5.5.2 Generalized Loop Iteration
Syntax
Name Resolution Rules
Term entry: iterator
— construct that is used to loop over the elements of an array
or container
Note: Iterators can be user defined, and can perform arbitrary computations
to access elements from a container.
Legality Rules
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If the reserved word
reverse appears, the
iterator_specification
is a
reverse iterator.
If the iterator_specification
is for a parallel construct, the iterator_specification
is a parallel iterator.
Otherwise,; otherwise it is a
forward
iterator.
Forward
and reverse iterators are collectively called sequential iterators.
In a reverse generalized iterator, the
iterator_name
shall be of a reversible iterator type.
In a parallel
generalized iterator, the iterator_name
shall be of a parallel iterator type. In a reverse container element
iterator, the default iterator type for the type of the
iterable_name
shall be a reversible iterator type.
In a parallel
container element iterator, the default iterator type for the type of
the iterable_name
shall be of a parallel iterator type.{
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In a container element iterator whose
iterable_name
has type
T, if the
iterable_name
denotes a constant or the Variable_Indexing aspect is not specified for
T, then the Constant_Indexing aspect shall be specified for
T.
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The iterator_name
or iterable_name
of an iterator_specification
shall not denote a subcomponent that depends on discriminants of an object
whose nominal subtype is unconstrained, unless the object is known to
be constrained.Reason: This is
the same rule that applies to renames; it serves the same purpose of
preventing the object from disappearing while the iterator is still using
it.
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A container element iterator is illegal if the
call of the default iterator function that creates the loop iterator
(see below) is illegal.Ramification: This
can happen if the parameter to the default iterator function is in
out and the iterable_name
is a constant. The wording applies to any reason that the call would
be illegal, as it's possible that one of the default parameters would
be illegal, or that some accessibility check would fail.
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A generalized iterator is illegal if the iteration
cursor subtype of the iterator_name
is a limited type at the point of the generalized iterator. A container
element iterator is illegal if the default cursor subtype of the type
of the iterable_name
is a limited type at the point of the container element iterator.Reason: If the
cursor type is limited, the assignment to the loop parameter for a generalized
iterator would be illegal. The same is true for a container element iterator.
We have to say "at the point of the iterator" as the limitedness
of a type can change due to visibility.
Static Semantics
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An
iterator_specification
declares a
loop parameter.
In a generalized
iterator,
the nominal subtype of the loop parameter
is the iteration cursor subtype. In an array component iterator
,
or a container element iterator, if a
loop_parameter_subtype_indication subtype_indication
is present, it determines the nominal subtype of the loop parameter.
In a generalized iterator, if a loop_parameter_subtype_indication
is not present, the nominal subtype of the loop parameter is the iteration
cursor subtype. In an array component iterator, if a
loop_parameter_subtype_indication subtype_indication
is not present, the nominal subtype of the loop parameter is the component
subtype of the type of the
iterable_name.
In a container element iterator, if a
loop_parameter_subtype_indication subtype_indication
is not present, the nominal subtype of the loop parameter is the default
element subtype for the type of the
iterable_name.
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In a generalized iterator, the loop parameter is a constant. In an array
component iterator, the loop parameter is a constant if the
iterable_name
denotes a constant; otherwise it denotes a variable. In a container element
iterator, the loop parameter is a constant if the
iterable_name
denotes a constant, or if the Variable_Indexing aspect is not specified
for the type of the
iterable_name;
otherwise it is a variable.
Ramification: {
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The loop parameter of a generalized iterator has
the same accessibility as the loop statement. This means that the loop
parameter object is finalized when the loop statement is left. (It also
may be finalized as part of assigning a new value to the loop parameter.)
For array component iterators and
container element iterators, the
loop parameter directly denotes an element of the array or
container and has the accessibility
of the associated array or container. For
container element iterators, the loop parameter denotes the result of
the indexing function call (in the case of a constant indexing) or a
generalized reference thereof (in the case of a variable indexing). Roughly
speaking, the loop parameter has the accessibility level of a single
iteration of the loop. More precisely, the function result (or the generalized
reference thereof) is considered to be renamed in the declarative part
of a notional block statement which immediately encloses the loop's sequence_of_statements;
the accessibility of the loop parameter is that of the block statement.
Dynamic Semantics
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For a
sequential generalized iterator, the
loop parameter is created, the
iterator_name
is evaluated, and the denoted iterator object becomes the
loop iterator.
In a forward generalized iterator, the operation First of the iterator
type is called on the loop iterator, to produce the initial value for
the loop parameter. If the result of calling Has_Element on the initial
value is False, then the execution of the
loop_statement
is complete. Otherwise, the
sequence_of_statements
is
conditionally executed and then the Next
operation of the iterator type is called with the loop iterator and the
current value of the loop parameter to produce the next value to be assigned
to the loop parameter. This repeats until the result of calling Has_Element
on the loop parameter is False, or the loop is left as a consequence
of a transfer of control. For a reverse generalized iterator, the operations
Last and Previous are called rather than First and Next.
