ada: New Local_Restrictions and User_Aspect aspects.

[gcc.git] / gcc / ada / doc / gnat_rm / implementation_defined_aspects.rst

.. _Implementation_Defined_Aspects:

******************************
Implementation Defined Aspects
******************************

Ada defines (throughout the Ada 2012 reference manual, summarized
in Annex K) a set of aspects that can be specified for certain entities.
These language defined aspects are implemented in GNAT in Ada 2012 mode
and work as described in the Ada 2012 Reference Manual.

In addition, Ada 2012 allows implementations to define additional aspects
whose meaning is defined by the implementation.  GNAT provides
a number of these implementation-defined aspects which can be used
to extend and enhance the functionality of the compiler.  This section of
the GNAT reference manual describes these additional aspects.

Note that any program using these aspects may not be portable to
other compilers (although GNAT implements this set of aspects on all
platforms).  Therefore if portability to other compilers is an important
consideration, you should minimize the use of these aspects.

Note that for many of these aspects, the effect is essentially similar
to the use of a pragma or attribute specification with the same name
applied to the entity. For example, if we write:


.. code-block:: ada

  type R is range 1 .. 100
    with Value_Size => 10;


then the effect is the same as:

.. code-block:: ada

  type R is range 1 .. 100;
  for R'Value_Size use 10;


and if we write:

.. code-block:: ada

  type R is new Integer
    with Shared => True;


then the effect is the same as:

.. code-block:: ada

  type R is new Integer;
  pragma Shared (R);


In the documentation below, such cases are simply marked
as being boolean aspects equivalent to the corresponding pragma
or attribute definition clause.

Aspect Abstract_State
=====================

.. index:: Abstract_State

This aspect is equivalent to :ref:`pragma Abstract_State<Pragma-Abstract_State>`.

Aspect Annotate
===============

.. index:: Annotate

There are three forms of this aspect (where ID is an identifier,
and ARG is a general expression),
corresponding to :ref:`pragma Annotate<Pragma-Annotate>`.



*Annotate => ID*
  Equivalent to ``pragma Annotate (ID, Entity => Name);``


*Annotate => (ID)*
  Equivalent to ``pragma Annotate (ID, Entity => Name);``


*Annotate => (ID ,ID {, ARG})*
  Equivalent to ``pragma Annotate (ID, ID {, ARG}, Entity => Name);``

Aspect Async_Readers
====================
.. index:: Async_Readers

This boolean aspect is equivalent to :ref:`pragma Async_Readers<Pragma-Async_Readers>`.

Aspect Async_Writers
====================
.. index:: Async_Writers

This boolean aspect is equivalent to :ref:`pragma Async_Writers<Pragma-Async_Writers>`.

Aspect Constant_After_Elaboration
=================================
.. index:: Constant_After_Elaboration

This aspect is equivalent to :ref:`pragma Constant_After_Elaboration<Pragma-Constant_After_Elaboration>`.

Aspect Contract_Cases
=====================
.. index:: Contract_Cases

This aspect is equivalent to :ref:`pragma Contract_Cases<Pragma-Contract_Cases>`, the sequence
of clauses being enclosed in parentheses so that syntactically it is an
aggregate.

Aspect Depends
==============
.. index:: Depends

This aspect is equivalent to :ref:`pragma Depends<Pragma-Depends>`.

Aspect Default_Initial_Condition
================================
.. index:: Default_Initial_Condition

This aspect is equivalent to :ref:`pragma Default_Initial_Condition<Pragma-Default_Initial_Condition>`.

Aspect Dimension
================
.. index:: Dimension

The ``Dimension`` aspect is used to specify the dimensions of a given
subtype of a dimensioned numeric type. The aspect also specifies a symbol
used when doing formatted output of dimensioned quantities. The syntax is::

  with Dimension =>
    ([Symbol =>] SYMBOL, DIMENSION_VALUE {, DIMENSION_Value})

  SYMBOL ::= STRING_LITERAL | CHARACTER_LITERAL

  DIMENSION_VALUE ::=
    RATIONAL
  | others               => RATIONAL
  | DISCRETE_CHOICE_LIST => RATIONAL

  RATIONAL ::= [-] NUMERIC_LITERAL [/ NUMERIC_LITERAL]


This aspect can only be applied to a subtype whose parent type has
a ``Dimension_System`` aspect. The aspect must specify values for
all dimensions of the system. The rational values are the powers of the
corresponding dimensions that are used by the compiler to verify that
physical (numeric) computations are dimensionally consistent. For example,
the computation of a force must result in dimensions (L => 1, M => 1, T => -2).
For further examples of the usage
of this aspect, see package ``System.Dim.Mks``.
Note that when the dimensioned type is an integer type, then any
dimension value must be an integer literal.

