2. 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.
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.
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).
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 non-static because Address
is a private type.
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.
13.1 Machine Code Insertions
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
.
13.1 Machine Code Insertions
AST_Entry
This attribute is implemented only in OpenVMS versions of GNAT. Applied to
the name of an entry, it yields a value of the predefined type AST_Handler
(declared in the predefined package System, as extended by the use of
pragma Extend_System (Aux_DEC)
). This value enables the given entry to
be called when an AST occurs. For further details, refer to the DEC Ada
Language Reference Manual, section 9.12a.
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 non-negative number not
exceeding the value of 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.
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.
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. A
typical result would be something like "GNAT 2012 (20090221)".
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.
Default_Bit_Order
Standard'Default_Bit_Order
(Standard
is the only
permissible 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
.
Descriptor_Size
Non-static 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 (Positive range <>) of Boolean;
Put_Line ("Descriptor size = " & Unconstr_Array'Descriptor_Size'Img);
|
The attribute takes into account any additional padding due to type alignment.
In the example above, the descriptor contains two values of type
Positive
representing the low and high bound. Since Positive
has
a size of 31 bits and an alignment of 4, the descriptor size is 2 *
Positive'Size + 2
or 64 bits.
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.
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.
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.
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.
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.
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.
Enum_Rep
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
.
Enum_Val
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).
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.
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
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.
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.
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.
Img
The Img
attribute differs from Image
in that it may be
applied to objects as well as types, in which case it gives the
Image
for the subtype of the object. This is convenient for
debugging:
| Put_Line ("X = " & X'Img);
|
has the same meaning as the more verbose:
| Put_Line ("X = " & T'Image (X));
|
where T is the (sub)type of the object X
.
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
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.
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.
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.
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.
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.
Max_Interrupt_Priority
Standard'Max_Interrupt_Priority
(Standard
is the only
permissible prefix), provides the same value as
System.Max_Interrupt_Priority
.
Max_Priority
Standard'Max_Priority
(Standard
is the only permissible
prefix) provides the same value as System.Max_Priority
.
Maximum_Alignment
Standard'Maximum_Alignment
(Standard
is the only
permissible 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.
Mechanism_Code
function'Mechanism_Code
yields an integer code for the
mechanism used for the result of function, and
subprogram'Mechanism_Code (n)
yields the mechanism
used for formal parameter number n (a static integer value with 1
meaning the first parameter) of subprogram. The code returned is:
- 1
- by copy (value)
- 2
- by reference
- 3
- by descriptor (default descriptor class)
- 4
- by descriptor (UBS: unaligned bit string)
- 5
- by descriptor (UBSB: aligned bit string with arbitrary bounds)
- 6
- by descriptor (UBA: unaligned bit array)
- 7
- by descriptor (S: string, also scalar access type parameter)
- 8
- by descriptor (SB: string with arbitrary bounds)
- 9
- by descriptor (A: contiguous array)
- 10
- by descriptor (NCA: non-contiguous array)
Values from 3 through 10 are only relevant to Digital OpenVMS implementations.
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.
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).
Old
The attribute Prefix'Old can be used within a
subprogram body or within a precondition or
postcondition pragma. The effect is to
refer to the value of the prefix on entry. So for
example if you have an argument of a record type X called Arg1,
you can refer to Arg1.Field'Old which yields the value of
Arg1.Field on entry. The implementation simply involves generating
an object declaration which captures the value on entry.
The prefix must denote an object of a nonlimited type (since limited types
cannot be copied to capture their values) and it must not reference a local
variable (since local variables do not exist at subprogram entry time). Note
that the variable introduced by a quantified expression is a local variable.
The following example shows the use of 'Old to implement
a test of a postcondition:
| with Old_Pkg;
procedure Old is
begin
Old_Pkg.Incr;
end Old;
package Old_Pkg is
procedure Incr;
end Old_Pkg;
package body Old_Pkg is
Count : Natural := 0;
procedure Incr is
begin
... code manipulating the value of Count
pragma Assert (Count = Count'Old + 1);
end Incr;
end Old_Pkg;
|
Note that it is allowed to apply 'Old to a constant entity, but this will
result in a warning, since the old and new values will always be the same.