Ramification: {
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The loop parameter of a generalized iterator is
a variable of which the user only has a constant view. It follows the
normal rules for a variable of its nominal subtype. In particular, if
the nominal subtype is indefinite, the variable is constrained by its
initial value. Similarly, if the nominal subtype is class-wide, the variable
(like all variables) has the tag of the initial value. Constraint_Error
may be raised by a subsequent iteration if Next or Previous return an
object with a different tag or constraint.
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For a parallel generalized iterator, the chunk_specification,
if any, of the associated parallel construct, is first elaborated, to
determine the maximum number of chunks (see 5.5),
and then the operation Split_Into_Chunks of the iterator type is called,
with the determined maximum passed as the Max_Chunks parameter, specifying
the upper bound for the number of loop parameter objects (and the number
of logical threads of control) to be associated with the iterator. In
the absence of a chunk_specification,
the maximum number of chunks is determined in an implementation-defined
manner.Implementation defined:
The maximum number of chunks for a parallel
generalized iterator without a chunk_specification.
Discussion: The
Max_Chunks parameter of the Split_Into_Chunks procedure is an upper bound
for the number of chunks to be associated with a loop. A container implementation
may opt for a lower value for the number of chunks if a more optimal
split can be determined. For instance, a tree-based container might create
the split based on the number of branches at the top levels of the tree.
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Upon return from Split_Into_Chunks, the actual
number of chunks for the loop is determined by calling the Chunk_Count
operation of the iterator, at which point one logical thread of control
is initiated for each chunk, with an associated chunk index in the range
from one to the actual number of chunks. {
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Within each logical thread of control, a loop parameter
is created. If a chunk_specification
with a discrete_subtype_definition
is present in the associated parallel construct, then a chunk parameter
is created and initialized with a value from the discrete subtype defined
by the discrete_subtype_definition,
so that the order of the chosen chunk parameter values correspond to
the order of the chunk indices associated with the logical threads of
control. The operation First of the iterator type that has a Chunk parameter
is called on the loop iterator, with Chunk initialized from the corresponding
chunk index, to produce the initial value for the loop parameter. If
the result of calling Has_Element on this initial value is False, then
the execution of the logical thread of control is complete. Otherwise,
the sequence_of_statements
is conditionally executed, and then the Next operation of the iterator
type that has a Chunk parameter is called with the loop iterator, the
current value of the loop parameter, and the corresponding chunk index,
to produce the next value to be assigned to the loop parameter. This
repeats until the result of calling Has_Element on the loop parameter
is False, or the associated parallel construct is left as a consequence
of a transfer of control. {
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In the absence of a transfer of control, the associated
parallel construct of a parallel generalized iterator is complete when
all of its logical threads of control are complete.{
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For an array component iterator,
the chunk_specification
of the associated parallel construct, if any, is first elaborated to
determine the maximum number of chunks (see 5.5),
and then the
iterable_name
is evaluated and the denoted array object becomes the
array for the
loop.
If the array for the loop is a null array,
then the execution of the
loop_statement
is complete. Otherwise, the
sequence_of_statements
is
conditionally executed with the loop
parameter denoting each component of the array for the loop, using a
canonical order of components,
which is last
dimension varying fastest (unless the array has convention Fortran, in
which case it is first dimension varying fastest). For a forward array
component iterator, the iteration starts with the component whose index
values are each the first in their index range, and continues in the
canonical order. For a reverse array component iterator, the iteration
starts with the component whose index values are each the last in their
index range, and continues in the reverse of the canonical order.
For
a parallel array component iterator, the iteration is broken up into
contiguous chunks of the canonical order, such that all components are
covered with no overlaps; each chunk has its own logical thread of control
with its own loop parameter and iteration within each chunk is in the
canonical order. The number of chunks is implementation defined, but
is limited in the presence of a chunk_specification
to the determined maximum. The loop iteration proceeds until the
sequence_of_statements
has been
conditionally executed for each
component of the array for the loop, or until the loop is left as a consequence
of a transfer of control.
Implementation defined:
The number of chunks for an array component
iterator.
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If a chunk_specification
with a discrete_subtype_definition
is present in the associated parallel construct, then the logical thread
of control associated with a given chunk has a chunk parameter initialized
with a distinct value from the discrete subtype defined by the discrete_subtype_definition.
