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Ada '83 Language Reference Manual

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4.7. Qualified Expressions


A qualified expression is used to state explicitly the type, and possibly the subtype, of an operand that is the given expression or aggregate.

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

The operand must have the same type as the base type of the type mark. The value of a qualified expression is the value of the operand. The evaluation of a qualified expression evaluates the operand and checks that its value belongs to the subtype denoted by the type mark. The exception CONSTRAINT_ERROR is raised if this check fails.


    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 


Whenever the type of an enumeration literal or aggregate is not known from the context, a qualified expression can be used to state the type explicitly. For example, an overloaded enumeration literal must be qualified in the following cases: when given as a parameter in a subprogram call to an overloaded subprogram that cannot otherwise be identified on the basis of remaining parameter or result types, in a relational expression where both operands are overloaded enumeration literals, or in an array or loop parameter range where both bounds are overloaded enumeration literals. Explicit qualification is also used to specify which one of a set of overloaded parameterless functions is meant, or to constrain a value to a given subtype.

References: aggregate, array, base type, bound of a range, constraint_error exception, context of overload resolution, enumeration literal, expression, function, loop parameter, overloading, raising of exceptions, range, relation, subprogram, subprogram call, subtype, type, type mark.

Rationale references: 3.5 Aggregates, 3.6 Expressions


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