Passed_By_Reference
type'Passed_By_Reference
for any subtype type 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 non-static.
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
.
Range_Length
type'Range_Length
for any discrete type type 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 Range
applied to the array itself.
Ref
The System.Address'Ref
(System.Address
is the only permissible prefix)
denotes a function identical to
System.Storage_Elements.To_Address
except that
it is a static attribute. See To_Address for more details.
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.
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.
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.
Scalar_Storage_Order
For every record subtype S, the representation attribute
Scalar_Storage_Order
denotes the order in which storage elements
that make up scalar components are ordered within S. Other properties are
as for standard representation attribute Bit_Order
, as defined by
Ada RM 13.5.3(4). The default is System.Default_Bit_Order
.
If S'Scalar_Storage_Order
is specified explicitly, it shall be
equal to S'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.
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 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_size
and covering
storage elements at least up to position + (last_bit +
storage_element_size - 1) / storage_element_size
- the position of the component is interpreted relative to that machine
scalar.
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".
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.
Storage_Unit
Standard'Storage_Unit
(Standard
is the only permissible
prefix) provides the same value as System.Storage_Unit
.
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.
System_Allocator_Alignment
Standard'System_Allocator_Alignment
(Standard
is the only
permissible prefix) provides the observable guaranted 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.
Target_Name
Standard'Target_Name
(Standard
is the only permissible
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").
Tick
Standard'Tick
(Standard
is the only permissible prefix)
provides the same value as System.Tick
,
To_Address
The System'To_Address
(System
is the only permissible 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. The result is 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 non-static, even if its
argument is static).
Type_Class
type'Type_Class
for any type or subtype type yields
the value of the type class for the full type of type. If
type 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.
UET_Address
The UET_Address
attribute can only be used for a prefix which
denotes a library package. It yields the address of the unit exception
table when zero cost exception handling is used. This attribute is
intended only for use within the GNAT implementation. See the unit
Ada.Exceptions
in files `a-except.ads' and `a-except.adb'
for details on how this attribute is used in the implementation.
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.
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;
|
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. 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 identical to
first applying the Address
attribute and then doing an unchecked
conversion to a desired access type. In GNAT, but not necessarily in
other implementations, the use of static chains for inner level
subprograms means that Unrestricted_Access
applied to a
subprogram yields a value that can be called as long as the subprogram
is in scope (normal Ada accessibility rules restrict this usage).
It is possible to use Unrestricted_Access
for any type, but care
must be exercised if it is used to create pointers to unconstrained
objects. In this case, the resulting pointer has the same scope as the
context of the attribute, and may not be returned to some enclosing
scope. For instance, a function cannot use Unrestricted_Access
to create a unconstrained pointer and then return that value to the
caller.
Valid_Scalars
The 'Valid_Scalars
attribute is intended to make it easier to
check the validity of scalar subcomponents of composite objects. It
is defined for any prefix X
that denotes an object.
The value of this attribute is of the predefined type Boolean.
X'Valid_Scalars
yields True if and only if evaluation of
P'Valid
yields True for every scalar part P of X or if X has
no scalar parts. It is not specified in what order the scalar parts
are checked, nor whether any more are checked after any one of them
is determined to be invalid. If the prefix X
is of a class-wide
type T'Class
(where T
is the associated specific type),
or if the prefix X
is of a specific tagged type T
, then
only the scalar parts of components of T
are traversed; in other
words, components of extensions of T
are not traversed even if
T'Class (X)'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 parts (e.g., if the prefix is of an access type, an interface type,
an undiscriminated task type, or an undiscriminated protected type).
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.
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.
Wchar_T_Size
Standard'Wchar_T_Size
(Standard
is the only permissible
prefix) provides the size in bits of the C wchar_t
type
primarily for constructing the definition of this type in
package Interfaces.C
.
Word_Size
Standard'Word_Size
(Standard
is the only permissible
prefix) provides the value System.Word_Size
.
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