3.3 Objects and Named Numbers
Objects are created at run time and contain a value
of a given type.
An object can be created and initialized
as part of elaborating a declaration, evaluating an
allocator,
aggregate,
or
function_call,
or passing a parameter by copy. Prior to reclaiming the storage for an
object, it is finalized if necessary (see
7.6.1).
Static Semantics
All
of the following are objects:
a formal parameter of a subprogram, entry, or generic
subprogram;
a generic formal object;
a loop parameter;
the result of dereferencing an access-to-object
value (see
4.1);
the return object of a function;
a component, slice, or view conversion of another
object.
An
object is either a
constant object or a
variable object.
Similarly, a view of an object is either a
constant or a
variable.
All views of a constant elementary object are constant. All views of
a constant composite object are constant, except for parts that are of
controlled or immutably limited types; variable views of those parts
and their subcomponents may exist. In this sense, objects of controlled
and immutably limited types are
inherently mutable.
A constant view of an object cannot be used to modify its value. The
terms constant and variable by themselves refer to constant and variable
views of objects.
The value of an object is
read
when the value of any part of the object is evaluated, or when the value
of an enclosing object is evaluated.
The value of
a variable is
updated when an assignment is performed to any part
of the variable, or when an assignment is performed to an enclosing object.
Whether a view of an
object is constant or variable is determined by the definition of the
view. The following (and no others) represent constants:
a formal parameter or generic formal object of
mode in;
a discriminant;
a loop parameter unless specified to be a variable
for a generalized loop (see
5.5.2);
a choice parameter or entry index;
the dereference of an access-to-constant value;
within the body of a protected function (or a function
declared immediately within a
protected_body),
the current instance of the enclosing protected unit;
At the place where a view of
an object is defined, a
nominal subtype is associated with the
view.
The object's
actual
subtype (that is, its subtype) can be more restrictive than the nominal
subtype of the view; it always is if the nominal subtype is an
indefinite
subtype.
A subtype is an
indefinite subtype if it is an unconstrained array subtype, or if it
has unknown discriminants or unconstrained discriminants without defaults
(see
3.7); otherwise, the subtype is a
definite
subtype (all elementary subtypes are definite subtypes). A class-wide
subtype is defined to have unknown discriminants, and is therefore an
indefinite subtype. An indefinite subtype does not by itself provide
enough information to create an object; an additional
constraint
or explicit initialization
expression
is necessary (see
3.3.1). A component cannot
have an indefinite nominal subtype.
A view of a composite
object is
known to be constrained if:
its nominal subtype is constrained, and is not
an untagged partial view; or
its nominal subtype is indefinite; or
its type is immutably limited (see
7.5);
or
it is part of a stand-alone constant (including
a generic formal object of mode in); or
it is part of a formal parameter of mode in;
or
it is a dereference of a pool-specific access type,
and there is no ancestor of its type that has a constrained partial view.
For the purposes
of determining within a generic body whether an object is known to be
constrained:
if a subtype is a descendant of an untagged generic
formal private or derived type, and the subtype is not an unconstrained
array subtype, it is not considered indefinite and is considered to have
a constrained partial view;
if a subtype is a descendant of a formal access
type, it is not considered pool-specific.
A
named number provides
a name for a numeric value known at compile time. It is declared by a
number_declaration.
7 A constant cannot be the target of an
assignment operation, nor be passed as an in out or out
parameter, between its initialization and finalization, if any.
8 The value of a constant object cannot
be changed after its initialization, except in some cases where the object
has a controlled or immutably limited part (see
7.5,
7.6, and
13.9.1).
9 The nominal and actual subtypes of an
elementary object are always the same. For a discriminated or array object,
if the nominal subtype is constrained, then so is the actual subtype.
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