The values of the chunk parameters are assigned such that they increase
in the canonical order of the starting array components for the chunks.{
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For a container element iterator,
the chunk_specification
of the associated parallel construct, if any, is first elaborated to
determine the maximum number of chunks (see 5.5),
and then the
iterable_name
is evaluated
. If the container type has Iterator_View
specified, an object of the Iterator_View type is created with the discriminant
referencing the iterable container object denoted by the iterable_name.
This is the iterable container object for the loop. Otherwise, and
the iterable container object
denoted by the iterable_name
becomes the iterable container object for the loop becomes
the iterable container object for the loop.
The default iterator function for the type of the iterable container
object for the loop is called on the iterable container object and the
result is the
loop iterator.
For
a sequential container element iterator, an An
object of the default cursor subtype is created (the
loop cursor).
For a parallel container element iterator, each chunk of iterations will
have its own loop cursor, again of the default cursor subtype.Reason: {
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If Iterator_View is specified, we add an extra
object and use that object for this iteration. This allows these iterators
to automatically use the stable view (defined in each of the language-defined
containers) to do the iteration. That eliminates the need to set and
clear the tampering with elements indication each time Reference is called;
that eliminates substantial overhead as finalization is typically used
to implement the tampering reset.
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A For a container
element iterator
then proceeds as described above
for a generalized iterator, except that each reference to a loop parameter
is replaced by a reference to the corresponding loop cursor. For a container
element iterator, the loop parameter for each iteration instead denotes,
the operation First of the iterator type is called on the loop iterator,
to produce the initial value for the loop cursor. If the result of calling
Has_Element on the initial value is False, then the execution of the
loop_statement
is complete. Otherwise, the sequence_of_statements
is executed with the loop parameter denoting an indexing (see
4.1.6) into the iterable container object
for the loop, with the only parameter to the indexing being the
current
value of the loop cursor
for the given iteration;
then the Next operation of the iterator type is called with the loop
iterator and the loop cursor to produce the next value to be assigned
to the loop cursor. This repeats until the result of calling Has_Element
on the loop cursor is False, or until the loop is left as a consequence
of a transfer of control. For a reverse container element iterator, the
operations Last and Previous are called rather than First and Next.
If the loop parameter is a constant (see above), then the indexing uses
the default constant indexing function for the type of the iterable container
object for the loop; otherwise it uses the default variable indexing
function.
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Any exception propagated by the execution of a
generalized iterator or container element iterator is propagated by the
immediately enclosing loop statement.Ramification: This
text covers exceptions raised by called functions that make up the execution
of the iterator as well as exceptions raised by the assignment to the
loop parameter or cursor.
Examples
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Example of a parallel generalized loop over
an array:{
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parallel -- Array component iterator example:
for Element
of Board
loop --
See 3.6.1.
Element := Element * 2.0; --
Double each element of Board, a two-dimensional array.
end loop;
Extensions to Ada 2005
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Generalized forms of loop iteration are new.
Incompatibilities With Ada 2012
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Corrigendum: Added a
rule to ensure that the object being iterated cannot be a component that
could disappear before the loop completes. This could be incompatible
by making a loop that was legal (and worked correctly, so long as the
enclosing object is not modified during the loop) from the original Ada
2012 illegal in corrected Ada 2012. Such loops should be pretty rare,
especially as these iterator forms are new to Ada 2012.{
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Corrigendum: Added rules to reject loops
if the call to the default iterator function for a container element
iterator is illegal, or if the cursor type of an iterator is limited.
These are formally incompatible with original Ada 2012, but as it's unlikely
that any Ada 2012 compiler ever allowed the illegal usages in an expansion
of a loop (it's much more likely that they would have just caused an
internal error in the compiler), this should have no effect in practice.{
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Corrigendum: Added a requirement that the
given subtype statically match the subtype of the element or component
for a component element iterator or array component iterator. Original
Ada 2012 text allowed any type that covered the subtype of the element
or component, but that led to questions of what the meaning was if they
are different. In this case, the element is essentially a renaming of
the container element, and it doesn't make sense for the constraints
to be different. Ignoring explicitly defined constraints in renames is
a mistake that we don't want to continue, thus we require static matching.
This means that some programs might be illegal, but those programs were
misleading at best, and potentially would raise unexpected exceptions
because the element values might have been invalid or abnormal with respect
to the declared constraint. Extensions to Ada 2012
Wording Changes from Ada 2012
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Corrigendum: Added wording to specify that
a loop propagates any exceptions propagated by the execution of an iterator.
Since that's what naturally would happen from a macro-style expansion
of the parts of an iterator, and no other interpretation makes sense
given the way the rest of Ada works, we consider it so unlikely that
any Ada 2012 implementation ever did anything else that we don't document
this as a possible inconsistency.{
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Added wording to include the use of the iterator
view in a container element iterator.{
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Added wording to describe the execution of parallel
iterators.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
