4. Implementation Defined Attributes#
Ada defines (throughout the Ada reference manual, summarized in Annex K), a set of attributes that provide useful additional functionality in all areas of the language. These language defined attributes are implemented in GNAT and work as described in the Ada Reference Manual.
In addition, Ada allows implementations to define additional attributes whose meaning is defined by the implementation. GNAT provides a number of these implementation-dependent attributes which can be used to extend and enhance the functionality of the compiler. This section of the GNAT reference manual describes these additional attributes. It also describes additional implementation-dependent features of standard language-defined attributes.
Note that any program using these attributes may not be portable to other compilers (although GNAT implements this set of attributes on all platforms). Therefore if portability to other compilers is an important consideration, you should minimize the use of these attributes.
4.1. Attribute Abort_Signal#
Standard'Abort_Signal (Standard is the only allowed
prefix) provides the entity for the special exception used to signal
task abort or asynchronous transfer of control. Normally this attribute
should only be used in the tasking runtime (it is highly peculiar, and
completely outside the normal semantics of Ada, for a user program to
intercept the abort exception).
4.2. Attribute Address_Size#
Standard'Address_Size (Standard is the only allowed
prefix) is a static constant giving the number of bits in an
Address. It is the same value as System.Address’Size,
but has the advantage of being static, while a direct
reference to System.Address’Size is nonstatic because Address
is a private type.
4.3. Attribute Asm_Input#
The Asm_Input attribute denotes a function that takes two
parameters. The first is a string, the second is an expression of the
type designated by the prefix. The first (string) argument is required
to be a static expression, and is the constraint for the parameter,
(e.g., what kind of register is required). The second argument is the
value to be used as the input argument. The possible values for the
constant are the same as those used in the RTL, and are dependent on
the configuration file used to built the GCC back end.
Machine Code Insertions
4.4. Attribute Asm_Output#
The Asm_Output attribute denotes a function that takes two
parameters. The first is a string, the second is the name of a variable
of the type designated by the attribute prefix. The first (string)
argument is required to be a static expression and designates the
constraint for the parameter (e.g., what kind of register is
required). The second argument is the variable to be updated with the
result. The possible values for constraint are the same as those used in
the RTL, and are dependent on the configuration file used to build the
GCC back end. If there are no output operands, then this argument may
either be omitted, or explicitly given as No_Output_Operands.
Machine Code Insertions
4.5. Attribute Atomic_Always_Lock_Free#
The prefix of the Atomic_Always_Lock_Free attribute is a type.
The result is a Boolean value which is True if the type has discriminants,
and False otherwise. The result indicate whether atomic operations are
supported by the target for the given type.
4.6. Attribute Bit#
obj'Bit, where obj is any object, yields the bit
offset within the storage unit (byte) that contains the first bit of
storage allocated for the object. The value of this attribute is of the
type universal_integer and is always a nonnegative number smaller
than System.Storage_Unit.
For an object that is a variable or a constant allocated in a register, the value is zero. (The use of this attribute does not force the allocation of a variable to memory).
For an object that is a formal parameter, this attribute applies to either the matching actual parameter or to a copy of the matching actual parameter.
For an access object the value is zero. Note that
obj.all'Bit is subject to an Access_Check for the
designated object. Similarly for a record component
X.C'Bit is subject to a discriminant check and
X(I).Bit and X(I1..I2)'Bit
are subject to index checks.
This attribute is designed to be compatible with the DEC Ada 83 definition
and implementation of the Bit attribute.
4.7. Attribute Bit_Position#
R.C'Bit_Position, where R is a record object and C is one
of the fields of the record type, yields the bit
offset within the record contains the first bit of
storage allocated for the object. The value of this attribute is of the
type universal_integer. The value depends only on the field
C and is independent of the alignment of
the containing record R.
4.8. Attribute Code_Address#
The 'Address
attribute may be applied to subprograms in Ada 95 and Ada 2005, but the
intended effect seems to be to provide
an address value which can be used to call the subprogram by means of
an address clause as in the following example:
procedure K is ...
procedure L;
for L'Address use K'Address;
pragma Import (Ada, L);
A call to L is then expected to result in a call to K.
In Ada 83, where there were no access-to-subprogram values, this was
a common work-around for getting the effect of an indirect call.
GNAT implements the above use of Address and the technique
illustrated by the example code works correctly.
However, for some purposes, it is useful to have the address of the start
of the generated code for the subprogram. On some architectures, this is
not necessarily the same as the Address value described above.
For example, the Address value may reference a subprogram
descriptor rather than the subprogram itself.
The 'Code_Address attribute, which can only be applied to
subprogram entities, always returns the address of the start of the
generated code of the specified subprogram, which may or may not be
the same value as is returned by the corresponding 'Address
attribute.
4.9. Attribute Compiler_Version#
Standard'Compiler_Version (Standard is the only allowed
prefix) yields a static string identifying the version of the compiler
being used to compile the unit containing the attribute reference.
4.10. Attribute Constrained#
In addition to the usage of this attribute in the Ada RM, GNAT
also permits the use of the 'Constrained attribute
in a generic template
for any type, including types without discriminants. The value of this
attribute in the generic instance when applied to a scalar type or a
record type without discriminants is always True. This usage is
compatible with older Ada compilers, including notably DEC Ada.
4.11. Attribute Default_Bit_Order#
Standard'Default_Bit_Order (Standard is the only
allowed prefix), provides the value System.Default_Bit_Order
as a Pos value (0 for High_Order_First, 1 for
Low_Order_First). This is used to construct the definition of
Default_Bit_Order in package System.
4.12. Attribute Default_Scalar_Storage_Order#
Standard'Default_Scalar_Storage_Order (Standard is the only
allowed prefix), provides the current value of the default scalar storage
order (as specified using pragma Default_Scalar_Storage_Order, or
equal to Default_Bit_Order if unspecified) as a
System.Bit_Order value. This is a static attribute.
