AARM95 - Formal Access Types
Author: JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor

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# 12.5.4 Formal Access Types

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[The class determined for a formal access type is the class of all access types.]

#### Syntax

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formal_access_type_definition ::= access_type_definition

#### Legality Rules

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For a formal access-to-object type, the designated subtypes of the formal and actual types shall statically match.
4
If and only if the general_access_modifier constant applies to the formal, the actual shall be an access-to-constant type. If the general_access_modifier all applies to the formal, then the actual shall be a general access-to-variable type (see 3.10).
4.a
Ramification: If no _modifier applies to the formal, then the actual type may be either a pool-specific or a general access-to-variable type.
4.a.1/1
Reason: {8652/0109} Matching an access-to-variable to a formal access-to-constant type cannot be allowed. If it were allowed, it would be possible to create an access-to-variable value designating a constant.
5
For a formal access-to-subprogram subtype, the designated profiles of the formal and the actual shall be mode-conformant, and the calling convention of the actual shall be protected if and only if that of the formal is protected.
5.a
Reason: We considered requiring subtype conformance here, but mode conformance is more flexible, given that there is no way in general to specify the convention of the formal.

#### Examples

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Example of formal access types:
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`-- the formal types of the generic package`
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`generic
type Node is private;
type Link is access Node;
package P is
...
end P;`
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`-- can be matched by the actual types`
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`type Car;
type Car_Name is access Car;`
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`type Car is
record
Pred, Succ : Car_Name;
Number : License_Number;
Owner : Person;
end record;`
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`-- in the following generic instantiation`
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`package R is new P(Node => Car, Link => Car_Name);`

#### Incompatibilities With Ada 83

13.a
The check for matching of designated subtypes is changed from a run-time check to a compile-time check. The Ada 83 rule that ``If the designated type is other than a scalar type, then the designated subtypes shall be either both constrained or both unconstrained'' is removed, since it is subsumed by static matching.

#### Extensions to Ada 83

13.b
Formal access-to-subprogram subtypes and formal general access types are new concepts.

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