7.5 Limited Types
{
AI95-00287-01}
[A limited type is (a view of) a type for which copying (such as for
an
assignment_statement)
is not allowed. A nonlimited type is a (view of a) type for which copying
is allowed.]
Discussion: The concept of the value
of a limited type is difficult to define, since the abstract value of
a limited type often extends beyond its physical representation. In some
sense, values of a limited type cannot be divorced from their object.
The value is the object.
{
AI95-00318-02}
In Ada 83, in the two places where limited types were defined by the
language, namely tasks and files, an implicit level of indirection was
implied by the semantics to avoid the separation of the value from an
associated object. In Ada 95, most limited types are passed by reference,
and even return-ed by reference. In Ada 2005, most limited types are
built-in-place upon return, rather than returned by reference. Thus the
object “identity” is part of the logical value of most limited
types.
To be honest: {
AI95-00287-01}
{
AI95-00419-01}
For a limited partial view whose full view is nonlimited, copying is
possible on parameter passing and function return. To prevent any copying
whatsoever, one should make both the partial
and full views limited.
Glossary entry: {
Limited type}
A limited type is a type for which copying (such as in an
assignment_statement)
is not allowed. A nonlimited type is a type for which copying is allowed.
Legality Rules
{
AI95-00419-01}
If a tagged record type has any limited components, then the reserved
word
limited shall appear in its
record_type_definition.
[If the reserved word
limited appears in the definition of a
derived_type_definition,
its parent type and any progenitor interfaces shall be limited.]
Proof: {
AI95-00419-01}
The rule about the parent type being required to be limited can be found
in
3.4. Rules about progenitor interfaces can
be found in
3.9.4, specifically, a nonlimited
interface can appear only on a nonlimited type. We repeat these rules
here to gather these scattered rules in one obvious place.
Reason: This
prevents tagged limited types from becoming nonlimited. Otherwise, the
following could happen:
package P is
type T is limited private;
type R is tagged
record -- Illegal!
-- This should say “limited record”.
X : T;
end record;
private
type T is new Integer; -- R becomes nonlimited here.
end P;
package Q is
type R2 is new R with
record
Y : Some_Task_Type;
end record;
end Q;
{
AI95-00230-01}
If the above were legal, then assignment would be defined for R'Class
in the body of P, which is bad news, given the task.
Discussion: All of these contexts normally
require copying; by restricting the uses as above, we can require the
new object to be built-in-place.
Static Semantics
{
AI95-00419-01}
{limited type} A
type is
limited if it is one of the following:
Ramification: Note that there is always
a “definition,” conceptually, even if there is no syntactic
category called “..._definition”.
{
AI95-00419-01}
This includes interfaces of the above kinds, derived types with the reserved
word
limited, as well as task and protected types.
{
AI95-00419-01}
a derived type whose parent is limited and is not an interface.
Ramification: {
AI95-00419-01}
Limitedness is not inherited from interfaces; it must be explicitly specified
when the parent is an interface.
To be honest: {
AI95-00419-01}
A derived type can become nonlimited if
limited does not appear
and the derivation takes place in the visible part of a child package,
and the parent type is nonlimited as viewed from the private part or
body of the child package.
Reason: {
AI95-00419-01}
We considered a rule where limitedness was always inherited from the
parent for derived types, but in the case of a type whose parent is an
interface, this meant that the first interface is treated differently
than other interfaces. It also would have forced users to declare dummy
nonlimited interfaces just to get the limitedness right. We also considered
a syntax like
not limited to specify nonlimitedness when the parent
was limited, but that was unsavory. The rule given is more uniform and
simpler to understand.
{
AI95-00419-01}
The rules for interfaces are asymmetrical, but the language is not: if
the parent interface is limited, the presence of the word
limited
determines the limitedness, and nonlimited progenitors are illegal by
the rules in
3.9.4. If the parent interface
is nonlimited, the word
limited is illegal by the rules in
3.4.
The net effect is that the order of the interfaces doesn't matter.