4.13. Attribute Deref#
The attribute typ'Deref(expr) where expr is of type System.Address yields
the variable of type typ that is located at the given address. It is similar
to (totyp (expr).all), where totyp is an unchecked conversion from address to
a named access-to-typ type, except that it yields a variable, so it can be
used on the left side of an assignment.
4.14. Attribute Descriptor_Size#
Nonstatic attribute Descriptor_Size returns the size in bits of the
descriptor allocated for a type. The result is non-zero only for unconstrained
array types and the returned value is of type universal integer. In GNAT, an
array descriptor contains bounds information and is located immediately before
the first element of the array.
type Unconstr_Array is array (Short_Short_Integer range <>) of Positive;
Put_Line ("Descriptor size = " & Unconstr_Array'Descriptor_Size'Img);
The attribute takes into account any padding due to the alignment of the
component type. In the example above, the descriptor contains two values
of type Short_Short_Integer representing the low and high bound. But,
since Positive has an alignment of 4, the size of the descriptor is
2 * Short_Short_Integer'Size rounded up to the next multiple of 32,
which yields a size of 32 bits, i.e. including 16 bits of padding.
4.15. Attribute Elaborated#
The prefix of the 'Elaborated attribute must be a unit name. The
value is a Boolean which indicates whether or not the given unit has been
elaborated. This attribute is primarily intended for internal use by the
generated code for dynamic elaboration checking, but it can also be used
in user programs. The value will always be True once elaboration of all
units has been completed. An exception is for units which need no
elaboration, the value is always False for such units.
4.16. Attribute Elab_Body#
This attribute can only be applied to a program unit name. It returns the entity for the corresponding elaboration procedure for elaborating the body of the referenced unit. This is used in the main generated elaboration procedure by the binder and is not normally used in any other context. However, there may be specialized situations in which it is useful to be able to call this elaboration procedure from Ada code, e.g., if it is necessary to do selective re-elaboration to fix some error.
4.17. Attribute Elab_Spec#
This attribute can only be applied to a program unit name. It returns the entity for the corresponding elaboration procedure for elaborating the spec of the referenced unit. This is used in the main generated elaboration procedure by the binder and is not normally used in any other context. However, there may be specialized situations in which it is useful to be able to call this elaboration procedure from Ada code, e.g., if it is necessary to do selective re-elaboration to fix some error.
4.18. Attribute Elab_Subp_Body#
This attribute can only be applied to a library level subprogram name and is only allowed in CodePeer mode. It returns the entity for the corresponding elaboration procedure for elaborating the body of the referenced subprogram unit. This is used in the main generated elaboration procedure by the binder in CodePeer mode only and is unrecognized otherwise.
4.19. Attribute Emax#
The Emax attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.
4.20. Attribute Enabled#
The Enabled attribute allows an application program to check at compile
time to see if the designated check is currently enabled. The prefix is a
simple identifier, referencing any predefined check name (other than
All_Checks) or a check name introduced by pragma Check_Name. If
no argument is given for the attribute, the check is for the general state
of the check, if an argument is given, then it is an entity name, and the
check indicates whether an Suppress or Unsuppress has been
given naming the entity (if not, then the argument is ignored).
Note that instantiations inherit the check status at the point of the
instantiation, so a useful idiom is to have a library package that
introduces a check name with pragma Check_Name, and then contains
generic packages or subprograms which use the Enabled attribute
to see if the check is enabled. A user of this package can then issue
a pragma Suppress or pragma Unsuppress before instantiating
the package or subprogram, controlling whether the check will be present.
4.21. Attribute Enum_Rep#
Note that this attribute is now standard in Ada 202x and is available as an implementation defined attribute for earlier Ada versions.
For every enumeration subtype S, S'Enum_Rep denotes a
function with the following spec:
function S'Enum_Rep (Arg : S'Base) return <Universal_Integer>;
It is also allowable to apply Enum_Rep directly to an object of an
enumeration type or to a non-overloaded enumeration
literal. In this case S'Enum_Rep is equivalent to
typ'Enum_Rep(S) where typ is the type of the
enumeration literal or object.
The function returns the representation value for the given enumeration
value. This will be equal to value of the Pos attribute in the
absence of an enumeration representation clause. This is a static
attribute (i.e., the result is static if the argument is static).
S'Enum_Rep can also be used with integer types and objects,
in which case it simply returns the integer value. The reason for this
is to allow it to be used for (<>) discrete formal arguments in
a generic unit that can be instantiated with either enumeration types
or integer types. Note that if Enum_Rep is used on a modular
type whose upper bound exceeds the upper bound of the largest signed
integer type, and the argument is a variable, so that the universal
integer calculation is done at run time, then the call to Enum_Rep
may raise Constraint_Error.
4.22. Attribute Enum_Val#
Note that this attribute is now standard in Ada 202x and is available as an implementation defined attribute for earlier Ada versions.
For every enumeration subtype S, S'Enum_Val denotes a
function with the following spec:
function S'Enum_Val (Arg : <Universal_Integer>) return S'Base;
The function returns the enumeration value whose representation matches the
argument, or raises Constraint_Error if no enumeration literal of the type
has the matching value.
This will be equal to value of the Val attribute in the
absence of an enumeration representation clause. This is a static
attribute (i.e., the result is static if the argument is static).
4.23. Attribute Epsilon#
The Epsilon attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.
4.24. Attribute Fast_Math#
Standard'Fast_Math (Standard is the only allowed
prefix) yields a static Boolean value that is True if pragma
Fast_Math is active, and False otherwise.
