3.3.1 Object Declarations
declares a stand-alone
object with a given nominal subtype and,
optionally, an explicit initial value given by an initialization expression.
For an array,
task, or protected object, the object_declaration
may include the definition of the (anonymous) type of the object.
Name Resolution Rules
with the reserved word constant
declares a constant object.
it has an initialization expression, then it is called a full constant
it is called a deferred constant declaration
. The rules for deferred
constant declarations are given in clause 7.4
The rules for full constant declarations are given in this subclause.
Any declaration that includes a defining_identifier_list
with more than one defining_identifier
is equivalent to a series of declarations each containing one defining_identifier
from the list, with the rest of the text of the declaration copied for
each declaration in the series, in the same order as the list. The remainder
of this International Standard relies on this equivalence; explanations
are given for declarations with a single defining_identifier
The phrase “full type definition” here includes the case
of an anonymous array, access,
A component of an object is
said to require late initialization if it has an access discriminant
value constrained by a per-object expression, or if it has an initialization
expression that includes a name denoting the current instance of the
type or denoting an access discriminant.
Reason: Such components
can depend on the values of other components of the object. We want to
initialize them as late and as reproducibly as possible.
If a composite object declared by an object_declaration
has an unconstrained nominal subtype, then if this subtype is indefinite
or the object is constant or aliased (see 3.10)
the actual subtype of this object is constrained. The constraint
is determined by the bounds or discriminants (if any) of its initial
the object is said to be constrained by
its initial value
the case of an aliased object, this initial value may be either explicit
or implicit; in the other cases, an explicit initial value is required.]
When not constrained by its initial value, the actual and nominal
subtypes of the object are the same.
its actual subtype is constrained, the object is called a constrained
without an initialization expression, any initial values for the object
or its subcomponents are determined by the implicit initial values
defined for its nominal subtype, as follows:
The implicit initial value for an access subtype
is the null value of the access type.
The implicit initial value for a scalar subtype
that has the Default_Value aspect specified is the value of that aspect
converted to the nominal subtype (which might raise Constraint_Error
— see 4.6, “Type
is a Dynamic Semantics rule, so the visibility of the aspect_specification
is not relevant — if the full type for a private type has the Default_Value
aspect specified, partial views of the type also have this implicit initial
The implicit initial (and only) value for each
discriminant of a constrained discriminated subtype is defined by the
For a (definite) composite subtype, the implicit initial value of each
component with a default_expression
is obtained by evaluation of this expression and conversion to the component's
nominal subtype (which might raise Constraint_Error — see 4.6, “Type
), unless the component is a discriminant
of a constrained subtype (the previous case), or is in an excluded variant
component that does not have a default_expression
if the composite subtype has the Default_Component_Value
aspect specified, the implicit initial value is the value of that aspect
converted to the component's nominal subtype; otherwise,
initial values are those determined by the component's nominal subtype.
For a protected or task subtype, there is an implicit
component (an entry queue) corresponding to each entry, with its implicit
initial value being an empty queue.
Implementation Note: The implementation
may add implicit components for its own use, which might have implicit
initial values. For a task subtype, such components might represent the
state of the associated thread of control. For a type with dynamic-sized
components, such implicit components might be used to hold the offset
to some explicit component.
is first elaborated. This creates the nominal subtype (and the anonymous
type in the last four latter
If the object_declaration
includes an initialization expression, the (explicit) initial value is
obtained by evaluating the expression and converting it to the nominal
subtype (which might raise Constraint_Error — see 4.6
The object is created, and, if there is not an initialization expression,
the object is initialized by default. When
an object is initialized by default,
any per-object constraints expressions
) are elaborated evaluated
and any implicit initial values for the object or for its subcomponents
are obtained as determined by the nominal subtype. Any initial values (whether explicit
or implicit) are assigned to the object or to the corresponding subcomponents.
As described in 5.2 and 7.6,
Initialize and Adjust procedures can be called.
For a per-object constraint
that contains some per-object expressions and some non-per-object expressions,
the values used for the constraint consist of the values of the non-per-object
expressions evaluated at the point of the type_declaration
and the values of the per-object expressions evaluated at the point of
the creation of the object.
The elaboration of per-object constraints was
presumably performed as part of the dependent compatibility check in
Ada 83. If the object is of a limited type with an access discriminant,
is elaborated at this time (see 3.7
Reason: The reason we say that evaluating
an explicit initialization expression happens before creating the object
is that in some cases it is impossible to know the size of the object
being created until its initial value is known, as in “X: String
:= Func_Call(...);”. The implementation can create the object early
in the common case where the size can be known early, since this optimization
is semantically neutral.
values (whether explicit or implicit) are assigned to the object or to
the corresponding subcomponents. As described in 5.2
and 7.6, Initialize and Adjust procedures can
Since the initial values
have already been converted to the appropriate nominal subtype, the only
Constraint_Errors that might occur as part of these assignments are for
values outside their base range that are used to initialize unconstrained
numeric subcomponents. See 3.5
For the third step above, the
object creation and any elaborations and
are performed in an arbitrary order subject to the following restrictions:,
except that if the default_expression
for a discriminant is evaluated to obtain its initial value, then this
evaluation is performed before that of the default_expression
for any component that depends on the discriminant, and also before that
of any default_expression
that includes the name of the discriminant. The evaluations of the third
step and the assignments of the fourth step are performed in an arbitrary
order, except that each evaluation is performed before the resulting
value is assigned.
Assignment to any part of the object is preceded
by the evaluation of the value that is to be assigned.
Reason: Duh. But
we ought to say it. Note that, like any rule in the International Standard,
it doesn't prevent an “as-if” optimization; as long as the
semantics as observed from the program are correct, the compiler can
generate any code it wants.
Reason: Duh again.
But we have to say this, too. It's odd that Ada 95 only required the
default expressions to be evaluated before the discriminant is used;
it says nothing about discriminant values that come from subtype_indications.
The evaluation of the default_expression
for any component that depends on a discriminant is preceded by the assignment
to that discriminant.
type R(D : Integer := F) is
S : String(1..D) := (others => G);
X : R;
For the elaboration of the declaration of X,
it is important that F be evaluated before the aggregate.
The assignments to any components, including implicit
components, not requiring late initialization must
precede the initial value evaluations
for any components requiring late initialization; if two components both
require late initialization, then assignments to parts of the component
occurring earlier in the order of the component declarations must
precede the initial value evaluations
of the component occurring later.
Components that require late initialization can refer to the entire
object during their initialization. We want them to be initialized as
late as possible to reduce the chance that their initialization depends
on uninitialized components. For instance:
type T (D : Natural) is
C1 : T1 (T'Access);
C2 : Natural := F (D);
C3 : String (1 .. D) := (others => ' ');
Component C1 requires
late initialization. The initialization could depend on the values of
any component of T, including D, C2, or C3. Therefore, we want to it
to be initialized last. Note that C2 and C3 do not require late initialization;
they only have to be initialized after D.
It is possible for there
to be more than one component that requires late initialization. In this
case, the language can't prevent problems, because all of the components
can't be the last one initialized. In this case, we specify the order
of initialization for components requiring late initialization; by doing
so, programmers can arrange their code to avoid accessing uninitialized
components, and such arrangements are portable. Note that if the program
accesses an uninitialized component, 13.9.1
defines the execution to be erroneous.
[There is no implicit initial value defined for a scalar subtype unless the Default_Value aspect has been specified for the type
In the absence of an explicit initialization or the specification of the Default_Value aspect
, a newly created
scalar object might have a value that does not belong to its subtype
To be honest:
It could even be represented
by a bit pattern that doesn't actually represent any value of the type
at all, such as an invalid internal code for an enumeration type, or
a NaN for a floating point type. It is a generally a bounded error to
reference scalar objects with such “invalid representations”,
as explained in 13.9.1
There is no requirement
that two objects of the same scalar subtype have the same implicit initial
“value” (or representation). It might even be the case that
two elaborations of the same object_declaration
produce two different initial values. However, any particular uninitialized
object is default-initialized to a single value (or invalid representation).
Thus, multiple reads of such an uninitialized object will produce the
same value each time (if the implementation chooses not to detect the
10 Implicit initial values are not defined
for an indefinite subtype, because if an object's nominal subtype is
indefinite, an explicit initial value is required.
12 The type of a stand-alone object cannot
be abstract (see 3.9.3
Example of a multiple
-- the multiple object declaration
-- is equivalent to the two single object declarations in the order given
John : not null
Person_Name := new
Person(Sex => M);
Paul : not null
Person_Name := new
Person(Sex => M);
Examples of variable
Count, Sum : Integer;
Size : Integer range
0 .. 10_000 := 0;
Sorted : Boolean := False;
Color_Table : array
(1 .. Max) of
Option : Bit_Vector(1 .. 10) := (others
Hello : aliased constant
String := "Hi, world.";
θ, φ : Float range -π .. +π;
Examples of constant
Limit : constant
Integer := 10_000;
Low_Limit : constant
Integer := Limit/10;
Tolerance : constant
Real := Dispersion(1.15);
Hello_Msg : constant access String := Hello'Access; -- see 3.10.2
Extensions to Ada 83
A variable declared by an object_declaration
can be constrained by its initial value; that is, a variable of a nominally
unconstrained array subtype, or discriminated type without defaults,
can be declared so long as it has an explicit initial value. In Ada 83,
this was permitted for constants, and for variables created by allocators,
but not for variables declared by object_declaration
This is particularly important for tagged class-wide types, since there
is no way to constrain them explicitly, and so an initial value is the
only way to provide a constraint. It is also important for generic formal
private types with unknown discriminants.
We now allow an unconstrained_array_definition
in an object_declaration
This allows an object of an anonymous array type to have its bounds determined
by its initial value. This is for uniformity: If one can write “X:
:= ...;” then it makes sense to also allow “X: constant
Integer := ...;”.
(Note that if anonymous array types are ever sensible, a common situation
is for a table implemented as an array. Tables are often constant, and
for constants, there's usually no point in forcing the user to count
the number of elements in the value.)
Wording Changes from Ada 83
Deferred constants no longer have a separate
syntax rule, but rather are incorporated in object_declaration
as constants declared without an initialization expression.
Inconsistencies With Ada 95
Unconstrained aliased objects
of types with discriminants with defaults are no longer constrained by
their initial values. This means that a program that raised Constraint_Error
from an attempt to change the discriminants will no longer do so. The
change only affects programs that depended on the raising of Constraint_Error
in this case, so the inconsistency is unlikely to occur outside of the
ACATS. This change may however cause compilers to implement these objects
differently, possibly taking additional memory or time. This is unlikely
to be worse than the differences caused by any major compiler upgrade.
Extensions to Ada 95
Wording Changes from Ada 95
Corrigendum: Corrected wording to say that
per-object constraints are elaborated (not evaluated).
The rules for evaluating default initialization
have been tightened. In particular, components whose default initialization
can refer to the rest of the object are required to be initialized last.
Extensions to Ada 2005
Wording Changes from Ada 2005
Implicit initial values can now be given for scalar
types and for scalar array components, using the Default_Value (see 3.5)
and Default_Component_Value (see 3.6) aspects;
the extension is documented there.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe