13.5.2 Storage Place Attributes
Static Semantics
{storage
place attributes (of a component)} For
a component C of a composite, non-array object R, the
storage place
attributes are defined:
Ramification: The storage place attributes
are not (individually) specifiable, but the user may control their values
by giving a
record_representation_clause.
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 the same value as R.C'Address – R'Address. The
value of this attribute is of the type
universal_integer.
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.
In “R.C'Address – R'Address”,
the "–" operator is the one in System.Storage_Elements
that takes two Addresses and returns a Storage_Offset.
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 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.
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 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.
R.C'Size = R.C'Last_Bit – R.C'First_Bit
+ 1. (Unless the implementation chooses an indirection representation.)
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
{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.
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.
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
{
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.
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