The visibility rules, given below, determine which declarations are visible and directly visible at each place within a program. The visibility rules apply to both explicit and implicit declarations.
A declaration is defined to be directly visible at places where a name consisting of only an identifier or operator_symbol is sufficient to denote the declaration; that is, no selected_component notation or special context (such as preceding => in a named association) is necessary to denote the declaration. A declaration is defined to be visible wherever it is directly visible, as well as at other places where some name (such as a selected_component) can denote the declaration.
The syntactic category direct_name is used to indicate contexts where direct visibility is required. The syntactic category selector_name is used to indicate contexts where visibility, but not direct visibility, is required.
There are two kinds of direct visibility: immediate visibility and use-visibility. A declaration is immediately visible at a place if it is directly visible because the place is within its immediate scope. A declaration is use-visible if it is directly visible because of a use_clause (see 8.4). Both conditions can apply.
A declaration can be hidden, either from direct visibility, or from all visibility, within certain parts of its scope. Where hidden from all visibility, it is not visible at all (neither using a direct_name nor a selector_name). Where hidden from direct visibility, only direct visibility is lost; visibility using a selector_name is still possible.
Two or more declarations are overloaded if they all have the same defining name and there is a place where they are all directly visible.
The declarations of callable entities (including enumeration literals) are overloadable, meaning that overloading is allowed for them.
Two declarations are homographs if they have the same defining name, and, if both are overloadable, their profiles are type conformant. An inner declaration hides any outer homograph from direct visibility.
Two homographs are not generally allowed immediately within the same declarative region unless one overrides the other (see Legality Rules below). The only declarations that are overridable are the implicit declarations for predefined operators and inherited primitive subprograms. A declaration overrides another homograph that occurs immediately within the same declarative region in the following cases:
- A declaration that is not overridable overrides one that is overridable, regardless of which declaration occurs first;
- The implicit declaration of an inherited operator overrides that of a predefined operator;
- An implicit declaration of an inherited subprogram overrides a previous implicit declaration of an inherited subprogram.
- For an implicit declaration of a primitive subprogram in a generic unit, there is a copy of this declaration in an instance. However, a whole new set of primitive subprograms is implicitly declared for each type declared within the visible part of the instance. These new declarations occur immediately after the type declaration, and override the copied ones. The copied ones can be called only from within the instance; the new ones can be called only from outside the instance, although for tagged types, the body of a new one can be executed by a call to an old one.
A declaration is visible within its scope, except where hidden from all visibility, as follows:
- An overridden declaration is hidden from all visibility within the scope of the overriding declaration.
- A declaration is hidden from all visibility until the end of the declaration, except:
- For a record type or record extension, the declaration is hidden from all visibility only until the reserved word record;
- For a package_declaration, task declaration, protected declaration, generic_package_declaration, or subprogram_body, the declaration is hidden from all visibility only until the reserved word is of the declaration.
- If the completion of a declaration is a declaration, then within the scope of the completion, the first declaration is hidden from all visibility. Similarly, a discriminant_specification or parameter_specification is hidden within the scope of a corresponding discriminant_specification or parameter_specification of a corresponding completion, or of a corresponding accept_statement.
- The declaration of a library unit (including a library_unit_renaming_declaration) is hidden from all visibility except at places that are within its declarative region or within the scope of a with_clause that mentions it. For each declaration or renaming of a generic unit as a child of some parent generic package, there is a corresponding declaration nested immediately within each instance of the parent. Such a nested declaration is hidden from all visibility except at places that are within the scope of a with_clause that mentions the child.
A declaration with a defining_identifier or defining_operator_symbol is immediately visible (and hence directly visible) within its immediate scope except where hidden from direct visibility, as follows:
- A declaration is hidden from direct visibility within the immediate scope of a homograph of the declaration, if the homograph occurs within an inner declarative region;
- A declaration is also hidden from direct visibility where hidden from all visibility.
Name Resolution Rules
A direct_name shall resolve to denote a directly visible declaration whose defining name is the same as the direct_name. A selector_name shall resolve to denote a visible declaration whose defining name is the same as the selector_name.
These rules on visibility and direct visibility do not apply in a context_clause, a parent_unit_name, or a pragma that appears at the place of a compilation_unit. For those contexts, see the rules in 10.1.6, Environment-Level Visibility Rules.
A non-overridable declaration is illegal if there is a homograph occurring immediately within the same declarative region that is visible at the place of the declaration, and is not hidden from all visibility by the non-overridable declaration. In addition, a type extension is illegal if somewhere within its immediate scope it has two visible components with the same name. Similarly, the context_clause for a subunit is illegal if it mentions (in a with_clause) some library unit, and there is a homograph of the library unit that is visible at the place of the corresponding stub, and the homograph and the mentioned library unit are both declared immediately within the same declarative region. These rules also apply to dispatching operations declared in the visible part of an instance of a generic unit. However, they do not apply to other overloadable declarations in an instance; such declarations may have type conformant profiles in the instance, so long as the corresponding declarations in the generic were not type conformant.
5 Visibility for compilation units follows from the definition of the environment in 10.1.4, except that it is necessary to apply a with_clause to obtain visibility to a library_unit_declaration or library_unit_renaming_declaration.
6 In addition to the visibility rules given above, the meaning of the occurrence of a direct_name or selector_name at a given place in the text can depend on the overloading rules (see 8.6).
7 Not all contexts where an identifier, character_literal, or operator_symbol are allowed require visibility of a corresponding declaration. Contexts where visibility is not required are identified by using one of these three syntactic categories directly in a syntax rule, rather than using direct_name or selector_name.
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Copyright © 2000 The MITRE Corporation, Inc. Ada Reference Manual