3. 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:

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

then the effect is the same as:

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

and if we write:

type R is new Integer
  with Shared => True;

then the effect is the same as:

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.

3.1. Aspect Abstract_State

This aspect is equivalent to pragma Abstract_State.

3.2. Aspect Annotate

There are three forms of this aspect (where ID is an identifier, and ARG is a general expression), corresponding to 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);

3.3. Aspect Async_Readers

This boolean aspect is equivalent to pragma Async_Readers.

3.4. Aspect Async_Writers

This boolean aspect is equivalent to pragma Async_Writers.

3.5. Aspect Constant_After_Elaboration

This aspect is equivalent to pragma Constant_After_Elaboration.

3.6. Aspect Contract_Cases

This aspect is equivalent to pragma Contract_Cases, the sequence of clauses being enclosed in parentheses so that syntactically it is an aggregate.

3.7. Aspect Depends

This aspect is equivalent to pragma Depends.

3.8. Aspect Default_Initial_Condition

This aspect is equivalent to pragma Default_Initial_Condition.

3.9. Aspect 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.

3.10. Aspect 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:

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.

3.11. Aspect 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.

3.12. Aspect Effective_Reads

This aspect is equivalent to pragma Effective_Reads.

3.13. Aspect Effective_Writes

This aspect is equivalent to pragma Effective_Writes.

3.14. Aspect Extensions_Visible

This aspect is equivalent to pragma Extensions_Visible.

3.15. Aspect Favor_Top_Level

This boolean aspect is equivalent to pragma Favor_Top_Level.

3.16. Aspect Ghost

This aspect is equivalent to pragma Ghost.

3.17. Aspect Global

This aspect is equivalent to pragma Global.

3.18. Aspect Initial_Condition

This aspect is equivalent to pragma Initial_Condition.

3.19. Aspect Initializes

This aspect is equivalent to pragma Initializes.

3.20. Aspect Inline_Always

This boolean aspect is equivalent to pragma Inline_Always.

3.21. Aspect Invariant

This aspect is equivalent to pragma Invariant. It is a synonym for the language defined aspect Type_Invariant except that it is separately controllable using pragma Assertion_Policy.

3.22. Aspect Invariant’Class

This aspect is equivalent to 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.

3.23. Aspect 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, or 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:

type List is private with
    Iterable => (First        => First_Cursor,
                 Next         => Advance,
                 Has_Element  => Cursor_Has_Element,
                [Element      => Get_Element]);
  • The value denoted by First must denote a primitive operation of the container type that returns a Cursor, which must a be a type declared in the container package or visible from it. For example:
function First_Cursor (Cont : Container) return Cursor;
  • The value of Next is a primitive operation of the container type that takes both a container and a cursor and yields a cursor. For example:
function Advance (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:
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:
function Get_Element (Cont : Container; Position : Cursor) return Element_Type;

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

3.24. Aspect Linker_Section

This aspect is equivalent to pragma Linker_Section.

3.25. Aspect Lock_Free

This boolean aspect is equivalent to pragma Lock_Free.

3.26. Aspect Max_Queue_Length

This aspect is equivalent to pragma Max_Queue_Length.

3.27. Aspect No_Elaboration_Code_All

This aspect is equivalent to pragma No_Elaboration_Code_All for a program unit.

3.28. Aspect No_Inline

This boolean aspect is equivalent to pragma No_Inline.

3.29. Aspect No_Tagged_Streams

This aspect is equivalent to pragma No_Tagged_Streams with an argument specifying a root tagged type (thus this aspect can only be applied to such a type).

3.30. Aspect Object_Size

This aspect is equivalent to attribute Object_Size.

3.31. Aspect Obsolescent

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

3.32. Aspect Part_Of

This aspect is equivalent to pragma Part_Of.

3.33. Aspect Persistent_BSS

This boolean aspect is equivalent to pragma Persistent_BSS.

3.34. Aspect Predicate

This aspect is equivalent to 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.

3.35. Aspect Pure_Function

This boolean aspect is equivalent to pragma Pure_Function.

3.36. Aspect Refined_Depends

This aspect is equivalent to pragma Refined_Depends.

3.37. Aspect Refined_Global

This aspect is equivalent to pragma Refined_Global.

3.38. Aspect Refined_Post

This aspect is equivalent to pragma Refined_Post.

3.39. Aspect Refined_State

This aspect is equivalent to pragma Refined_State.

3.40. Aspect Remote_Access_Type

This aspect is equivalent to pragma Remote_Access_Type.

3.41. Aspect Secondary_Stack_Size

This aspect is equivalent to pragma Secondary_Stack_Size.

3.42. Aspect Scalar_Storage_Order

This aspect is equivalent to a attribute Scalar_Storage_Order.

3.43. Aspect Shared

This boolean aspect is equivalent to pragma Shared and is thus a synonym for aspect Atomic.

3.44. Aspect Simple_Storage_Pool

This aspect is equivalent to attribute Simple_Storage_Pool.

3.45. Aspect Simple_Storage_Pool_Type

This boolean aspect is equivalent to pragma Simple_Storage_Pool_Type.

3.46. Aspect SPARK_Mode

This aspect is equivalent to pragma SPARK_Mode and may be specified for either or both of the specification and body of a subprogram or package.

3.47. Aspect Suppress_Debug_Info

This boolean aspect is equivalent to pragma Suppress_Debug_Info.

3.48. Aspect Suppress_Initialization

This boolean aspect is equivalent to pragma Suppress_Initialization.

3.49. Aspect Test_Case

This aspect is equivalent to pragma Test_Case.

3.50. Aspect Thread_Local_Storage

This boolean aspect is equivalent to pragma Thread_Local_Storage.

3.51. Aspect Universal_Aliasing

This boolean aspect is equivalent to pragma Universal_Aliasing.

3.52. Aspect Universal_Data

This aspect is equivalent to pragma Universal_Data.

3.53. Aspect Unmodified

This boolean aspect is equivalent to pragma Unmodified.

3.54. Aspect Unreferenced

This boolean aspect is equivalent to pragma Unreferenced. Note that in the case of formal parameters, it is not permitted to have aspects for a formal parameter, so in this case the pragma form must be used.

3.55. Aspect Unreferenced_Objects

This boolean aspect is equivalent to pragma Unreferenced_Objects.

3.56. Aspect Value_Size

This aspect is equivalent to attribute Value_Size.

3.57. Aspect Volatile_Full_Access

This boolean aspect is equivalent to pragma Volatile_Full_Access.

3.58. Aspect Volatile_Function

This boolean aspect is equivalent to pragma Volatile_Function.

3.59. Aspect Warnings

This aspect is equivalent to the two argument form of pragma Warnings, where the first argument is ON or OFF and the second argument is the entity.