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
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 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 Ada 83.
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
This aspect shall be specified by a static expression, and that expression
shall be explicit, even if the aspect has a boolean type. Default_Component_Value
shall be specified only on a full_type_declaration
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 13.1.1
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
could be different whether or not a Default_Value is specified (see 6.4.1
Aspect Description 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_Component_Value aspect, the aspect
may be specified to have any value for the derived type.
This overrides the 13.1.1
rule that says that a boolean aspect with a value True cannot be changed.
Name Resolution Rules
49 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:
Grid : array
(1 .. 80, 1 .. 100) of
Mix : array
Red .. Green) of
Msg_Table : constant array
(Error_Code) of access constant
(Too_Big => new
String'("Result too big"), Too_Small => ...);
Page : array
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 (see 3.7.1
Wording Changes from Ada 95
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