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2. 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 equivalent to the corresponding pragma or attribute definition clause.

Aspect Abstract_State  
Annotate  
Aspect Async_Readers  
Aspect Async_Writers  
Aspect Contract_Cases  
Aspect Depends  
Aspect Dimension  
Aspect Dimension_System  
Aspect Effective_Reads  
Aspect Effective_Writes  
Aspect Favor_Top_Level  
Aspect Global  
Aspect Initial_Condition  
Aspect Initializes  
Aspect Inline_Always  
Aspect Invariant  
Aspect Iterable  
Aspect Linker_Section  
Aspect Lock_Free  
Aspect Object_Size  
Aspect Part_Of  
Aspect Persistent_BSS  
Aspect Predicate  
Aspect Pure_Function  
Aspect Refined_Depends  
Aspect Refined_Global  
Aspect Refined_Post  
Aspect Refined_State  
Aspect Remote_Access_Type  
Aspect Scalar_Storage_Order  
Aspect Shared  
Aspect Simple_Storage_Pool  
Aspect Simple_Storage_Pool_Type  
Aspect SPARK_Mode  
Aspect Suppress_Debug_Info  
Aspect Test_Case  
Aspect Thread_Local_Storage  
Aspect Universal_Aliasing  
Aspect Universal_Data  
Aspect Unmodified  
Aspect Unreferenced  
Aspect Unreferenced_Objects  
Aspect Value_Size  
Aspect Warnings  


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Aspect Abstract_State

This aspect is equivalent to pragma Abstract_State.


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Annotate

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

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);


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Aspect Async_Readers

This aspect is equivalent to pragma Async_Readers.


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Aspect Async_Writers

This aspect is equivalent to pragma Async_Writers.


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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.


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Aspect Depends

This aspect is equivalent to pragma Depends.


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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_Systen 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.


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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 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.


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Aspect Effective_Reads

This aspect is equivalent to pragma Effective_Reads.


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Aspect Effective_Writes

This aspect is equivalent to pragma Effective_Writes.


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Aspect Favor_Top_Level

This aspect is equivalent to pragma Favor_Top_Level.


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Aspect Global

This aspect is equivalent to pragma Global.


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Aspect Initial_Condition

This aspect is equivalent to pragma Initial_Condition.


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Aspect Initializes

This aspect is equivalent to pragma Initializes.


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Aspect Inline_Always

This aspect is equivalent to pragma Inline_Always.


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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.


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Aspect Iterable

This aspect is used in the GNAT-defined formal container packages, to provide a light-weight mechanism for loops over such containers, without the overhead imposed by the tampering checks of standard Ada2012 iterators. The value of the aspect is a aggregate with four named components: First, Next, Has_Element, and Element. The following is a typical example of use:

 
type List is private with
    Iterable => (First        => First_Element,
                 Next         => Advance,
                 Has_Element  => Get_Element,
                 Element      => List_Element);


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Aspect Linker_Section

This aspect is equivalent to an Linker_Section pragma.


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Aspect Lock_Free

This aspect is equivalent to pragma Lock_Free.


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Aspect Object_Size

This aspect is equivalent to an Object_Size attribute definition clause.


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Aspect Part_Of

This aspect is equivalent to pragma Part_Of.


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Aspect Persistent_BSS

This aspect is equivalent to pragma Persistent_BSS.


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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.


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Aspect Pure_Function

This aspect is equivalent to pragma Pure_Function.


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Aspect Refined_Depends

This aspect is equivalent to pragma Refined_Depends.


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Aspect Refined_Global

This aspect is equivalent to pragma Refined_Global.


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Aspect Refined_Post

This aspect is equivalent to pragma Refined_Post.


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Aspect Refined_State

This aspect is equivalent to pragma Refined_State.


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Aspect Remote_Access_Type

This aspect is equivalent to pragma Remote_Access_Type.


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Aspect Scalar_Storage_Order

This aspect is equivalent to a Scalar_Storage_Order attribute definition clause.


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Aspect Shared

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


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Aspect Simple_Storage_Pool

This aspect is equivalent to a Simple_Storage_Pool attribute definition clause.


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Aspect Simple_Storage_Pool_Type

This aspect is equivalent to pragma Simple_Storage_Pool_Type.


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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.


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Aspect Suppress_Debug_Info

This aspect is equivalent to pragma Suppress_Debug_Info.


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Aspect Test_Case

This aspect is equivalent to pragma Test_Case.


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Aspect Thread_Local_Storage

This aspect is equivalent to pragma Thread_Local_Storage.


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Aspect Universal_Aliasing

This aspect is equivalent to pragma Universal_Aliasing.


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Aspect Universal_Data

This aspect is equivalent to pragma Universal_Data.


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Aspect Unmodified

This aspect is equivalent to pragma Unmodified.


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Aspect Unreferenced

This aspect is equivalent to pragma Unreferenced.


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Aspect Unreferenced_Objects

This aspect is equivalent to pragma Unreferenced_Objects.


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Aspect Value_Size

This aspect is equivalent to a Value_Size attribute definition clause.


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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.


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