Selected Components
Author: JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor

Ada Reference Manual — Legal Information

# 4.1.3 Selected Components

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Selected_components are used to denote components (including discriminants), entries, entry families, and protected subprograms; they are also used as expanded names as described below.

#### Syntax

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selected_component ::= prefix . selector_name
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selector_name ::= identifier | character_literal | operator_symbol

#### Name Resolution Rules

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A selected_component is called an expanded name if, according to the visibility rules, at least one possible interpretation of its prefix denotes a package or an enclosing named construct (directly, not through a subprogram_renaming_declaration or generic_renaming_declaration).
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A selected_component that is not an expanded name shall resolve to denote one of the following:
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- A component (including a discriminant):

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The prefix shall resolve to denote an object or value of some non-array composite type (after any implicit dereference). The selector_name shall resolve to denote a discriminant_specification of the type, or, unless the type is a protected type, a component_declaration of the type. The selected_component denotes the corresponding component of the object or value.

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- A single entry, an entry family, or a protected subprogram:

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The prefix shall resolve to denote an object or value of some task or protected type (after any implicit dereference). The selector_name shall resolve to denote an entry_declaration or subprogram_declaration occurring (implicitly or explicitly) within the visible part of that type. The selected_component denotes the corresponding entry, entry family, or protected subprogram.

9.1/2

- A view of a subprogram whose first formal parameter is of a tagged type or is an access parameter whose designated type is tagged:

9.2/2

The prefix (after any implicit dereference) shall resolve to denote an object or value of a specific tagged type T or class-wide type T'Class. The selector_name shall resolve to denote a view of a subprogram declared immediately within the declarative region in which an ancestor of the type T is declared. The first formal parameter of the subprogram shall be of type T, or a class-wide type that covers T, or an access parameter designating one of these types. The designator of the subprogram shall not be the same as that of a component of the tagged type visible at the point of the selected_component. The selected_component denotes a view of this subprogram that omits the first formal parameter. This view is called a prefixed view of the subprogram, and the prefix of the selected_component (after any implicit dereference) is called the prefix of the prefixed view.

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An expanded name shall resolve to denote a declaration that occurs immediately within a named declarative region, as follows:
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- The prefix shall resolve to denote either a package (including the current instance of a generic package, or a rename of a package), or an enclosing named construct.

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- The selector_name shall resolve to denote a declaration that occurs immediately within the declarative region of the package or enclosing construct (the declaration shall be visible at the place of the expanded name — see 8.3). The expanded name denotes that declaration.

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- If the prefix does not denote a package, then it shall be a direct_name or an expanded name, and it shall resolve to denote a program unit (other than a package), the current instance of a type, a block_statement, a loop_statement, or an accept_statement (in the case of an accept_statement or entry_body, no family index is allowed); the expanded name shall occur within the declarative region of this construct. Further, if this construct is a callable construct and the prefix denotes more than one such enclosing callable construct, then the expanded name is ambiguous, independently of the selector_name.

#### Legality Rules

13.1/2
For a subprogram whose first parameter is an access parameter, the prefix of any prefixed view shall denote an aliased view of an object.
13.2/2
For a subprogram whose first parameter is of mode in out or out, or of an anonymous access-to-variable type, the prefix of any prefixed view shall denote a variable.

#### Dynamic Semantics

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The evaluation of a selected_component includes the evaluation of the prefix.
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For a selected_component that denotes a component of a variant, a check is made that the values of the discriminants are such that the value or object denoted by the prefix has this component. The exception Constraint_Error is raised if this check fails.

#### Examples

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Examples of selected components:
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Tomorrow.Month -- a record component (see 3.8)
Next_Car.Owner -- a record component (see 3.10.1)
Next_Car.Owner.Age -- a record component (see 3.10.1)
-- the previous two lines involve implicit dereferences
Writer.Unit -- a record component (a discriminant) (see 3.8.1)
Min_Cell(H).Value -- a record component of the result (see 6.1)
-- of the function call Min_Cell(H)
Cashier.Append -- a prefixed view of a procedure (see 3.9.4)
Control.Seize -- an entry of a protected object (see 9.4)
Pool(K).Write -- an entry of the task Pool(K) (see 9.4)
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Examples of expanded names:
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Key_Manager."<" -- an operator of the visible part of a package (see 7.3.1)
Dot_Product.Sum -- a variable declared in a function body (see 6.1)
Buffer.Pool -- a variable declared in a protected unit (see 9.11)
Buffer.Read -- an entry of a protected unit (see 9.11)
Swap.Temp -- a variable declared in a block statement (see 5.6)
Standard.Boolean -- the name of a predefined type (see A.1)

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