Ada Reference Manual (Ada 2022)
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
Name Resolution Rules
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.
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 3.2.4 that the value satifies the predicates of the subtype.
NOTE 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);
type Code is (Fix, Cla, Dec, Tnz, Sub);
Print (Mask'(Dec)); -- Dec is of type Mask
Print (Code'(Dec)); -- Dec is of type Code
Print (Code'(Dec)); -- Dec is of type Code
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 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