4.25. Attribute Finalization_Size#
The prefix of attribute Finalization_Size must be an object or
a non-class-wide type. This attribute returns the size of any hidden data
reserved by the compiler to handle finalization-related actions. The type of
the attribute is universal_integer.
Finalization_Size yields a value of zero for a type with no controlled
parts, an object whose type has no controlled parts, or an object of a
class-wide type whose tag denotes a type with no controlled parts.
Note that only heap-allocated objects contain finalization data.
4.26. Attribute Fixed_Value#
For every fixed-point type S, S'Fixed_Value denotes a
function with the following specification:
function S'Fixed_Value (Arg : <Universal_Integer>) return S;
The value returned is the fixed-point value V such that:
V = Arg * S'Small
The effect is thus similar to first converting the argument to the
integer type used to represent S, and then doing an unchecked
conversion to the fixed-point type. The difference is
that there are full range checks, to ensure that the result is in range.
This attribute is primarily intended for use in implementation of the
input-output functions for fixed-point values.
4.27. Attribute From_Any#
This internal attribute is used for the generation of remote subprogram stubs in the context of the Distributed Systems Annex.
4.28. Attribute Has_Access_Values#
The prefix of the Has_Access_Values attribute is a type. The result
is a Boolean value which is True if the is an access type, or is a composite
type with a component (at any nesting depth) that is an access type, and is
False otherwise.
The intended use of this attribute is in conjunction with generic
definitions. If the attribute is applied to a generic private type, it
indicates whether or not the corresponding actual type has access values.
4.29. Attribute Has_Discriminants#
The prefix of the Has_Discriminants attribute is a type. The result
is a Boolean value which is True if the type has discriminants, and False
otherwise. The intended use of this attribute is in conjunction with generic
definitions. If the attribute is applied to a generic private type, it
indicates whether or not the corresponding actual type has discriminants.
4.30. Attribute Has_Tagged_Values#
The prefix of the Has_Tagged_Values attribute is a type. The result is a
Boolean value which is True if the type is a composite type (array or record)
that is either a tagged type or has a subcomponent that is tagged, and is False
otherwise. The intended use of this attribute is in conjunction with generic
definitions. If the attribute is applied to a generic private type, it
indicates whether or not the corresponding actual type has access values.
4.31. Attribute Img#
The Img attribute differs from Image in that, while both can be
applied directly to an object, Img cannot be applied to types.
Example usage of the attribute:
Put_Line ("X = " & X'Img);
which has the same meaning as the more verbose:
Put_Line ("X = " & T'Image (X));
where T is the (sub)type of the object X.
Note that technically, in analogy to Image,
X'Img returns a parameterless function
that returns the appropriate string when called. This means that
X'Img can be renamed as a function-returning-string, or used
in an instantiation as a function parameter.
4.32. Attribute Initialized#
For the syntax and semantics of this attribute, see the SPARK 2014 Reference Manual, section 6.10.
4.33. Attribute Integer_Value#
For every integer type S, S'Integer_Value denotes a
function with the following spec:
function S'Integer_Value (Arg : <Universal_Fixed>) return S;
The value returned is the integer value V, such that:
Arg = V * T'Small
where T is the type of Arg.
The effect is thus similar to first doing an unchecked conversion from
the fixed-point type to its corresponding implementation type, and then
converting the result to the target integer type. The difference is
that there are full range checks, to ensure that the result is in range.
This attribute is primarily intended for use in implementation of the
standard input-output functions for fixed-point values.
4.34. Attribute Invalid_Value#
For every scalar type S, S’Invalid_Value returns an undefined value of the type. If possible this value is an invalid representation for the type. The value returned is identical to the value used to initialize an otherwise uninitialized value of the type if pragma Initialize_Scalars is used, including the ability to modify the value with the binder -Sxx flag and relevant environment variables at run time.
4.35. Attribute Iterable#
Equivalent to Aspect Iterable.
4.36. Attribute Large#
The Large attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.
4.37. Attribute Library_Level#
P'Library_Level, where P is an entity name,
returns a Boolean value which is True if the entity is declared
at the library level, and False otherwise. Note that within a
generic instantiation, the name of the generic unit denotes the
instance, which means that this attribute can be used to test
if a generic is instantiated at the library level, as shown
in this example:
generic
...
package Gen is
pragma Compile_Time_Error
(not Gen'Library_Level,
"Gen can only be instantiated at library level");
...
end Gen;
4.38. Attribute Loop_Entry#
Syntax:
X'Loop_Entry [(loop_name)]
The Loop_Entry attribute is used to refer to the value that an
expression had upon entry to a given loop in much the same way that the
Old attribute in a subprogram postcondition can be used to refer
to the value an expression had upon entry to the subprogram. The
relevant loop is either identified by the given loop name, or it is the
innermost enclosing loop when no loop name is given.
A Loop_Entry attribute can only occur within an Assert,
Assert_And_Cut, Assume, Loop_Variant or Loop_Invariant pragma.
In addition, such a pragma must be one of the items in the sequence
of statements of a loop body, or nested inside block statements that
appear in the sequence of statements of a loop body.
A common use of Loop_Entry is to compare the current value of objects with
their initial value at loop entry, in a Loop_Invariant pragma.
The effect of using X'Loop_Entry is the same as declaring
a constant initialized with the initial value of X at loop
entry. This copy is not performed if the loop is not entered, or if the
corresponding pragmas are ignored or disabled.
4.39. Attribute Machine_Size#
This attribute is identical to the Object_Size attribute. It is
provided for compatibility with the DEC Ada 83 attribute of this name.
4.40. Attribute Mantissa#
The Mantissa attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.
4.41. Attribute Maximum_Alignment#
Standard'Maximum_Alignment (Standard is the only
allowed prefix) provides the maximum useful alignment value for the
target. This is a static value that can be used to specify the alignment
for an object, guaranteeing that it is properly aligned in all
cases.
4.42. Attribute Max_Integer_Size#
Standard'Max_Integer_Size (Standard is the only allowed
prefix) provides the size of the largest supported integer type for
the target. The result is a static constant.
4.43. Attribute Mechanism_Code#
func'Mechanism_Code yields an integer code for the
mechanism used for the result of function func, and
subprog'Mechanism_Code (n) yields the mechanism
used for formal parameter number n (a static integer value, with 1
meaning the first parameter) of subprogram subprog. The code returned is:
- 1
by copy (value)
- 2
by reference
4.44. Attribute Null_Parameter#
A reference T'Null_Parameter denotes an imaginary object of
type or subtype T allocated at machine address zero. The attribute
is allowed only as the default expression of a formal parameter, or as
an actual expression of a subprogram call. In either case, the
subprogram must be imported.
The identity of the object is represented by the address zero in the argument list, independent of the passing mechanism (explicit or default).
This capability is needed to specify that a zero address should be
passed for a record or other composite object passed by reference.
There is no way of indicating this without the Null_Parameter
attribute.
4.45. Attribute Object_Size#
The size of an object is not necessarily the same as the size of the type
of an object. This is because by default object sizes are increased to be
a multiple of the alignment of the object. For example,
Natural'Size is
31, but by default objects of type Natural will have a size of 32 bits.
Similarly, a record containing an integer and a character:
type Rec is record
I : Integer;
C : Character;
end record;
will have a size of 40 (that is Rec'Size will be 40). The
alignment will be 4, because of the
integer field, and so the default size of record objects for this type
will be 64 (8 bytes).
If the alignment of the above record is specified to be 1, then the object size will be 40 (5 bytes). This is true by default, and also an object size of 40 can be explicitly specified in this case.
A consequence of this capability is that different object sizes can be given to subtypes that would otherwise be considered in Ada to be statically matching. But it makes no sense to consider such subtypes as statically matching. Consequently, GNAT adds a rule to the static matching rules that requires object sizes to match. Consider this example:
1. procedure BadAVConvert is
2. type R is new Integer;
3. subtype R1 is R range 1 .. 10;
4. subtype R2 is R range 1 .. 10;
5. for R1'Object_Size use 8;
6. for R2'Object_Size use 16;
7. type R1P is access all R1;
8. type R2P is access all R2;
9. R1PV : R1P := new R1'(4);
10. R2PV : R2P;
11. begin
12. R2PV := R2P (R1PV);
|
>>> target designated subtype not compatible with
type "R1" defined at line 3
13. end;
In the absence of lines 5 and 6,
types R1 and R2 statically match and
hence the conversion on line 12 is legal. But since lines 5 and 6
cause the object sizes to differ, GNAT considers that types
R1 and R2 are not statically matching, and line 12
generates the diagnostic shown above.
Similar additional checks are performed in other contexts requiring statically matching subtypes.
4.46. Attribute Old#
In addition to the usage of Old defined in the Ada 2012 RM (usage
within Post aspect), GNAT also permits the use of this attribute
in implementation defined pragmas Postcondition,
Contract_Cases and Test_Case. Also usages of
Old which would be illegal according to the Ada 2012 RM
definition are allowed under control of
implementation defined pragma Unevaluated_Use_Of_Old.
4.47. Attribute Passed_By_Reference#
typ'Passed_By_Reference for any subtype typ returns
a value of type Boolean value that is True if the type is
normally passed by reference and False if the type is normally
passed by copy in calls. For scalar types, the result is always False
and is static. For non-scalar types, the result is nonstatic.
4.48. Attribute Pool_Address#
X'Pool_Address for any object X returns the address
of X within its storage pool. This is the same as
X'Address, except that for an unconstrained array whose
bounds are allocated just before the first component,
X'Pool_Address returns the address of those bounds,
whereas X'Address returns the address of the first
component.
Here, we are interpreting ‘storage pool’ broadly to mean
wherever the object is allocated, which could be a
user-defined storage pool,
the global heap, on the stack, or in a static memory area.
For an object created by new, Ptr.all'Pool_Address is
what is passed to Allocate and returned from Deallocate.
4.49. Attribute Range_Length#
typ'Range_Length for any discrete type typ yields
the number of values represented by the subtype (zero for a null
range). The result is static for static subtypes. Range_Length
applied to the index subtype of a one dimensional array always gives the
same result as Length applied to the array itself.
4.50. Attribute Restriction_Set#
This attribute allows compile time testing of restrictions that are currently in effect. It is primarily intended for specializing code in the run-time based on restrictions that are active (e.g. don’t need to save fpt registers if restriction No_Floating_Point is known to be in effect), but can be used anywhere.
There are two forms:
System'Restriction_Set (partition_boolean_restriction_NAME)
System'Restriction_Set (No_Dependence => library_unit_NAME);
In the case of the first form, the only restriction names
allowed are parameterless restrictions that are checked
for consistency at bind time. For a complete list see the
subtype System.Rident.Partition_Boolean_Restrictions.
The result returned is True if the restriction is known to be in effect, and False if the restriction is known not to be in effect. An important guarantee is that the value of a Restriction_Set attribute is known to be consistent throughout all the code of a partition.
This is trivially achieved if the entire partition is compiled with a consistent set of restriction pragmas. However, the compilation model does not require this. It is possible to compile one set of units with one set of pragmas, and another set of units with another set of pragmas. It is even possible to compile a spec with one set of pragmas, and then WITH the same spec with a different set of pragmas. Inconsistencies in the actual use of the restriction are checked at bind time.
In order to achieve the guarantee of consistency for the Restriction_Set pragma, we consider that a use of the pragma that yields False is equivalent to a violation of the restriction.