Aspect Dimension_System
=======================
.. index:: Dimension_System

The ``Dimension_System`` aspect is used to define a system of
dimensions that will be used in subsequent subtype declarations with
``Dimension`` aspects that reference this system. The syntax is::

  with Dimension_System => (DIMENSION {, DIMENSION});

  DIMENSION ::= ([Unit_Name   =>] IDENTIFIER,
                 [Unit_Symbol =>] SYMBOL,
                 [Dim_Symbol  =>] SYMBOL)

  SYMBOL ::= CHARACTER_LITERAL | STRING_LITERAL


This aspect is applied to a type, which must be a numeric derived type
(typically a floating-point type), that
will represent values within the dimension system. Each ``DIMENSION``
corresponds to one particular dimension. A maximum of 7 dimensions may
be specified. ``Unit_Name`` is the name of the dimension (for example
``Meter``). ``Unit_Symbol`` is the shorthand used for quantities
of this dimension (for example ``m`` for ``Meter``).
``Dim_Symbol`` gives
the identification within the dimension system (typically this is a
single letter, e.g. ``L`` standing for length for unit name ``Meter``).
The ``Unit_Symbol`` is used in formatted output of dimensioned quantities.
The ``Dim_Symbol`` is used in error messages when numeric operations have
inconsistent dimensions.

GNAT provides the standard definition of the International MKS system in
the run-time package ``System.Dim.Mks``. You can easily define
similar packages for cgs units or British units, and define conversion factors
between values in different systems. The MKS system is characterized by the
following aspect:

.. code-block:: ada

     type Mks_Type is new Long_Long_Float with
       Dimension_System => (
         (Unit_Name => Meter,    Unit_Symbol => 'm',   Dim_Symbol => 'L'),
         (Unit_Name => Kilogram, Unit_Symbol => "kg",  Dim_Symbol => 'M'),
         (Unit_Name => Second,   Unit_Symbol => 's',   Dim_Symbol => 'T'),
         (Unit_Name => Ampere,   Unit_Symbol => 'A',   Dim_Symbol => 'I'),
         (Unit_Name => Kelvin,   Unit_Symbol => 'K',   Dim_Symbol => '@'),
         (Unit_Name => Mole,     Unit_Symbol => "mol", Dim_Symbol => 'N'),
         (Unit_Name => Candela,  Unit_Symbol => "cd",  Dim_Symbol => 'J'));


Note that in the above type definition, we use the ``at`` symbol (``@``) to
represent a theta character (avoiding the use of extended Latin-1
characters in this context).

See section 'Performing Dimensionality Analysis in GNAT' in the GNAT Users
Guide for detailed examples of use of the dimension system.

Aspect Disable_Controlled
=========================
.. index:: Disable_Controlled

The aspect  ``Disable_Controlled`` is defined for controlled record types. If
active, this aspect causes suppression of all related calls to ``Initialize``,
``Adjust``, and ``Finalize``. The intended use is for conditional compilation,
where for example you might want a record to be controlled or not depending on
whether some run-time check is enabled or suppressed.

Aspect Effective_Reads
======================
.. index:: Effective_Reads

This aspect is equivalent to :ref:`pragma Effective_Reads<Pragma-Effective_Reads>`.

Aspect Effective_Writes
=======================
.. index:: Effective_Writes

This aspect is equivalent to :ref:`pragma Effective_Writes<Pragma-Effective_Writes>`.

Aspect Extensions_Visible
=========================
.. index:: Extensions_Visible

This aspect is equivalent to :ref:`pragma Extensions_Visible<Pragma-Extensions_Visible>`.

Aspect Favor_Top_Level
======================
.. index:: Favor_Top_Level

This boolean aspect is equivalent to :ref:`pragma Favor_Top_Level<Pragma-Favor_Top_Level>`.

Aspect Ghost
=============
.. index:: Ghost

This aspect is equivalent to :ref:`pragma Ghost<Pragma-Ghost>`.

Aspect Ghost_Predicate
======================
.. index:: Ghost_Predicate

This aspect introduces a subtype predicate that can reference ghost
entities. The subtype cannot appear as a subtype_mark in a membership test.

For the detailed semantics of this aspect, see the entry for subtype predicates
in the SPARK Reference Manual, section 3.2.4.

Aspect Global
=============
.. index:: Global

This aspect is equivalent to :ref:`pragma Global<Pragma-Global>`.

Aspect Initial_Condition
========================
.. index:: Initial_Condition

This aspect is equivalent to :ref:`pragma Initial_Condition<Pragma-Initial_Condition>`.

