Ada 2012 Reference Manual

4.7 Qualified Expressions

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

qualified_expression ::=
subtype_mark'(expression) | subtype_mark'aggregate

Name Resolution Rules

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.

Static Semantics

If the operand of a qualified_expression denotes an object, the qualified_expression denotes a constant view of that object. The nominal subtype of a qualified_expression is the subtype denoted by the subtype_mark.

Dynamic Semantics

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 subclause 3.2.4, “Subtype Predicates” that the value satifies the predicates of the subtype.

NOTES

23 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

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