So for example if you write
if System'Restriction_Set (No_Floating_Point) then
...
else
...
end if;
And the result is False, so that the else branch is executed, you can assume that this restriction is not set for any unit in the partition. This is checked by considering this use of the restriction pragma to be a violation of the restriction No_Floating_Point. This means that no other unit can attempt to set this restriction (if some unit does attempt to set it, the binder will refuse to bind the partition).
Technical note: The restriction name and the unit name are intepreted entirely syntactically, as in the corresponding Restrictions pragma, they are not analyzed semantically, so they do not have a type.
4.51. Attribute Result#
function'Result can only be used with in a Postcondition pragma
for a function. The prefix must be the name of the corresponding function. This
is used to refer to the result of the function in the postcondition expression.
For a further discussion of the use of this attribute and examples of its use,
see the description of pragma Postcondition.
4.52. Attribute Safe_Emax#
The Safe_Emax attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.
4.53. Attribute Safe_Large#
The Safe_Large attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.
4.54. Attribute Safe_Small#
The Safe_Small attribute is provided for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.
4.55. Attribute Scalar_Storage_Order#
For every array or record type S, the representation attribute
Scalar_Storage_Order denotes the order in which storage elements
that make up scalar components are ordered within S. The value given must
be a static expression of type System.Bit_Order. The following is an example
of the use of this feature:
-- Component type definitions
subtype Yr_Type is Natural range 0 .. 127;
subtype Mo_Type is Natural range 1 .. 12;
subtype Da_Type is Natural range 1 .. 31;
-- Record declaration
type Date is record
Years_Since_1980 : Yr_Type;
Month : Mo_Type;
Day_Of_Month : Da_Type;
end record;
-- Record representation clause
for Date use record
Years_Since_1980 at 0 range 0 .. 6;
Month at 0 range 7 .. 10;
Day_Of_Month at 0 range 11 .. 15;
end record;
-- Attribute definition clauses
for Date'Bit_Order use System.High_Order_First;
for Date'Scalar_Storage_Order use System.High_Order_First;
-- If Scalar_Storage_Order is specified, it must be consistent with
-- Bit_Order, so it's best to always define the latter explicitly if
-- the former is used.
Other properties are as for the standard representation attribute Bit_Order
defined by Ada RM 13.5.3(4). The default is System.Default_Bit_Order.
For a record type T, if T'Scalar_Storage_Order is
specified explicitly, it shall be equal to T'Bit_Order. Note:
this means that if a Scalar_Storage_Order attribute definition
clause is not confirming, then the type’s Bit_Order shall be
specified explicitly and set to the same value.
Derived types inherit an explicitly set scalar storage order from their parent types. This may be overridden for the derived type by giving an explicit scalar storage order for it. However, for a record extension, the derived type must have the same scalar storage order as the parent type.
A component of a record type that is itself a record or an array and that does not start and end on a byte boundary must have have the same scalar storage order as the record type. A component of a bit-packed array type that is itself a record or an array must have the same scalar storage order as the array type.
No component of a type that has an explicit Scalar_Storage_Order
attribute definition may be aliased.
A confirming Scalar_Storage_Order attribute definition clause (i.e.
with a value equal to System.Default_Bit_Order) has no effect.
If the opposite storage order is specified, then whenever the value of
a scalar component of an object of type S is read, the storage
elements of the enclosing machine scalar are first reversed (before
retrieving the component value, possibly applying some shift and mask
operatings on the enclosing machine scalar), and the opposite operation
is done for writes.
In that case, the restrictions set forth in 13.5.1(10.3/2) for scalar components are relaxed. Instead, the following rules apply:
the underlying storage elements are those at positions
(position + first_bit / storage_element_size) .. (position + (last_bit + storage_element_size - 1) / storage_element_size)the sequence of underlying storage elements shall have a size no greater than the largest machine scalar
the enclosing machine scalar is defined as the smallest machine scalar starting at a position no greater than
position + first_bit / storage_element_sizeand covering storage elements at least up toposition + (last_bit + storage_element_size - 1) / storage_element_sizethe position of the component is interpreted relative to that machine scalar.
If no scalar storage order is specified for a type (either directly, or by
inheritance in the case of a derived type), then the default is normally
the native ordering of the target, but this default can be overridden using
pragma Default_Scalar_Storage_Order.
If a component of T is itself of a record or array type, the specfied
Scalar_Storage_Order does not apply to that nested type: an explicit
attribute definition clause must be provided for the component type as well
if desired.
Representation changes that explicitly or implicitly toggle the scalar storage
order are not supported and may result in erroneous execution of the program,
except when performed by means of an instance of Ada.Unchecked_Conversion.
In particular, overlays are not supported and a warning is given for them:
type Rec_LE is record
I : Integer;
end record;
for Rec_LE use record
I at 0 range 0 .. 31;
end record;
for Rec_LE'Bit_Order use System.Low_Order_First;
for Rec_LE'Scalar_Storage_Order use System.Low_Order_First;
type Rec_BE is record
I : Integer;
end record;
for Rec_BE use record
I at 0 range 0 .. 31;
end record;
for Rec_BE'Bit_Order use System.High_Order_First;
for Rec_BE'Scalar_Storage_Order use System.High_Order_First;
R_LE : Rec_LE;
R_BE : Rec_BE;
for R_BE'Address use R_LE'Address;
warning: overlay changes scalar storage order [enabled by default]
In most cases, such representation changes ought to be replaced by an
instantiation of a function or procedure provided by GNAT.Byte_Swapping.
Note that the scalar storage order only affects the in-memory data representation. It has no effect on the representation used by stream attributes.
Note that debuggers may be unable to display the correct value of scalar components of a type for which the opposite storage order is specified.
