In Ada 95, the
Size of a discrete type is the minimum number of bits
required to hold values of the type. Although this interpretation was
allowed in Ada 83, it was not required, and this requirement in practice
can cause some significant difficulties. For example, in most Ada 83
Natural'Size was 32. However, in Ada-95,
typically 31. This means that code may change in behavior when moving
from Ada 83 to Ada 95. For example, consider:
type Rec is record; A : Natural; B : Natural; end record; for Rec use record for A use at 0 range 0 .. Natural'Size - 1; for B use at 0 range Natural'Size .. 2 * Natural'Size - 1; end record;
In the above code, since the typical size of
is 32 bits and
Natural'Size is 31, the above code can cause
unexpected inefficient packing in Ada 95, and in general there are
surprising cases where the fact that the object size can exceed the
size of the type causes surprises.
To help get around this problem GNAT provides two implementation
applied to a type, these attributes yield the size of the type
(corresponding to the RM defined size attribute), and the size of
objects of the type respectively.
Object_Size is used for determining the default size of
objects and components. This size value can be referred to using the
Object_Size attribute. The phrase “is used” here means that it is
the basis of the determination of the size. The backend is free to
pad this up if necessary for efficiency, e.g. an 8-bit stand-alone
character might be stored in 32 bits on a machine with no efficient
byte access instructions such as the Alpha.
The default rules for the value of
Object_Size for fixed-point and
discrete types are as follows:
Object_Sizefor base subtypes reflect the natural hardware size in bits (run the utility
gnatpstato find those values for numeric types). Enumeration types and fixed-point base subtypes have 8, 16, 32 or 64 bits for this size, depending on the range of values to be stored.
Object_Sizeof a subtype is the same as the
Object_Sizeof the type from which it is obtained.
Object_Sizeof a derived base type is copied from the parent base type, and the
Object_Sizeof a derived first subtype is copied from the parent first subtype.
is the number of bits required to store a value
of the type. This size can be referred to using the
attribute. This value is used to determine how tightly to pack
records or arrays with components of this type, and also affects
the semantics of unchecked conversion (unchecked conversions where
Value_Size values differ generate a warning, and are potentially
The default rules for the value of
Value_Size are as follows:
Value_Sizefor a base subtype is the minimum number of bits required to store all values of the type (including the sign bit only if negative values are possible).
Value_Sizeas the first subtype. This is a consequence of RM 13.1(14) (“if two subtypes statically match, then their subtype-specific aspects are the same”.)
Value_Sizecorresponding to the minimum number of bits required to store all values of the subtype. For dynamic bounds, it is assumed that the value can range down or up to the corresponding bound of the ancestor
The RM defined attribute
Size corresponds to the
Size attribute may be defined for a first-named subtype. This sets
the first-named subtype to the given value, and the
Object_Size of this first-named subtype to the given value padded up
to an appropriate boundary. It is a consequence of the default rules
above that this
Object_Size will apply to all further subtypes. On the
Value_Size is affected only for the first subtype, any
dynamic subtypes obtained from it directly, and any statically matching
Value_Size of any other static subtypes is not affected.
Object_Size may be explicitly set for any subtype using
an attribute definition clause. Note that the use of these attributes
can cause the RM 13.1(14) rule to be violated. If two access types
reference aliased objects whose subtypes have differing
values as a result of explicit attribute definition clauses, then it
is erroneous to convert from one access subtype to the other.
At the implementation level, Esize stores the Object_SIze and the
RM_Size field stores the
Value_Size (and hence the value of the
which, as noted above, is equivalent to
To get a feel for the difference, consider the following examples (note that in each case the base is short_short_integer with a size of 8):
Object_Size Value_Size type x1 is range 0 .. 5; 8 3 type x2 is range 0 .. 5; for x2'size use 12; 12 12 subtype x3 is x2 range 0 .. 3; 12 2 subtype x4 is x2'base range 0 .. 10; 8 4 subtype x5 is x2 range 0 .. dynamic; 12 (7) subtype x6 is x2'base range 0 .. dynamic; 8 (7)
Note: the entries marked (7) are not actually specified by the Ada 95 RM, but it seems in the spirit of the RM rules to allocate the minimum number of bits known to be large enough to hold the given range of values.
So far, so good, but GNAT has to obey the RM rules, so the question is
under what conditions must the RM
Size be used.
The following is a list
of the occasions on which the RM
Size must be used:
Sizefor a type
For types other than discrete and fixed-point types, the
and Value_Size are the same (and equivalent to the RM attribute
Size may be specified for such types.