4.7 Qualified Expressions
1
A
qualified_expression
is used to state explicitly the type, and to verify the subtype, of an
operand that is either an
expression or an
aggregate.
Syntax
2
qualified_expression ::=
subtype_mark'(
expression) |
subtype_mark'
aggregate
Name Resolution Rules
3
The
operand (the
expression
or
aggregate) shall resolve to be of the type
determined by the
subtype_mark, or a universal
type that covers it.
Dynamic Semantics
4
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.
5
22 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 might be necessary to qualify the operand to resolve its type.
Examples
6
Examples of disambiguating
expressions using qualification:
7
type Mask is (Fix, Dec, Exp, Signif);
type Code is (Fix, Cla, Dec, Tnz, Sub);
8
Print (Mask'(Dec)); -- Dec is of type Mask
Print (Code'(Dec)); -- Dec is of type Code
9
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
10
Dozen'(1 | 3 | 5 | 7 => 2,
others => 0)
-- see 4.6
Sponsored by Ada-Europe