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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).
The type'Object_Size attribute
has been added to GNAT to allow the
default object size of a type to be easily determined. For example,
Natural'Object_Size is 32, and
Rec'Object_Size (for the record type in the above example) will be
64. Note also that, unlike the situation with the
Size attribute as defined in the Ada RM, the
Object_Size attribute can be specified individually
for different subtypes. For example:
type R is new Integer;
subtype R1 is R range 1 .. 10;
subtype R2 is R range 1 .. 10;
for R2'Object_Size use 8;
In this example, R'Object_Size and R1'Object_Size are both
32 since the default object size for a subtype is the same as the object size
for the parent subtype. This means that objects of type R
or R1 will
by default be 32 bits (four bytes). But objects of type
R2 will be only
8 bits (one byte), since R2'Object_Size has been set to 8.