A.18.18 The Generic Package Containers.Indefinite_Holders
The language-defined generic package Containers.Indefinite_Holders
provides a private type Holder and a set of operations for that type.
A holder container holds a single element of an indefinite type.
A holder container allows the declaration of an object
that can be used like an uninitialized variable or component of an indefinite
type.
A holder container may be
empty. An empty
holder does not contain an element.
Static Semantics
The generic library
package Containers.Indefinite_Holders has the following declaration:
generic
type Element_Type (<>)
is private;
with function "=" (Left, Right : Element_Type)
return Boolean
is <>;
package Ada.Containers.Indefinite_Holders
is
pragma Preelaborate(Indefinite_Holders);
pragma Remote_Types(Indefinite_Holders);
type Holder
is tagged private;
pragma Preelaborable_Initialization (Holder);
Empty_Holder :
constant Holder;
function "=" (Left, Right : Holder) return Boolean;
function To_Holder (New_Item : Element_Type)
return Holder;
function Is_Empty (Container : Holder)
return Boolean;
procedure Clear (Container :
in out Holder);
function Element (Container : Holder)
return Element_Type;
procedure Replace_Element (Container :
in out Holder;
New_Item :
in Element_Type);
procedure Query_Element
(Container :
in Holder;
Process :
not null access procedure (Element :
in Element_Type));
procedure Update_Element
(Container :
in out Holder;
Process :
not null access procedure (Element :
in out Element_Type));
type Constant_Reference_Type
(Element :
not null access constant Element_Type)
is private
with Implicit_Dereference => Element;
type Reference_Type (Element :
not null access Element_Type)
is private
with Implicit_Dereference => Element;
function Constant_Reference (Container :
aliased in Holder)
return Constant_Reference_Type;
function Reference (Container :
aliased in out Holder)
return Reference_Type;
procedure Assign (Target :
in out Holder; Source :
in Holder);
function Copy (Source : Holder)
return Holder;
procedure Move (Target :
in out Holder; Source :
in out Holder);
private
... -- not specified by the language
end Ada.Containers.Indefinite_Holders;
The actual function for the generic formal function
"=" on Element_Type values is expected to define a reflexive
and symmetric relationship and return the same result value each time
it is called with a particular pair of values. If it behaves in some
other manner, the function "=" on holder values returns an
unspecified value. The exact arguments and number of calls of this generic
formal function by the function "=" on holder values are unspecified.
The type Holder is used to represent holder containers.
The type Holder needs finalization
(see
7.6).
Empty_Holder represents an empty holder object. If
an object of type Holder is not otherwise initialized, it is initialized
to the same value as Empty_Holder.
Some operations of this generic package have access-to-subprogram
parameters. To ensure such operations are well-defined, they guard against
certain actions by the designated subprogram. In particular, some operations
check for “tampering with the element” of a container because
they depend on the element of the container not being replaced.
A
subprogram is said to
tamper with the element of a holder object
H if:
It clears the element contained by H, that
is, it calls the Clear procedure with H as a parameter;
It replaces the element contained by H,
that is, it calls the Replace_Element procedure with H as a parameter;
It calls the Move procedure with H as a
parameter;
It finalizes H.
When tampering
with the element is
prohibited for a particular holder object
H, Program_Error is propagated by a call of any language-defined
subprogram that is defined to tamper with the element of
H, leaving
H unmodified. These checks are made before any other defined behavior
of the body of the language-defined subprogram.
function "=" (Left, Right : Holder) return Boolean;
If Left and Right
denote the same holder object, then the function returns True. Otherwise,
it compares the element contained in Left to the element contained in
Right using the generic formal equality operator, returning the result
of that operation. Any exception raised during the evaluation of element
equality is propagated.
function To_Holder (New_Item : Element_Type) return Holder;
Returns a nonempty
holder containing an element initialized to New_Item. To_Holder performs
indefinite insertion (see
A.18).
function Is_Empty (Container : Holder) return Boolean;
Returns True if
Container is empty, and False if it contains an element.
procedure Clear (Container : in out Holder);
Removes the element
from Container. Container is empty after a successful Clear operation.
function Element (Container : Holder) return Element_Type;
If Container is
empty, Constraint_Error is propagated. Otherwise, returns the element
stored in Container.
procedure Replace_Element (Container : in out Holder;
New_Item : in Element_Type);
Replace_Element
assigns the value New_Item into Container, replacing any preexisting
content of Container; Replace_Element performs indefinite insertion (see
A.18). Container is not empty after a successful
call to Replace_Element.
procedure Query_Element
(Container : in Holder;
Process : not null access procedure (Element : in Element_Type));
If Container is
empty, Constraint_Error is propagated. Otherwise, Query_Element calls
Process.all with the contained element as the argument. Tampering
with the element of Container is prohibited during the execution of the
call on Process.all. Any exception raised by Process.all
is propagated.
procedure Update_Element
(Container : in out Holder;
Process : not null access procedure (Element : in out Element_Type));
If Container is
empty, Constraint_Error is propagated. Otherwise, Update_Element calls
Process.all with the contained element as the argument. Tampering
with the element of Container is prohibited during the execution of the
call on Process.all. Any exception raised by Process.all
is propagated.
type Constant_Reference_Type
(Element : not null access constant Element_Type) is private
with Implicit_Dereference => Element;
type Reference_Type (Element : not null access Element_Type) is private
with Implicit_Dereference => Element;
The types Constant_Reference_Type
and Reference_Type need finalization.
The default initialization of an object of type
Constant_Reference_Type or Reference_Type propagates Program_Error.
function Constant_Reference (Container : aliased in Holder)
return Constant_Reference_Type;
This function (combined
with the Implicit_Dereference aspect) provides a convenient way to gain
read access to the contained element of a holder container.
If Container is empty, Constraint_Error is propagated.
Otherwise, Constant_Reference returns an object whose discriminant is
an access value that designates the contained element. Tampering with
the elements of Container is prohibited while the object returned by
Constant_Reference exists and has not been finalized.
function Reference (Container : aliased in out Holder)
return Reference_Type;
This function (combined
with the Implicit_Dereference aspects) provides a convenient way to gain
read and write access to the contained element of a holder container.
If Container is empty, Constraint_Error is propagated.
Otherwise, Reference returns an object whose discriminant is an access
value that designates the contained element. Tampering with the elements
of Container is prohibited while the object returned by Reference exists
and has not been finalized.
procedure Assign (Target : in out Holder; Source : in Holder);
If Target denotes
the same object as Source, the operation has no effect. If Source is
empty, Clear (Target) is called. Otherwise, Replace_Element (Target,
Element (Source)) is called.
function Copy (Source : Holder) return Holder;
If Source is empty,
returns an empty holder container; otherwise, returns To_Holder (Element
(Source)).
procedure Move (Target : in out Holder; Source : in out Holder);
If Target denotes
the same object as Source, then the operation has no effect. Otherwise,
the element contained by Source (if any) is removed from Source and inserted
into Target, replacing any preexisting content. Source is empty after
a successful call to Move.
Bounded (Run-Time) Errors
It is a bounded error for the
actual function associated with a generic formal subprogram, when called
as part of an operation of this package, to tamper with the element of
any Holder parameter of the operation. Either Program_Error is raised,
or the operation works as defined on the value of the Holder either prior
to, or subsequent to, some or all of the modifications to the Holder.
It is a bounded error to call
any subprogram declared in the visible part of Containers.Indefinite_Holders
when the associated container has been finalized. If the operation takes
Container as an
in out parameter, then it raises Constraint_Error
or Program_Error. Otherwise, the operation either proceeds as it would
for an empty container, or it raises Constraint_Error or Program_Error.
Erroneous Execution
Execution is erroneous if the holder container associated
with the result of a call to Reference or Constant_Reference is finalized
before the result object returned by the call to Reference or Constant_Reference
is finalized.
Implementation Requirements
No storage associated with a holder object shall
be lost upon assignment or scope exit.
The execution of an
assignment_statement
for a holder container shall have the effect of copying the element (if
any) from the source holder object to the target holder object.
Implementation Advice
Move should not copy the element, and should minimize
copying of internal data structures.
If an exception is propagated from a holder operation,
no storage should be lost, nor should the element be removed from a holder
container unless specified by the operation.
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