4.3 Aggregates


4.3 Aggregates

1 [{aggregate} An aggregate combines component values into a composite value of an array type, record type, or record extension.] {literal: See also aggregate}


2 aggregate ::= record_aggregate | extension_aggregate | array_aggregate

Name Resolution Rules

3/2 {expected type (aggregate) [partial]} The expected type for an aggregate shall be a single array type, record type, or record extension. 

3.a Discussion: See 8.6, “The Context of Overload Resolution” for the meaning of “shall be a single ... type.” 

Legality Rules

4 An aggregate shall not be of a class-wide type. 

4.a Ramification: When the expected type in some context is class-wide, an aggregate has to be explicitly qualified by the specific type of value to be created, so that the expected type for the aggregate itself is specific. 

4.b Discussion: We used to disallow aggregates of a type with unknown discriminants. However, that was unnecessarily restrictive in the case of an extension aggregate, and irrelevant to a record aggregate (since a type that is legal for a record aggregate could not possibly have unknown discriminants) and to an array aggregate (the only specific types that can have unknown discriminants are private types, private extensions, and types derived from them). 

Dynamic Semantics

5 {evaluation (aggregate) [partial]} For the evaluation of an aggregate, an anonymous object is created and values for the components or ancestor part are obtained (as described in the subsequent subclause for each kind of the aggregate) and assigned into the corresponding components or ancestor part of the anonymous object. {assignment operation (during evaluation of an aggregate)} Obtaining the values and the assignments occur in an arbitrary order. The value of the aggregate is the value of this object.

5.a Discussion: The ancestor part is the set of components inherited from the ancestor type. The syntactic category ancestor_part is the expression or subtype_mark that specifies how the ancestor part of the anonymous object should be initialized. 

5.b Ramification: The assignment operations do the necessary value adjustment, as described in 7.6. Note that the value as a whole is not adjusted — just the subcomponents (and ancestor part, if any). 7.6 also describes when this anonymous object is finalized.

5.c If the ancestor_part is a subtype_mark the Initialize procedure for the ancestor type is applied to the ancestor part after default-initializing it, unless the procedure is abstract, as described in 7.6. The Adjust procedure for the ancestor type is not called in this case, since there is no assignment to the ancestor part as a whole. 

6 {Discriminant_Check [partial]} {check, language-defined (Discriminant_Check)} If an aggregate is of a tagged type, a check is made that its value belongs to the first subtype of the type. {Constraint_Error (raised by failure of run-time check)} Constraint_Error is raised if this check fails. 

6.a Ramification: This check ensures that no values of a tagged type are ever outside the first subtype, as required for inherited dispatching operations to work properly (see 3.4). This check will always succeed if the first subtype is unconstrained. This check is not extended to untagged types to preserve upward compatibility.

Extensions to Ada 83

6.b {extensions to Ada 83} We now allow extension_aggregates. 

Wording Changes from Ada 83

6.c We have adopted new wording for expressing the rule that the type of an aggregate shall be determinable from the outside, though using the fact that it is nonlimited record (extension) or array.

6.d An aggregate now creates an anonymous object. This is necessary so that controlled types will work (see 7.6). 

Incompatibilities With Ada 95

6.e/2 {incompatibilities with Ada 95} In Ada 95, a limited type is not considered when resolving an aggregate. Since Ada 2005 now allows limited aggregates, we can have incompatibilities. For example: 


type Lim is limited
      Comp: Integer;
   end record;


type Not_Lim is
      Comp: Integer;
   end record;


procedure P(X: Lim);
procedure P(X: Not_Lim);

6.i/2 P((Comp => 123)); -- Illegal in Ada 2005, legal in Ada 95

6.j/2 The call to P is ambiguous in Ada 2005, while it would not be ambiguous in Ada 95 as the aggregate could not have a limited type. Qualifying the aggregate will eliminate any ambiguity. This construction would be rather confusing to a maintenance programmer, so it should be avoided, and thus we expect it to be rare. 

Extensions to Ada 95

6.k/2 {extensions to Ada 95} Aggregates can be of a limited type.

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