4.56. Attribute Simple_Storage_Pool#
For every nonformal, nonderived access-to-object type Acc, the
representation attribute Simple_Storage_Pool may be specified
via an attribute_definition_clause (or by specifying the equivalent aspect):
My_Pool : My_Simple_Storage_Pool_Type;
type Acc is access My_Data_Type;
for Acc'Simple_Storage_Pool use My_Pool;
The name given in an attribute_definition_clause for the
Simple_Storage_Pool attribute shall denote a variable of
a ‘simple storage pool type’ (see pragma Simple_Storage_Pool_Type).
The use of this attribute is only allowed for a prefix denoting a type for which it has been specified. The type of the attribute is the type of the variable specified as the simple storage pool of the access type, and the attribute denotes that variable.
It is illegal to specify both Storage_Pool and Simple_Storage_Pool
for the same access type.
If the Simple_Storage_Pool attribute has been specified for an access
type, then applying the Storage_Pool attribute to the type is flagged
with a warning and its evaluation raises the exception Program_Error.
If the Simple_Storage_Pool attribute has been specified for an access
type S, then the evaluation of the attribute S'Storage_Size
returns the result of calling Storage_Size (S'Simple_Storage_Pool),
which is intended to indicate the number of storage elements reserved for
the simple storage pool. If the Storage_Size function has not been defined
for the simple storage pool type, then this attribute returns zero.
If an access type S has a specified simple storage pool of type
SSP, then the evaluation of an allocator for that access type calls
the primitive Allocate procedure for type SSP, passing
S'Simple_Storage_Pool as the pool parameter. The detailed
semantics of such allocators is the same as those defined for allocators
in section 13.11 of the Ada Reference Manual, with the term
simple storage pool substituted for storage pool.
If an access type S has a specified simple storage pool of type
SSP, then a call to an instance of the Ada.Unchecked_Deallocation
for that access type invokes the primitive Deallocate procedure
for type SSP, passing S'Simple_Storage_Pool as the pool
parameter. The detailed semantics of such unchecked deallocations is the same
as defined in section 13.11.2 of the Ada Reference Manual, except that the
term simple storage pool is substituted for storage pool.
4.57. Attribute Small#
The Small attribute is defined in Ada 95 (and Ada 2005) only for
fixed-point types.
GNAT also allows this attribute to be applied to floating-point types
for compatibility with Ada 83. See
the Ada 83 reference manual for an exact description of the semantics of
this attribute when applied to floating-point types.
4.58. Attribute Small_Denominator#
typ'Small_Denominator for any fixed-point subtype typ yields the
denominator in the representation of typ'Small as a rational number
with coprime factors (i.e. as an irreducible fraction).
4.59. Attribute Small_Numerator#
typ'Small_Numerator for any fixed-point subtype typ yields the
numerator in the representation of typ'Small as a rational number
with coprime factors (i.e. as an irreducible fraction).
4.60. Attribute Storage_Unit#
Standard'Storage_Unit (Standard is the only allowed
prefix) provides the same value as System.Storage_Unit.
4.61. Attribute Stub_Type#
The GNAT implementation of remote access-to-classwide types is organized as described in AARM section E.4 (20.t): a value of an RACW type (designating a remote object) is represented as a normal access value, pointing to a “stub” object which in turn contains the necessary information to contact the designated remote object. A call on any dispatching operation of such a stub object does the remote call, if necessary, using the information in the stub object to locate the target partition, etc.
For a prefix T that denotes a remote access-to-classwide type,
T'Stub_Type denotes the type of the corresponding stub objects.
By construction, the layout of T'Stub_Type is identical to that of
type RACW_Stub_Type declared in the internal implementation-defined
unit System.Partition_Interface. Use of this attribute will create
an implicit dependency on this unit.
4.62. Attribute System_Allocator_Alignment#
Standard'System_Allocator_Alignment (Standard is the only
allowed prefix) provides the observable guaranteed to be honored by
the system allocator (malloc). This is a static value that can be used
in user storage pools based on malloc either to reject allocation
with alignment too large or to enable a realignment circuitry if the
alignment request is larger than this value.
4.63. Attribute Target_Name#
Standard'Target_Name (Standard is the only allowed
prefix) provides a static string value that identifies the target
for the current compilation. For GCC implementations, this is the
standard gcc target name without the terminating slash (for
example, GNAT 5.0 on windows yields “i586-pc-mingw32msv”).
4.64. Attribute To_Address#
The System'To_Address
(System is the only allowed prefix)
denotes a function identical to
System.Storage_Elements.To_Address except that
it is a static attribute. This means that if its argument is
a static expression, then the result of the attribute is a
static expression. This means that such an expression can be
used in contexts (e.g., preelaborable packages) which require a
static expression and where the function call could not be used
(since the function call is always nonstatic, even if its
argument is static). The argument must be in the range
-(2**(m-1)) .. 2**m-1, where m is the memory size
(typically 32 or 64). Negative values are intepreted in a
modular manner (e.g., -1 means the same as 16#FFFF_FFFF# on
a 32 bits machine).
4.65. Attribute To_Any#
This internal attribute is used for the generation of remote subprogram stubs in the context of the Distributed Systems Annex.
4.66. Attribute Type_Class#
typ'Type_Class for any type or subtype typ yields
the value of the type class for the full type of typ. If
typ is a generic formal type, the value is the value for the
corresponding actual subtype. The value of this attribute is of type
System.Aux_DEC.Type_Class, which has the following definition:
type Type_Class is
(Type_Class_Enumeration,
Type_Class_Integer,
Type_Class_Fixed_Point,
Type_Class_Floating_Point,
Type_Class_Array,
Type_Class_Record,
Type_Class_Access,
Type_Class_Task,
Type_Class_Address);
Protected types yield the value Type_Class_Task, which thus
applies to all concurrent types. This attribute is designed to
be compatible with the DEC Ada 83 attribute of the same name.
