Ada Programming/Attributes/'Descriptor Size

Description

Nonstatic 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 (Short_Short_Integer range <>) of Positive;
Put_Line ("Descriptor size = " & Unconstr_Array'Descriptor_Size'Img);

The attribute takes into account any padding due to the alignment of the component type. In the example above, the descriptor contains two values of type Short_Short_Integer representing the low and high bound. But, since Positive has an alignment of 4, the size of the descriptor is 2 * Short_Short_Integer'Size rounded up to the next multiple of 32, which yields a size of 32 bits, i.e. including 16 bits of padding.