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

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# 12.5.3 Formal Array Types

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

#### Syntax

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formal_array_type_definition ::= array_type_definition

#### Legality Rules

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The only form of discrete_subtype_definition that is allowed within the declaration of a generic formal (constrained) array subtype is a subtype_mark.
3.a
Reason: The reason is the same as for forbidding constraints in subtype_indications (see 12.1).
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For a formal array subtype, the actual subtype shall satisfy the following conditions:
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- The formal array type and the actual array type shall have the same dimensionality; the formal subtype and the actual subtype shall be either both constrained or both unconstrained.

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- For each index position, the index types shall be the same, and the index subtypes (if unconstrained), or the index ranges (if constrained), shall statically match (see 4.9.1).

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- The component subtypes of the formal and actual array types shall statically match.

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- If the formal type has aliased components, then so shall the actual.

8.a
Ramification: On the other hand, if the formal's components are not aliased, then the actual's components can be either aliased or not.

#### Examples

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Example of formal array types:
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`-- given the generic package`
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`generic
type Item is private;
type Index is (<>);
type Vector is array (Index range <>) of Item;
type Table is array (Index) of Item;
package P is
...
end P;`
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`-- and the types`
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`type Mix is array (Color range <>) of Boolean;
type Option is array (Color) of Boolean;`
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`-- then Mix can match Vector and Option can match Table`
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`package R is new P(Item => Boolean, Index => Color,
Vector => Mix, Table => Option);`
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`-- Note that Mix cannot match Table and Option cannot match Vector`

#### Incompatibilities With Ada 83

16.a
The check for matching of component subtypes and index subtypes or index ranges is changed from a run-time check to a compile-time check. The Ada 83 rule that ``If the component type is not a scalar type, then the component subtypes shall be either both constrained or both unconstrained'' is removed, since it is subsumed by static matching. Likewise, the rules requiring that component types be the same is subsumed.

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