4.67. Attribute Type_Key#
The Type_Key attribute is applicable to a type or subtype and
yields a value of type Standard.String containing encoded information
about the type or subtype. This provides improved compatibility with
other implementations that support this attribute.
4.68. Attribute TypeCode#
This internal attribute is used for the generation of remote subprogram stubs in the context of the Distributed Systems Annex.
4.69. Attribute Unconstrained_Array#
The Unconstrained_Array attribute can be used with a prefix that
denotes any type or subtype. It is a static attribute that yields
True if the prefix designates an unconstrained array,
and False otherwise. In a generic instance, the result is
still static, and yields the result of applying this test to the
generic actual.
4.70. Attribute Universal_Literal_String#
The prefix of Universal_Literal_String must be a named
number. The static result is the string consisting of the characters of
the number as defined in the original source. This allows the user
program to access the actual text of named numbers without intermediate
conversions and without the need to enclose the strings in quotes (which
would preclude their use as numbers).
For example, the following program prints the first 50 digits of pi:
with Text_IO; use Text_IO;
with Ada.Numerics;
procedure Pi is
begin
Put (Ada.Numerics.Pi'Universal_Literal_String);
end;
4.71. Attribute Unrestricted_Access#
The Unrestricted_Access attribute is similar to Access
except that all accessibility and aliased view checks are omitted. This
is a user-beware attribute.
For objects, it is similar to Address, for which it is a
desirable replacement where the value desired is an access type.
In other words, its effect is similar to first applying the
Address attribute and then doing an unchecked conversion to a
desired access type.
For subprograms, P'Unrestricted_Access may be used where
P'Access would be illegal, to construct a value of a
less-nested named access type that designates a more-nested
subprogram. This value may be used in indirect calls, so long as the
more-nested subprogram still exists; once the subprogram containing it
has returned, such calls are erroneous. For example:
package body P is
type Less_Nested is not null access procedure;
Global : Less_Nested;
procedure P1 is
begin
Global.all;
end P1;
procedure P2 is
Local_Var : Integer;
procedure More_Nested is
begin
... Local_Var ...
end More_Nested;
begin
Global := More_Nested'Unrestricted_Access;
P1;
end P2;
end P;
When P1 is called from P2, the call via Global is OK, but if P1 were called after P2 returns, it would be an erroneous use of a dangling pointer.
For objects, it is possible to use Unrestricted_Access for any
type. However, if the result is of an access-to-unconstrained array
subtype, then the resulting pointer has the same scope as the context
of the attribute, and must not be returned to some enclosing scope.
For instance, if a function uses Unrestricted_Access to create
an access-to-unconstrained-array and returns that value to the caller,
the result will involve dangling pointers. In addition, it is only
valid to create pointers to unconstrained arrays using this attribute
if the pointer has the normal default ‘fat’ representation where a
pointer has two components, one points to the array and one points to
the bounds. If a size clause is used to force ‘thin’ representation
for a pointer to unconstrained where there is only space for a single
pointer, then the resulting pointer is not usable.
In the simple case where a direct use of Unrestricted_Access attempts to make a thin pointer for a non-aliased object, the compiler will reject the use as illegal, as shown in the following example:
with System; use System;
procedure SliceUA2 is
type A is access all String;
for A'Size use Standard'Address_Size;
procedure P (Arg : A) is
begin
null;
end P;
X : String := "hello world!";
X2 : aliased String := "hello world!";
AV : A := X'Unrestricted_Access; -- ERROR
|
>>> illegal use of Unrestricted_Access attribute
>>> attempt to generate thin pointer to unaliased object
begin
P (X'Unrestricted_Access); -- ERROR
|
>>> illegal use of Unrestricted_Access attribute
>>> attempt to generate thin pointer to unaliased object
P (X(7 .. 12)'Unrestricted_Access); -- ERROR
|
>>> illegal use of Unrestricted_Access attribute
>>> attempt to generate thin pointer to unaliased object
P (X2'Unrestricted_Access); -- OK
end;
but other cases cannot be detected by the compiler, and are considered to be erroneous. Consider the following example:
with System; use System;
with System; use System;
procedure SliceUA is
type AF is access all String;
type A is access all String;
for A'Size use Standard'Address_Size;
procedure P (Arg : A) is
begin
if Arg'Length /= 6 then
raise Program_Error;
end if;
end P;
X : String := "hello world!";
Y : AF := X (7 .. 12)'Unrestricted_Access;
begin
P (A (Y));
end;
A normal unconstrained array value
or a constrained array object marked as aliased has the bounds in memory
just before the array, so a thin pointer can retrieve both the data and
the bounds. But in this case, the non-aliased object X does not have the
bounds before the string. If the size clause for type A
were not present, then the pointer
would be a fat pointer, where one component is a pointer to the bounds,
and all would be well. But with the size clause present, the conversion from
fat pointer to thin pointer in the call loses the bounds, and so this
is erroneous, and the program likely raises a Program_Error exception.
In general, it is advisable to completely
avoid mixing the use of thin pointers and the use of
Unrestricted_Access where the designated type is an
unconstrained array. The use of thin pointers should be restricted to
cases of porting legacy code that implicitly assumes the size of pointers,
and such code should not in any case be using this attribute.
Another erroneous situation arises if the attribute is applied to a constant. The resulting pointer can be used to access the constant, but the effect of trying to modify a constant in this manner is not well-defined. Consider this example:
P : constant Integer := 4;
type R is access all Integer;
RV : R := P'Unrestricted_Access;
..
RV.all := 3;
Here we attempt to modify the constant P from 4 to 3, but the compiler may
or may not notice this attempt, and subsequent references to P may yield
either the value 3 or the value 4 or the assignment may blow up if the
compiler decides to put P in read-only memory. One particular case where
Unrestricted_Access can be used in this way is to modify the
value of an in parameter:
procedure K (S : in String) is
type R is access all Character;
RV : R := S (3)'Unrestricted_Access;
begin
RV.all := 'a';
end;
In general this is a risky approach. It may appear to “work” but such uses of
Unrestricted_Access are potentially non-portable, even from one version
of GNAT to another, so are best avoided if possible.
