13.5.2 Storage Place Attributes
Static Semantics
1
{storage
place attributes (of a component)} For
a component C of a composite, non-array object R, the
storage place
attributes are defined:
1.a
Ramification: The storage place attributes
are not (individually) specifiable, but the user may control their values
by giving a record_representation_clause.
2/2
R.C'Position
{
AI95-00133-01}
If the nondefault bit ordering applies to the composite
type, and if a component_clause specifies
the placement of C, denotes the value given for the position
of the component_clause; otherwise, denotes Denotes
the same value as R.C'Address – R'Address. The value of this attribute
is of the type
universal_integer.
2.a/2
Ramification: {
AI95-00133-01}
Thus,
for the default bit order, R.C'Position
is the offset of C in storage elements from the beginning of the object,
where the first storage element of an object is numbered zero. R'Address
+ R.C'Position = R.C'Address. For record extensions, the offset is not
measured from the beginning of the extension part, but from the beginning
of the whole object, as usual.
2.b
In “R.C'Address – R'Address”,
the "–" operator is the one in System.Storage_Elements
that takes two Addresses and returns a Storage_Offset.
3/2
R.C'First_Bit
{
AI95-00133-01}
If the nondefault bit ordering applies to the composite
type, and if a component_clause specifies
the placement of C, denotes the value given for the first_bit
of the component_clause; otherwise, denotes Denotes
the offset, from the start of the first of the storage elements occupied
by C, of the first bit occupied by C. This offset is measured in bits.
The first bit of a storage element is numbered zero. The value of this
attribute is of the type
universal_integer.
4/2
R.C'Last_Bit
{
AI95-00133-01}
If the nondefault bit ordering applies to the composite
type, and if a component_clause specifies
the placement of C, denotes the value given for the last_bit
of the component_clause; otherwise, denotes Denotes
the offset, from the start of the first of the storage elements occupied
by C, of the last bit occupied by C. This offset is measured in bits.
The value of this attribute is of the type
universal_integer.
4.a/2
4.b
R.C'Size = R.C'Last_Bit – R.C'First_Bit
+ 1. (Unless the implementation chooses an indirection representation.)
4.c
If a component_clause
applies to a component, then that component will be at the same relative
storage place in all objects of the type. Otherwise, there is no such
requirement.
Implementation Advice
5
{contiguous representation
[partial]} {discontiguous
representation [partial]} If a component
is represented using some form of pointer (such as an offset) to the
actual data of the component, and this data is contiguous with the rest
of the object, then the storage place attributes should reflect the place
of the actual data, not the pointer. If a component is allocated discontiguously
from the rest of the object, then a warning should be generated upon
reference to one of its storage place attributes.
5.a
Reason: For discontiguous components,
these attributes make no sense. For example, an implementation might
allocate dynamic-sized components on the heap. For another example, an
implementation might allocate the discriminants separately from the other
components, so that multiple objects of the same subtype can share discriminants.
Such representations cannot happen if there is a component_clause
for that component.
5.b/2
Implementation Advice:
If a component is represented using
a pointer to the actual data of the component which is contiguous with
the rest of the object, then the storage place attributes should reflect
the place of the actual data. If a component is allocated discontiguously
from the rest of the object, then a warning should be generated upon
reference to one of its storage place attributes.
Incompatibilities With Ada 95
5.c/2
{
AI95-00133-01}
{incompatibilities with Ada 95} Amendment
Correction: The meaning of the storage place attributes for the nondefault
bit order is now clearly defined, and can be different than that given
by strictly following the Ada 95 wording. Any code which depends on the
original Ada 95 values for a type using the nondefault bit order where
they are different will break.
Sponsored by Ada-Europe