'Valid_Scalars attribute is intended to make it easier to
check the validity of scalar subcomponents of composite objects. It
is defined for any prefix
X that denotes an object.
The value of this attribute is of the predefined type Boolean.
X'Valid_Scalars yields True if and only if evaluation of
P'Valid yields True for every scalar part P of X or if X has
no scalar parts. It is not specified in what order the scalar parts
are checked, nor whether any more are checked after any one of them
is determined to be invalid. If the prefix
X is of a class-wide
T is the associated specific type),
or if the prefix
X is of a specific tagged type
only the scalar parts of components of
T are traversed; in other
words, components of extensions of
T are not traversed even if
T'Class (X)'Tag /= T'Tag . The compiler will issue a warning if it can
be determined at compile time that the prefix of the attribute has no
scalar parts (e.g., if the prefix is of an access type, an interface type,
an undiscriminated task type, or an undiscriminated protected type).
For scalar types,
Valid_Scalars is equivalent to
Valid. The use
of this attribute is not permitted for
Unchecked_Union types for which
in general it is not possible to determine the values of the discriminants.
Valid_Scalars can generate a lot of code, especially in the case
of a large variant record. If the attribute is called in many places in the
same program applied to objects of the same type, it can reduce program size
to write a function with a single use of the attribute, and then call that
function from multiple places.