8.3.1 Overriding Indicators
the operation shall be a
primitive operation for some type;
if the overriding_indicator
is overriding, then the operation shall override a homograph at
the place of the declaration or body;
To be honest:
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.
if the overriding_indicator
is not overriding, then the operation shall not override any homograph
(at any place).
to the places where Legality Rules normally apply, these rules also apply
in the private part of an instance of a generic unit.
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
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:
type Security_Queue is new Queue with record ...;
procedure Append(Q : in out Security_Queue; Person : in Person_Name);
procedure Remove_First(Q : in out Security_Queue; Person : in Person_Name);
function Cur_Count(Q : in Security_Queue) return Natural;
function Max_Count(Q : in Security_Queue) return Natural;
procedure Arrest(Q : in out Security_Queue; Person : in Person_Name);
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.
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
Extensions to Ada 95
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
Expression functions can have overriding indicators.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe