13.14 Freezing Rules
This subclause 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).
This subclause also defines
a place in the program text where the profile of each declared callable
entity becomes
frozen. A use of a callable entity causes freezing
of its profile in some contexts, as described below. At the place where
the profile of a callable entity becomes frozen, the entity itself becomes
frozen.
The end of a
declarative_part,
protected_body,
or a declaration of a library package or generic library package, causes
freezing of each entity and profile declared within it, except
for incomplete types.
A
proper_body,
body_stub,
or
entry_body
causes freezing of each entity and profile declared before it within
the same
declarative_part
that is not an incomplete type; it only causes freezing of an incomplete
type if the body is within the immediate scope of the incomplete type.
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, except that a
name
which is a generic actual parameter whose corresponding generic formal
parameter is a formal incomplete type (see
12.5.1)
does not cause freezing. In addition, if a parameter of the instantiation
is defaulted, the
default_expression
or
default_name
for that parameter causes freezing.
At the occurrence of a renames-as-body whose
callable_entity_name
denotes an expression function, the return expression of the expression
function 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.
The declaration of a record extension, interface
type, task unit, or protected unit causes freezing of any progenitor
types specified in the declaration.
A static expression (other than
within an
aspect_specification)
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,
the return expression of an expression function, an
aspect_specification,
or a per-object expression of a component's
constraint,
in which case, the freezing occurs later as part of another construct
or at the freezing point of an associated entity.
An implicit call freezes the
same entities and profiles 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 a function call causes freezing, the profile of the function
is frozen. Furthermore, if a parameter of the call is defaulted, the
default_expression
for that parameter causes freezing. If the function call is to an expression
function, the return expression of the expression function causes freezing.
At the
place where a
generic_instantiation
causes freezing of a callable entity, the profile of that entity is frozen
unless the formal subprogram corresponding to the callable entity has
a parameter or result of a formal untagged incomplete type; if the callable
entity is an expression function, the return expression of the expression
function causes freezing.
At the place where a use of
the Access or Unchecked_Access attribute whose
prefix
denotes an expression function causes freezing, the return expression
of the expression function causes freezing.
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 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 profile is frozen, each subtype of the profile is frozen.
If the corresponding callable entity is a member of an entry family,
the index subtype of the family is frozen.
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.
At the place where
a specific tagged type is frozen, the primitive subprograms of the type
are frozen. At the place where a type is frozen, any subprogram named
in an
attribute_definition_clause
for the type is frozen.
Legality Rules
The explicit declaration
of a primitive subprogram of a tagged type shall occur before the type
is frozen (see
3.9.2).
A type shall be completely
defined before it is frozen (see
3.11.1
and
7.3).
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).
Dynamic Semantics
The tag (see
3.9) of a
tagged type T is created at the point where T is frozen.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
