.6
Instance Constructor
ABAP_BACKGROUND When you define an ABAP class, you specify who creates an instance of this class or who may access the instance constructor of the class. To do this, you use the CREATE addition of the CLASS ... DEFINITION statement. The CREATE PUBLIC addition is the default setting and allows for the instancing by any user of the class. By specifying CREATE PROTECTED, you can restrict the object creation to the class itself and its subclasses. With the CREATE PRIVATE addition, objects can only be created by the class itself. The restriction of the object creation to the class itself is useful in connection with the singleton design pattern, for example, where the class itself performs the object creation. From a technical point of view, the instance constructor can be declared in all visibility sections that are more general or equal to the instantiation specified in the CREATE addition of the CLASS ... DEFINITION statement, using the METHODS constructor statement. However, the actual visibility is controlled by the CREATE addition.
ABAP_RULE Declare the instance constructor in the public visibility section. Always declare the instance constructor of a global class in its public visibility section and independently of the instantiation specified by the CREATE addition in the class definition.
ABAP_DETAILS The components of global classes are stored internally, separated according to the visibility section they belong to. Depending on the usage type of the class, only parts of the class are considered by the ABAP Compiler during compilation. This procedure requires that the constructor of a global class is always declared in the public visibility section of the class. For these technical reasons, the instance constructor of a global class is always supposed to be declared in the public visibility section (PUBLIC SECTION). If it is declared in another visibility section, in individual cases this may result in unjustified syntax errors when global classes are used. Exception The technical restrictions mentioned only apply to the processing of global classes. Within local classes, the instance constructor can also be defined in other visibility sections. However, this positioning should correspond with the visibility section specified using the CREATE addition. Such a strategy enables you to use types for the interface of the instance constructor of a local class that are only accessible in a restricted visibility section.
Exception The technical restrictions mentioned only apply to the processing of global classes. Within local classes, the instance constructor can also be defined in other visibility sections. However, this positioning should correspond with the visibility section specified using the CREATE addition. Such a strategy enables you to use types for the interface of the instance constructor of a local class that are only acc essible in a restricted visibility section.
