13.14 Freezing Rules
This clause defines a place in the program text where each declared entity becomes frozen. A use of an entity, such as a reference to it by name, or (for a type) an expression of the type, causes freezing of the entity in some contexts, as described below. The Legality Rules forbid certain kinds of uses of an entity in the region of text where it is frozen.
The freezing of an entity occurs at one or more places (freezing points) in the program text where the representation for the entity has to be fully determined. Each entity is frozen from its first freezing point to the end of the program text (given the ordering of compilation units defined in 10.1.4).
The end of a declarative_part, protected_body, or a declaration of a library package or generic library package, causes freezing of each entity declared within it, except for incomplete types. A noninstance body other than a renames-as-body causes freezing of each entity declared before it within the same declarative_part.
A construct that (explicitly or implicitly) references an entity can cause the freezing of the entity, as defined by subsequent paragraphs. At the place where a construct causes freezing, each name, expression, implicit_dereference, or range within the construct causes freezing:
- The occurrence of a generic_instantiation causes freezing; also, if a parameter of the instantiation is defaulted, the default_expression or default_name for that parameter causes freezing.
- The occurrence of an object_declaration that has no corresponding completion causes freezing.
- The declaration of a record extension causes freezing of the parent subtype.
A static expression causes freezing where it occurs. An object name or nonstatic expression causes freezing where it occurs, unless the name or expression is part of a default_expression, a default_name, or a per-object expression of a component's constraint, in which case, the freezing occurs later as part of another construct.
An implicit call freezes the same entities that would be frozen by an explicit call. This is true even if the implicit call is removed via implementation permissions.
If an expression is implicitly converted to a type or subtype T, then at the place where the expression causes freezing, T is frozen.
The following rules define which entities are frozen at the place where a construct causes freezing:
- At the place where an expression causes freezing, the type of the expression is frozen, unless the expression is an enumeration literal used as a discrete_choice of the array_aggregate of an enumeration_representation_clause.
- At the place where a name causes freezing, the entity denoted by the name is frozen, unless the name is a prefix of an expanded name; at the place where an object name causes freezing, the nominal subtype associated with the name is frozen.
- At the place where an implicit_dereference causes freezing, the nominal subtype associated with the implicit_dereference is frozen.
- At the place where a range causes freezing, the type of the range is frozen.
- At the place where an allocator causes freezing, the designated subtype of its type is frozen. If the type of the allocator is a derived type, then all ancestor types are also frozen.
- At the place where a callable entity is frozen, each subtype of its profile is frozen. If the callable entity is a member of an entry family, the index subtype of the family is frozen. At the place where a function call causes freezing, if a parameter of the call is defaulted, the default_expression for that parameter causes freezing.
- At the place where a subtype is frozen, its type is frozen. At the place where a type is frozen, any expressions or names within the full type definition cause freezing; the first subtype, and any component subtypes, index subtypes, and parent subtype of the type are frozen as well. For a specific tagged type, the corresponding class-wide type is frozen as well. For a class-wide type, the corresponding specific type is frozen as well.
The explicit declaration of a primitive subprogram of a tagged type shall occur before the type is frozen (see 3.9.2).
The completion of a deferred constant declaration shall occur before the constant is frozen (see 7.4).
An operational or representation item that directly specifies an aspect of an entity shall appear before the entity is frozen (see 13.1).
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Copyright © 2000 The MITRE Corporation, Inc. Ada Reference Manual