4.7 Qualified Expressions
Syntax
Name Resolution Rules
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The expected type for the
operand (the
expression
or
aggregate)
is determined by the
subtype_mark.
Furthermore, the operand shall resolve to be either the specified expected
type or a universal type that covers it.
Reason: {
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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.
Static Semantics
Dynamic Semantics
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The
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
3.2.4
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.
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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.
Reason: 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
type_conversion
can be used rather than a
qualified_expression
if subtype conversion is desired.
NOTE {
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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
or
aggregate
is passed to an overloaded subprogram, it can be necessary to qualify
the operand to resolve its type.
Examples
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
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for J
in Code'(Fix) .. Code'(Dec)
loop ...
-- qualification is necessary 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 DecDozen'(1 | 3 | 5 | 7 => 2,
others => 0)
-- see 4.6 Wording Changes from Ada 2005
Inconsistencies With Ada 2012
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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 where this scenario arises 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
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Reworded the resolution rule so that the operand of a
qualified_expression
has an expected type. This eliminates an annoying inconsistency in the
language definition.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