Aspect Initializes
==================
.. index:: Initializes

This aspect is equivalent to :ref:`pragma Initializes<Pragma-Initializes>`.

Aspect Inline_Always
====================
.. index:: Inline_Always

This boolean aspect is equivalent to :ref:`pragma Inline_Always<Pragma-Inline_Always>`.

Aspect Invariant
================
.. index:: Invariant

This aspect is equivalent to :ref:`pragma Invariant<Pragma-Invariant>`. It is a
synonym for the language defined aspect ``Type_Invariant`` except
that it is separately controllable using pragma ``Assertion_Policy``.

Aspect Invariant'Class
======================
.. index:: Invariant'Class

This aspect is equivalent to :ref:`pragma Type_Invariant_Class<Pragma-Type_Invariant_Class>`. It is a
synonym for the language defined aspect ``Type_Invariant'Class`` except
that it is separately controllable using pragma ``Assertion_Policy``.

Aspect Iterable
===============
.. index:: Iterable

This aspect provides a light-weight mechanism for loops and quantified
expressions over container types, without the overhead imposed by the tampering
checks of standard Ada 2012 iterators. The value of the aspect is an aggregate
with six named components, of which the last three are optional: ``First``,
``Next``, ``Has_Element``, ``Element``, ``Last``, and ``Previous``.
When only the first three components are specified, only the
``for .. in`` form of iteration over cursors is available. When ``Element``
is specified, both this form and the ``for .. of`` form of iteration over
elements are available. If the last two components are specified, reverse
iterations over the container can be specified (analogous to what can be done
over predefined containers that support the ``Reverse_Iterator`` interface).
The following is a typical example of use:

.. code-block:: ada

  type List is private with
      Iterable => (First       => First_Cursor,
                   Next        => Advance,
                   Has_Element => Cursor_Has_Element
                 [,Element     => Get_Element]
                 [,Last        => Last_Cursor]
                 [,Previous    => Retreat]);

* The values of ``First`` and ``Last`` are primitive operations of the
  container type that return a ``Cursor``, which must be a type declared in
  the container package or visible from it. For example:

.. code-block:: ada

  function First_Cursor (Cont : Container) return Cursor;
  function Last_Cursor  (Cont : Container) return Cursor;

* The values of ``Next`` and ``Previous`` are primitive operations of the container type that take
  both a container and a cursor and yield a cursor. For example:

.. code-block:: ada

  function Advance (Cont : Container; Position : Cursor) return Cursor;
  function Retreat (Cont : Container; Position : Cursor) return Cursor;

* The value of ``Has_Element`` is a primitive operation of the container type
  that takes both a container and a cursor and yields a boolean. For example:

.. code-block:: ada

  function Cursor_Has_Element (Cont : Container; Position : Cursor) return Boolean;

* The value of ``Element`` is a primitive operation of the container type that
  takes both a container and a cursor and yields an ``Element_Type``, which must
  be a type declared in the container package or visible from it. For example:

.. code-block:: ada

  function Get_Element (Cont : Container; Position : Cursor) return Element_Type;

This aspect is used in the GNAT-defined formal container packages.

Aspect Linker_Section
=====================
.. index:: Linker_Section

This aspect is equivalent to :ref:`pragma Linker_Section<Pragma-Linker_Section>`.

Aspect Local_Restrictions
=========================
.. index:: Local_Restrictions

This aspect may be specified for a subprogram (and for other declarations
as described below). It is used to specify that a particular subprogram does
not violate one or more local restrictions, nor can it call a subprogram
that is not subject to the same requirement. Positional aggregate syntax
(with parentheses, not square brackets) may be used to specify more than one
local restriction, as in

.. code-block:: ada

  procedure Do_Something
    with Local_Restrictions => (Some_Restriction, Another_Restriction);

Parentheses are currently required even in the case of specifying a single
local restriction (this requirement may be relaxed in the future).
Supported local restrictions currently include (only) No_Heap_Allocations and
No_Secondary_Stack.
No_Secondary_Stack corresponds to the GNAT-defined (global) restriction
of the same name. No_Heap_Allocations corresponds to the conjunction of the
Ada-defined restrictions No_Allocators and No_Implicit_Heap_Allocations.

Additional requirements are imposed in order to ensure that restriction
violations cannot be achieved via overriding dispatching operations,
calling through an access-to-subprogram value, calling a generic formal
subprogram, or calling through a subprogram renaming.
For a dispatching operation, an overrider must be subject to (at least) the
same restrictions as the overridden inherited subprogram; similarly, the
actual subprogram corresponding to a generic formal subprogram
in an instantiation must be subject to (at least) the same restrictions
as the formal subprogram. A call through an access-to-subprogram value
is conservatively assumed to violate all local restrictions; tasking-related
constructs (notably entry calls) are treated similarly. A renaming-as-body is
treated like a subprogram body containing a call to the renamed subprogram.

