4.7 Qualified Expressions
Name Resolution Rules
The expected type for the operand
is shall resolve to
be of the type
determined by the subtype_mark,
or a universal type that covers it
the operand shall resolve to be either the specified expected type or
a universal type that covers it.
The first sentence defines the expected type for
rules that assume one is defined. The second sentence prevents the use
of the various implicit conversions that are usually allowed for expected
types (except the one for numeric literals). The intent is that a qualified
expression is similar to an assertion about the subtype of the operand,
and thus implicit conversions would interfere with that intent.
evaluation of a qualified_expression
evaluates the operand (and if of a universal type, converts it to the
type determined by the subtype_mark
and checks that its value belongs to the subtype denoted by the subtype_mark
The exception Constraint_Error
is raised if this check fails. Furthermore, if
predicate checks are enabled for the subtype denoted by the subtype_mark,
a check is performed as defined in subclause 3.2.4,
“Subtype Predicates” that the
value satifies the predicates of the subtype.
Ramification: This is one of the few
contexts in Ada 95 where implicit subtype conversion is not performed
prior to a constraint check, and hence no “sliding” of array
bounds is provided.
The effect of a failed predicate check is as defined
in 3.2.4; such a check could raise any exception,
not just Constraint_Error or Assertion_Error.
Implicit subtype conversion is
not provided because a qualified_expression
with a constrained target subtype is essentially an assertion about the
subtype of the operand, rather than a request for conversion. An explicit
can be used rather than a qualified_expression
if subtype conversion is desired.
24 When a given context does not uniquely
identify an expected type, a qualified_expression
can be used to do so. In particular, if an overloaded name
is passed to an overloaded subprogram, it might be necessary to qualify
the operand to resolve its type.
Examples of disambiguating
expressions using qualification:
type Mask is (Fix, Dec, Exp, Signif);
type Code is (Fix, Cla, Dec, Tnz, Sub);
Print (Mask'(Dec)); -- Dec is of type Mask
Print (Code'(Dec)); -- Dec is of type Code
for J in Code'(Fix) .. Code'(Dec) loop ... -- qualification needed for either Fix or Dec
for J in Code range Fix .. Dec loop ... -- qualification unnecessary
for J in Code'(Fix) .. Dec loop ... -- qualification unnecessary for Dec
Dozen'(1 | 3 | 5 | 7 => 2, others
=> 0) -- see 4.6
Wording Changes from Ada 2005
Inconsistencies With Ada 2012
Corrigendum: A qualified_expression
now performs a predicate check for the named subtype (if it is enabled).
Original Ada 2012 did not include that check (an omission). While this
is formally inconsistent (an exception could be raised when none would
be raised by original Ada 2012), cases when this could be the case are
likely to be rare (the qualified expression would have to have a stricter
subtype than the following usage) and the check is more likely to detect
bugs than be unexpected.
Wording Changes from Ada 2012
Reworded the resolution rule so that the operand
of a qualified_expression
has an expected type. This eliminates an annoying inconsistency in the
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe