Ada Reference Manual — Legal Information

4.1.3 Selected Components

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

selected_component ::= prefix . selector_name

selector_name ::= identifier | character_literal | operator_symbol

Name Resolution Rules

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).

A selected_component that is not an expanded name shall resolve to denote one of the following:

A component (including a discriminant):

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.

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

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:

9.2/3

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 subprogram shall not be an implicitly declared primitive operation of type T that overrides an inherited subprogram implemented by an entry or protected subprogram 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.

An expanded name shall resolve to denote a declaration that occurs immediately within a named declarative region, as follows:

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.

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.

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

The evaluation of a selected_component includes the evaluation of the prefix.

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

Examples of selected components:

17/2

  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)

Examples of expanded names:

  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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