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8.3 Visibility

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. 

Static Semantics

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 is visible within its scope, except where hidden from all visibility, as follows: 
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: 
   An attribute_definition_clause is visible everywhere within its scope.

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”.

Legality 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 compilation unit 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 compilation unit, 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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