{nonlimited type}
Otherwise, the type is nonlimited.
[There are no predefined equality operators for a
limited type.]
Implementation Requirements
{
AI95-00287-01}
{
AI95-00318-02}
For an
aggregate
of a limited type used to initialize an object as allowed above, the
implementation shall not create a separate anonymous object for the
aggregate.
For a
function_call
of a type with a part that is of a task, protected, or explicitly limited
record type that is used to initialize an object as allowed above, the
implementation shall not create a separate return object (see 6.5) for
the
function_call.
The
aggregate
or
function_call
shall be constructed directly in the new object.
Discussion: {
AI95-00318-02}
For a
function_call,
we only require
build-in-place{
build-in-place [partial]}
for a limited type that would have been a return-by-reference
type in Ada 95. We do this because we want to minimize disruption to
Ada 95 implementations and users.
To be honest: This isn't quite true if
the type can become nonlimited (see below);
function_calls
only are required to be build-in-place for “really” limited
types.
Paragraphs 10 through
15 were deleted.
15
{become
nonlimited} {nonlimited
type (becoming nonlimited)} {limited
type (becoming nonlimited)} As illustrated
in
7.3.1, an untagged limited type can become
nonlimited under certain circumstances.
Ramification: Limited private types do
not become nonlimited; instead, their full view can be nonlimited, which
has a similar effect.
It is important to remember that a single nonprivate
type can be both limited and nonlimited in different parts of its scope.
In other words, “limited” is a property that depends on where
you are in the scope of the type. We don't call this a “view property”
because there is no particular declaration to declare the nonlimited
view.
Tagged types never become nonlimited.
Examples
Example of a package
with a limited type:
package IO_Package is
type File_Name is limited private;
procedure Open (F : in out File_Name);
procedure Close(F : in out File_Name);
procedure Read (F : in File_Name; Item : out Integer);
procedure Write(F : in File_Name; Item : in Integer);
private
type File_Name is
limited record
Internal_Name : Integer := 0;
end record;
end IO_Package;
package body IO_Package is
Limit : constant := 200;
type File_Descriptor is record ... end record;
Directory : array (1 .. Limit) of File_Descriptor;
...
procedure Open (F : in out File_Name) is ... end;
procedure Close(F : in out File_Name) is ... end;
procedure Read (F : in File_Name; Item : out Integer) is ... end;
procedure Write(F : in File_Name; Item : in Integer) is ... end;
begin
...
end IO_Package;
16 Notes on the example: In the
example above, an outside subprogram making use of IO_Package may obtain
a file name by calling Open and later use it in calls to Read and Write.
Thus, outside the package, a file name obtained from Open acts as a kind
of password; its internal properties (such as containing a numeric value)
are not known and no other operations (such as addition or comparison
of internal names) can be performed on a file name. Most importantly,
clients of the package cannot make copies of objects of type File_Name.
This example is characteristic of any case where complete
control over the operations of a type is desired. Such packages serve
a dual purpose. They prevent a user from making use of the internal structure
of the type. They also implement the notion of an encapsulated data type
where the only operations on the type are those given in the package
specification.
{
AI95-00318-02}
The fact that the full view of File_Name is explicitly declared
limited
means that parameter passing will always be by reference and function
results will always be built directly in the result object (see
6.2
and
6.5).
Extensions to Ada 83
{
extensions to Ada 83}
The
restrictions in RM83-7.4.4(4), which disallowed
out parameters
of limited types in certain cases, are removed.
Wording Changes from Ada 83
Since limitedness and privateness are orthogonal
in Ada 95 (and to some extent in Ada 83), this is now its own clause
rather than being a subclause of
7.3, “
Private
Types and Private Extensions”.
Extensions to Ada 95
Wording Changes from Ada 95
{
AI95-00411-01}
{
AI95-00419-01}
Rewrote the definition of limited to ensure that interfaces are covered,
but that limitedness is not inherited from interfaces. Derived types
that explicitly include
limited are now also covered.
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