5.5.2 Generalized Loop Iteration
Syntax
Name Resolution Rules
Glossary entry: An iterator is a construct
that is used to loop over the elements of an array or container. Iterators
may be user defined, and may 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;
otherwise it is a
forward iterator.
In a reverse generalized iterator, the
iterator_name
shall be of a reversible 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.
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The subtype defined by the
subtype_indication,
if any, of an array component iterator shall statically match the component
subtype of the type of the
iterable_name.
The subtype defined by the
subtype_indication,
if any, of a container element iterator shall statically match the default
element subtype for the type of the
iterable_name.
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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
subtype_indication
is present, it determines the nominal subtype of the loop parameter.
In an array component iterator, if a
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
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.
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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.
Dynamic Semantics
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For a 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 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.
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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 an array component iterator, 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 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. The
loop iteration proceeds until the
sequence_of_statements
has been executed for each component of the array for the loop, or until
the loop is left as a consequence of a transfer of control.
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For a container element iterator, the
iterable_name
is evaluated and the denoted iterable container object 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.
An object of the default cursor subtype
is created (the
loop cursor).
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For a forward container element iterator, 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; 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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--
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.
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.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