The Local_Restrictions aspect can be specified for a package specification,
in which case the aspect specification also applies to all eligible entities
declared with the package. This includes types. Default initialization of an
object of a given type is treated like a call to an implicitly-declared
initialization subprogram. Such a "call" is subject to the same local
restriction checks as any other call. If a type is subject to a local
restriction, then any violations of that restriction within the default
initialization expressions (if any) of the type are rejected. This may
include "calls" to the default initialization subprograms of other types.

Local_Restrictions aspect specifications are additive (for example, in the
case of a declaration that occurs within nested packages that each have
a Local_Restrictions specification).


Aspect Lock_Free
================
.. index:: Lock_Free

This boolean aspect is equivalent to :ref:`pragma Lock_Free<Pragma-Lock_Free>`.

Aspect Max_Queue_Length
=======================
.. index:: Max_Queue_Length

This aspect is equivalent to :ref:`pragma Max_Queue_Length<Pragma-Max_Queue_Length>`.

Aspect No_Caching
=================
.. index:: No_Caching

This boolean aspect is equivalent to :ref:`pragma No_Caching<Pragma-No_Caching>`.

Aspect No_Elaboration_Code_All
==============================
.. index:: No_Elaboration_Code_All

This aspect is equivalent to :ref:`pragma No_Elaboration_Code_All<Pragma-No_Elaboration_Code_All>`
for a program unit.

Aspect No_Inline
================
.. index:: No_Inline

This boolean aspect is equivalent to :ref:`pragma No_Inline<Pragma-No_Inline>`.

Aspect No_Tagged_Streams
========================
.. index:: No_Tagged_Streams

This aspect is equivalent to :ref:`pragma No_Tagged_Streams<Pragma-No_Tagged_Streams>` with an
argument specifying a root tagged type (thus this aspect can only be
applied to such a type).

Aspect No_Task_Parts
========================
.. index:: No_Task_Parts

Applies to a type. If True, requires that the type and any descendants
do not have any task parts. The rules for this aspect are the same as
for the language-defined No_Controlled_Parts aspect (see RM-H.4.1),
replacing "controlled" with "task".

If No_Task_Parts is True for a type T, then the compiler can optimize
away certain tasking-related code that would otherwise be needed
for T'Class, because descendants of T might contain tasks.

Aspect Object_Size
==================
.. index:: Object_Size

This aspect is equivalent to :ref:`attribute Object_Size<Attribute-Object_Size>`.

Aspect Obsolescent
==================
.. index:: Obsolescent

This aspect is equivalent to :ref:`pragma Obsolescent<Pragma_Obsolescent>`. Note that the
evaluation of this aspect happens at the point of occurrence, it is not
delayed until the freeze point.

Aspect Part_Of
==============
.. index:: Part_Of

This aspect is equivalent to :ref:`pragma Part_Of<Pragma-Part_Of>`.

Aspect Persistent_BSS
=====================
.. index:: Persistent_BSS

This boolean aspect is equivalent to :ref:`pragma Persistent_BSS<Pragma-Persistent_BSS>`.

Aspect Predicate
================
.. index:: Predicate

This aspect is equivalent to :ref:`pragma Predicate<Pragma-Predicate>`. It is thus
similar to the language defined aspects ``Dynamic_Predicate``
and ``Static_Predicate`` except that whether the resulting
predicate is static or dynamic is controlled by the form of the
expression. It is also separately controllable using pragma
``Assertion_Policy``.

Aspect Pure_Function
====================
.. index:: Pure_Function

This boolean aspect is equivalent to :ref:`pragma Pure_Function<Pragma-Pure_Function>`.

Aspect Refined_Depends
======================
.. index:: Refined_Depends

This aspect is equivalent to :ref:`pragma Refined_Depends<Pragma-Refined_Depends>`.

Aspect Refined_Global
=====================
.. index:: Refined_Global

This aspect is equivalent to :ref:`pragma Refined_Global<Pragma-Refined_Global>`.

Aspect Refined_Post
===================
.. index:: Refined_Post

This aspect is equivalent to :ref:`pragma Refined_Post<Pragma-Refined_Post>`.

Aspect Refined_State
====================
.. index:: Refined_State

This aspect is equivalent to :ref:`pragma Refined_State<Pragma-Refined_State>`.

