3.6 Array Types
object is a composite object consisting of components which all have
the same subtype. The name for a component of an array uses one or more
index values belonging to specified discrete types. The value of an array
object is a composite value consisting of the values of the components.
Name Resolution Rules
For a discrete_subtype_definition
that is a range
shall resolve to be of some specific discrete type[; which discrete type
shall be determined without using any context other than the bounds of
itself (plus the preference for root_integer
— see 8.6
is a discrete quantity used to select along a given dimension of an array.
A component is selected by specifying corresponding values for each of
This paragraph was
Within the definition of a nonlimited composite
type (or a limited composite type that later in its immediate scope becomes
nonlimited — see 7.3.1 and 7.5),
if a component_definition
contains the reserved word aliased and the type of the component
is discriminated, then the nominal subtype of the component shall be
Reason: If we allowed
the subtype to be unconstrained, then the discriminants might change
because of an assignment to the containing (nonlimited) object, thus
causing a potential violation of an access subtype constraint of an access
value designating the aliased component.
Note that the rule elsewhere
defining all aliased discriminated objects to be constrained does not
help — that rule prevents assignments to the component itself from
doing any harm, but not assignments to the containing object.
allow this for components within limited types since assignment to the
enclosing object is not a problem. Furthermore, it is important to be
able to use a default expression for a discriminant in arrays of limited
components, since that is the only way to give the components different
values for their discriminants. For example:
protected type Counter_Type(Initial_Value : Integer := 1) is
procedure Get_Next(Next_Value : out Integer);
-- Returns the next value on each call, bumping Count
-- before returning.
Count : Integer := Initial_Value;
protected body Counter_Type is ...
function Next_Id(Counter : access Counter_Type) return Integer is
Result : Integer;
C : aliased Counter_Type;
task type T(Who_Am_I : Integer := Next_Id(C'Access));
task body T is ...
Task_Array : array(1..100) of aliased T;
-- Array of task elements, each with its own unique ID.
-- We specify "aliased" so we can use Task_Array(I)'Access.
-- This is safe because Task_Array is of a limited type,
-- so there is no way an assignment to it could change
-- the discriminants of one of its components.
that this rule applies to array components and record components, but
not to protected type components (since they are always limited).
array is characterized by the number of indices (the dimensionality
of the array), the type and position of each index, the lower and upper
bounds for each index, and the subtype of the components. The order of
the indices is significant.
A one-dimensional array has a distinct component
for each possible index value. A multidimensional array has a distinct
component for each possible sequence of index values that can be formed
by selecting one value for each index position (in the given order).
The possible values for a given index are all the values between the
lower and upper bounds, inclusive;
this range of
values is called the index range
of an array are the bounds of its index ranges.
of a dimension of an array is the number of values of the
index range of the dimension (zero for a null range).
of a one-dimensional array is the length of its only dimension.
defines an array type and its first subtype. For each object of this
array type, the number of indices, the type and position of each index,
and the subtype of the components are as in the type definition[; the
values of the lower and upper bounds for each index belong to the corresponding
index subtype of its type, except for null arrays (see 3.6.1
defines an array type with a constrained first subtype. Each discrete_subtype_definition
defines the corresponding index subtype, as well as the corresponding
index range for the constrained first subtype.
of the first subtype consists of the bounds of the
Although there is no nameable namable
unconstrained array subtype in this case, the predefined slicing and
concatenation operations can operate on and yield values that do not
necessarily belong to the first array subtype. This is also true for
If the type of the range
resolves to root_integer
, then the discrete_subtype_definition
defines a subtype of the predefined type Integer with bounds given by
a conversion to Integer of the bounds of the range
Reason: This ensures that indexing over
the discrete subtype can be performed with regular Integers, rather than
We considered doing this
by simply creating a “preference” for Integer when resolving
However, this can introduce Beaujolais
when the simple_expression
involve calls on functions visible due to use
In this case, the nominal subtype cannot be an
unconstrained discriminated subtype. See 3.8.
This aspect shall be specified by a static expression,
and that expression shall be explicit, even if the aspect has a boolean
type. Default_Value shall be specified only on a full_type_declaration.
Reason: The part
about requiring an explicit expression is to disallow omitting the value
for this aspect, which would otherwise be allowed by the rules of 13.3.1.
This is a representation
attribute in order to disallow specifying it on a derived type that has
inherited primitive subprograms; that is necessary as the sizes of out
parameters could be different whether or not a Default_Value is specified
for Default_Component_Value: Default
value for the components of an array-of-scalar subtype.
If a derived type with no primitive subprograms
inherits a boolean Default_Value aspect, the aspect may be specified
to have any value for the derived type.
Reason: This overrides
the 13.3.1 rule that says that a boolean
aspect with a value True cannot be changed.
Name Resolution Rules
46 All components of an array have the
same subtype. In particular, for an array of components that are one-dimensional
arrays, this means that all components have the same bounds and hence
the same length.
Examples of type
declarations with unconstrained array definitions:
Vector is array
Matrix is array
<>, Integer range
Bit_Vector is array
Roman is array
Roman_Digit; -- see 3.5.2
Examples of type
declarations with constrained array definitions:
type Table is array(1 .. 10) of Integer;
type Schedule is array(Day) of Boolean;
type Line is array(1 .. Max_Line_Size) of Character;
Examples of object
declarations with array type definitions:
(1 .. 80, 1 .. 100) of
Red .. Green) of
Msg_Table : constant array(Error_Code) of access constant String :=
(Too_Big => new String'("Result too big"), Too_Small => ...);
Line := -- an array of arrays
(1 | 50 => Line'(1 | Line'Last => '+', others
=> '-'), -- see 4.3.3
2 .. 49 => Line'(1 | Line'Last => '|', others
=> ' '));
-- Page is constrained by its initial value to (1..50)
Extensions to Ada 83
in a discrete_subtype_definition
may use arbitrary universal expressions for each bound (e.g. –1
.. 3+5), rather than strictly "implicitly convertible" operands.
The subtype defined will still be a subtype of Integer.
Wording Changes from Ada 83
We introduce a new syntactic category, discrete_subtype_definition
as distinct from discrete_range
These two constructs have the same syntax, but their semantics are quite
different (one defines a subtype, with a preference for Integer subtypes,
while the other just selects a subrange of an existing subtype). We use
this new syntactic category in for
loops and entry families.
The syntax rule for component_definition
moved here from RM83-3.7.
Extensions to Ada 95
The prohibition against unconstrained discriminated
aliased components has been lifted. It has been replaced by a prohibition
against the actual troublemakers: general access discriminant constraints
Wording Changes from Ada 95
Corrigendum: Added wording to allow the
elaboration of per-object constraints for constrained arrays.
Extensions to Ada 2005
The new aspect Default_Component_Value
allows defining implicit initial values (see 3.3.1)
for arrays of scalar types.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe