Ada Reference Manual — Legal Information

8.3.1 Overriding Indicators

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An overriding_indicator is used to declare that an operation is intended to override (or not override) an inherited operation.

Syntax

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overriding_indicator ::= [not] overriding

Legality Rules

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If an abstract_subprogram_declaration, null_procedure_declaration, subprogram_body, subprogram_body_stub, subprogram_renaming_declaration, generic_instantiation of a subprogram, or subprogram_declaration other than a protected subprogram has an overriding_indicator, then:

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the operation shall be a primitive operation for some type;

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if the overriding_indicator is overriding, then the operation shall override a homograph at the place of the declaration or body;

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if the overriding_indicator is not overriding, then the operation shall not override any homograph (at any place).

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In addition to the places where Legality Rules normally apply, these rules also apply in the private part of an instance of a generic unit.

NOTES

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8 Rules for overriding_indicators of task and protected entries and of protected subprograms are found in 9.5.2 and 9.4, respectively.

Examples

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The use of overriding_indicators allows the detection of errors at compile-time that otherwise might not be detected at all. For instance, we might declare a security queue derived from the Queue interface of 3.9.4 as:

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type Security_Queue is new Queue with record ...;

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overriding
procedure Append(Q : in out Security_Queue; Person : in Person_Name);

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overriding
procedure Remove_First(Q : in out Security_Queue; Person : in Person_Name);

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overriding
function Cur_Count(Q : in Security_Queue) return Natural;

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overriding
function Max_Count(Q : in Security_Queue) return Natural;

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not overriding
procedure Arrest(Q : in out Security_Queue; Person : in Person_Name);

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The first four subprogram declarations guarantee that these subprograms will override the four subprograms inherited from the Queue interface. A misspelling in one of these subprograms will be detected by the implementation. Conversely, the declaration of Arrest guarantees that this is a new operation.