Aspect Relaxed_Initialization
=============================
.. index:: Refined_Initialization

For the syntax and semantics of this aspect, see the SPARK 2014 Reference
Manual, section 6.10.

Aspect Remote_Access_Type
=========================
.. index:: Remote_Access_Type

This aspect is equivalent to :ref:`pragma Remote_Access_Type<Pragma-Remote_Access_Type>`.

Aspect Secondary_Stack_Size
===========================

.. index:: Secondary_Stack_Size

This aspect is equivalent to :ref:`pragma Secondary_Stack_Size<Pragma-Secondary_Stack_Size>`.


Aspect Scalar_Storage_Order
===========================
.. index:: Scalar_Storage_Order

This aspect is equivalent to a :ref:`attribute Scalar_Storage_Order<Attribute-Scalar_Storage_Order>`.

Aspect Shared
=============
.. index:: Shared

This boolean aspect is equivalent to :ref:`pragma Shared<Pragma-Shared>`
and is thus a synonym for aspect ``Atomic``.

Aspect Side_Effects
===================
.. index:: Side_Effects

This aspect is equivalent to :ref:`pragma Side_Effects<Pragma-Side_Effects>`.

Aspect Simple_Storage_Pool
==========================
.. index:: Simple_Storage_Pool

This aspect is equivalent to :ref:`attribute Simple_Storage_Pool<Attribute_Simple_Storage_Pool>`.

Aspect Simple_Storage_Pool_Type
===============================
.. index:: Simple_Storage_Pool_Type

This boolean aspect is equivalent to :ref:`pragma Simple_Storage_Pool_Type<Pragma-Simple_Storage_Pool_Type>`.

Aspect SPARK_Mode
=================
.. index:: SPARK_Mode

This aspect is equivalent to :ref:`pragma SPARK_Mode<Pragma-SPARK_Mode>` and
may be specified for either or both of the specification and body
of a subprogram or package.

Aspect Suppress_Debug_Info
==========================
.. index:: Suppress_Debug_Info

This boolean aspect is equivalent to :ref:`pragma Suppress_Debug_Info<Pragma-Suppress_Debug_Info>`.

Aspect Suppress_Initialization
==============================
.. index:: Suppress_Initialization

This boolean aspect is equivalent to :ref:`pragma Suppress_Initialization<Pragma-Suppress_Initialization>`.

Aspect Test_Case
================
.. index:: Test_Case

This aspect is equivalent to :ref:`pragma Test_Case<Pragma-Test_Case>`.

Aspect Thread_Local_Storage
===========================
.. index:: Thread_Local_Storage

This boolean aspect is equivalent to :ref:`pragma Thread_Local_Storage<Pragma-Thread_Local_Storage>`.

Aspect Universal_Aliasing
=========================
.. index:: Universal_Aliasing

This boolean aspect is equivalent to :ref:`pragma Universal_Aliasing<Pragma-Universal_Aliasing>`.

Aspect Unmodified
=================
.. index:: Unmodified

This boolean aspect is equivalent to :ref:`pragma Unmodified<Pragma-Unmodified>`.

Aspect Unreferenced
===================
.. index:: Unreferenced

This boolean aspect is equivalent to :ref:`pragma Unreferenced<Pragma-Unreferenced>`.

When using the ``-gnat2022`` switch, this aspect is also supported on formal
parameters, which is in particular the only form possible for expression
functions.

Aspect Unreferenced_Objects
===========================
.. index:: Unreferenced_Objects

This boolean aspect is equivalent to :ref:`pragma Unreferenced_Objects<Pragma-Unreferenced_Objects>`.

Aspect User_Aspect
==================
.. index:: User_Aspect

This aspect takes an argument that is the name of an aspect defined by a
User_Aspect_Definition configuration pragma.
A User_Aspect aspect specification is semantically equivalent to
replicating the set of aspect specifications associated with the named
pragma-defined aspect.

Aspect Value_Size
=================
.. index:: Value_Size

This aspect is equivalent to :ref:`attribute Value_Size<Attribute-Value_Size>`.

Aspect Volatile_Full_Access
===========================
.. index:: Volatile_Full_Access

This boolean aspect is equivalent to :ref:`pragma Volatile_Full_Access<Pragma-Volatile_Full_Access>`.

Aspect Volatile_Function
===========================
.. index:: Volatile_Function

This boolean aspect is equivalent to :ref:`pragma Volatile_Function<Pragma-Volatile_Function>`.

Aspect Warnings
===============
.. index:: Warnings

This aspect is equivalent to the two argument form of :ref:`pragma Warnings<Pragma_Warnings>`,
where the first argument is ``ON`` or ``OFF`` and the second argument
is the entity.