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Annotated Ada Reference Manual (Ada 202x Draft 11)Legal Information
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8.3.1 Overriding Indicators

1/2
{AI95-00218-03} An overriding_indicator is used to declare that an operation is intended to override (or not override) an inherited operation. 

Syntax

2/2
{AI95-00218-03} overriding_indicator ::= [notoverriding

Legality Rules

3/3
{AI95-00218-03} {AI95-00348-01} {AI95-00397-01} {AI05-0177-1} If an abstract_subprogram_declaration, null_procedure_declaration, expression_function_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:
4/2
the operation shall be a primitive operation for some type;
5/2
if the overriding_indicator is overriding, then the operation shall override a homograph at the place of the declaration or body;
5.a/3
To be honest: {AI05-0005-1} This doesn't require that the overriding happen at precisely the place of the declaration or body; it only requires that the region in which the overriding is known to have happened includes this place. That is, the overriding can happen at or before the place of the declaration or body. 
6/2
if the overriding_indicator is not overriding, then the operation shall not override any homograph (at any place). 
7/2
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.
7.a/2
Discussion: The overriding and not overriding rules differ slightly. For overriding, we want the indicator to reflect the overriding state at the place of the declaration; otherwise the indicator would be “lying”. Whether a homograph is implicitly declared after the declaration (see 7.3.1 to see how this can happen) has no impact on this check. However, not overriding is different; “lying” would happen if a homograph declared later actually is overriding. So, we require this check to take into account later overridings. That can be implemented either by looking ahead, or by rechecking when additional operations are declared.
7.b/2
The “no lying” rules are needed to prevent a subprogram_declaration and subprogram_body from having contradictory overriding_indicators.
NOTES
8/2
8  {AI95-00397-01} Rules for overriding_indicators of task and protected entries and of protected subprograms are found in 9.5.2 and 9.4, respectively. 

Examples

9/2
{AI95-00433-01} 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:
10/2
type Security_Queue is new Queue with record ...;
11/2
overriding
procedure Append(Q : in out Security_Queue; Person : in Person_Name);
12/5
{AI12-0178-1} overriding
procedure Remove_First(Q : in out Security_Queue; Person : out in Person_Name);
13/2
overriding
function Cur_Count(Q : in Security_Queue) return Natural;
14/2
overriding
function Max_Count(Q : in Security_Queue) return Natural;
15/2
not overriding
procedure Arrest(Q : in out Security_Queue; Person : in Person_Name);
16/2
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.
16.a/2
Discussion: In this case, the subprograms are abstract, so misspellings will get detected anyway. But for other subprograms (especially when deriving from concrete types), the error might never be detected, and a body other than the one the programmer intended might be executed without warning. Thus our new motto: “Overriding indicators — don't derive a type without them!” 

Extensions to Ada 95

16.b/2
{AI95-00218-03} Overriding_indicators are new. These let the programmer state her overriding intentions to the compiler; if the compiler disagrees, an error will be produced rather than a hard to find bug. 

Wording Changes from Ada 2005

16.c/3
{AI95-0177-1} Expression functions can have overriding indicators. 

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