3.3 Objects and Named Numbers
[Objects are created at run time and contain a value
of a given type.
{creation (of an object)}
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
{object}
All of the following are objects:
Glossary entry: {
Object} An object
is either a constant or a variable. An object contains a value. An object
is created by an
object_declaration
or by an
allocator.
A formal parameter is (a view of) an object. A subcomponent of an object
is an object.
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);
a component, slice, or view conversion of another
object.
{constant}
{variable}
{constant object}
{variable object}
{constant view}
{variable view}
An object is either a
constant object or a
variable object. The value of a constant object cannot be changed
between its initialization and its finalization, whereas the value of
a variable object can be changed. Similarly, a view of an object is either
a
constant or a
variable. All views of a constant object
are constant. A constant view of a variable object cannot be used to
modify the value of the variable. The terms constant and variable by
themselves refer to constant and variable views of objects.
{read (the value
of an object)} 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.
{update
(the value of an object)} 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.
Ramification: Reading and updating are
intended to include read/write references of any kind, even if they are
not associated with the evaluation of a particular construct. Consider,
for example, the expression “X.all(F)”, where X is
an access-to-array object, and F is a function. The implementation is
allowed to first evaluate “X.all” and then F. Finally,
a read is performed to get the value of the F'th component of the array.
Note that the array is not necessarily read as part of the evaluation
of “X.all”. This is important, because if F were to
free X using Unchecked_Deallocation, we want the execution of the final
read to be erroneous.
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:
X : access constant T;
is not a constant.
a formal parameter or generic formal object of
mode in;
a discriminant;
a loop parameter, choice parameter, or entry index;
the dereference of an access-to-constant value;
{nominal subtype}
At the place where a view of an object is defined,
a
nominal subtype is associated with the view.
{actual
subtype} {subtype
(of an object): See actual subtype of an object} 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.
{indefinite
subtype} {definite
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.]
{named number}
A
named number provides a name for a numeric
value known at compile time. It is declared by a
number_declaration.
5 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.
6 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.
Extensions to Ada 83
{
extensions to Ada 83}
There
are additional kinds of objects (choice parameters and entry indices
of entry bodies).
The result of a function and of evaluating an
aggregate are considered (constant) objects. This is necessary to explain
the action of finalization on such things. Because a
function_call
is also syntactically a
name
(see
4.1), the result of a
function_call
can be renamed, thereby allowing repeated use of the result without calling
the function again.
Wording Changes from Ada 83
This clause and its subclauses now follow the
clause and subclauses on types and subtypes, to cut down on the number
of forward references.
The term nominal subtype is new. It is used
to distinguish what is known at compile time about an object's constraint,
versus what its "true" run-time constraint is.
The terms definite and indefinite (which apply
to subtypes) are new. They are used to aid in the description of generic
formal type matching, and to specify when an explicit initial value is
required in an
object_declaration.
We have moved the syntax for
object_declaration
and
number_declaration
down into their respective subclauses, to keep the syntax close to the
description of the associated semantics.
We talk about variables and constants here,
since the discussion is not specific to
object_declarations,
and it seems better to have the list of the kinds of constants juxtaposed
with the kinds of objects.
We no longer talk about indirect updating due
to parameter passing. Parameter passing is handled in 6.2 and 6.4.1 in
a way that there is no need to mention it here in the definition of read
and update. Reading and updating now includes the case of evaluating
or assigning to an enclosing object.
Wording Changes from Ada 95
{
AI95-00416-01}
Clarified that the return object is the object created by a function
call.
Sponsored by Ada-Europe