4.72. Attribute Update#
The Update attribute creates a copy of an array or record value
with one or more modified components. The syntax is:
PREFIX'Update ( RECORD_COMPONENT_ASSOCIATION_LIST )
PREFIX'Update ( ARRAY_COMPONENT_ASSOCIATION {, ARRAY_COMPONENT_ASSOCIATION } )
PREFIX'Update ( MULTIDIMENSIONAL_ARRAY_COMPONENT_ASSOCIATION
{, MULTIDIMENSIONAL_ARRAY_COMPONENT_ASSOCIATION } )
MULTIDIMENSIONAL_ARRAY_COMPONENT_ASSOCIATION ::= INDEX_EXPRESSION_LIST_LIST => EXPRESSION
INDEX_EXPRESSION_LIST_LIST ::= INDEX_EXPRESSION_LIST {| INDEX_EXPRESSION_LIST }
INDEX_EXPRESSION_LIST ::= ( EXPRESSION {, EXPRESSION } )
where PREFIX is the name of an array or record object, the
association list in parentheses does not contain an others
choice and the box symbol <> may not appear in any
expression. The effect is to yield a copy of the array or record value
which is unchanged apart from the components mentioned in the
association list, which are changed to the indicated value. The
original value of the array or record value is not affected. For
example:
type Arr is Array (1 .. 5) of Integer;
...
Avar1 : Arr := (1,2,3,4,5);
Avar2 : Arr := Avar1'Update (2 => 10, 3 .. 4 => 20);
yields a value for Avar2 of 1,10,20,20,5 with Avar1
begin unmodified. Similarly:
type Rec is A, B, C : Integer;
...
Rvar1 : Rec := (A => 1, B => 2, C => 3);
Rvar2 : Rec := Rvar1'Update (B => 20);
yields a value for Rvar2 of (A => 1, B => 20, C => 3),
with Rvar1 being unmodifed.
Note that the value of the attribute reference is computed
completely before it is used. This means that if you write:
Avar1 := Avar1'Update (1 => 10, 2 => Function_Call);
then the value of Avar1 is not modified if Function_Call
raises an exception, unlike the effect of a series of direct assignments
to elements of Avar1. In general this requires that
two extra complete copies of the object are required, which should be
kept in mind when considering efficiency.
The Update attribute cannot be applied to prefixes of a limited
type, and cannot reference discriminants in the case of a record type.
The accessibility level of an Update attribute result object is defined
as for an aggregate.
In the record case, no component can be mentioned more than once. In the array case, two overlapping ranges can appear in the association list, in which case the modifications are processed left to right.
Multi-dimensional arrays can be modified, as shown by this example:
A : array (1 .. 10, 1 .. 10) of Integer;
..
A := A'Update ((1, 2) => 20, (3, 4) => 30);
which changes element (1,2) to 20 and (3,4) to 30.
4.73. Attribute Valid_Value#
The 'Valid_Value attribute is defined for enumeration types other than
those in package Standard. This attribute is a function that takes
a String, and returns Boolean. T'Valid_Value (S) returns True
if and only if T'Value (S) would not raise Constraint_Error.
4.74. Attribute Valid_Scalars#
The 'Valid_Scalars attribute is intended to make it easier to check the
validity of scalar subcomponents of composite objects. The attribute is defined
for any prefix P which denotes an object. Prefix P can be any type
except for tagged private or Unchecked_Union types. The value of the
attribute is of type Boolean.
P'Valid_Scalars yields True if and only if the evaluation of
C'Valid yields True for every scalar subcomponent C of P, or if
P has no scalar subcomponents. Attribute 'Valid_Scalars is equivalent
to attribute 'Valid for scalar types.
It is not specified in what order the subcomponents are checked, nor whether
any more are checked after any one of them is determined to be invalid. If the
prefix P is of a class-wide type T'Class (where T is the associated
specific type), or if the prefix P is of a specific tagged type T, then
only the subcomponents of T are checked; in other words, components of
extensions of T are not checked even if T'Class (P)'Tag /= T'Tag.
The compiler will issue a warning if it can be determined at compile time that the prefix of the attribute has no scalar subcomponents.
Note: Valid_Scalars can generate a lot of code, especially in the case of
a large variant record. If the attribute is called in many places in the same
program applied to objects of the same type, it can reduce program size to
write a function with a single use of the attribute, and then call that
function from multiple places.
4.75. Attribute VADS_Size#
The 'VADS_Size attribute is intended to make it easier to port
legacy code which relies on the semantics of 'Size as implemented
by the VADS Ada 83 compiler. GNAT makes a best effort at duplicating the
same semantic interpretation. In particular, 'VADS_Size applied
to a predefined or other primitive type with no Size clause yields the
Object_Size (for example, Natural'Size is 32 rather than 31 on
typical machines). In addition 'VADS_Size applied to an object
gives the result that would be obtained by applying the attribute to
the corresponding type.
4.76. Attribute Value_Size#
type'Value_Size is the number of bits required to represent
a value of the given subtype. It is the same as type'Size,
but, unlike Size, may be set for non-first subtypes.
4.77. Attribute Wchar_T_Size#
Standard'Wchar_T_Size (Standard is the only allowed
prefix) provides the size in bits of the C wchar_t type
primarily for constructing the definition of this type in
package Interfaces.C. The result is a static constant.
4.78. Attribute Word_Size#
Standard'Word_Size (Standard is the only allowed
prefix) provides the value System.Word_Size. The result is
a static constant.