source: trunk/src/xmlpatterns/expr/qfirstitempredicate_p.h@ 846

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//
//  W A R N I N G
//  -------------
//
// This file is not part of the Qt API.  It exists purely as an
// implementation detail.  This header file may change from version to
// version without notice, or even be removed.
//
// We mean it.

#ifndef Patternist_FirstItemPredicate_H
#define Patternist_FirstItemPredicate_H

#include "qsinglecontainer_p.h"

QT_BEGIN_HEADER

QT_BEGIN_NAMESPACE

namespace QPatternist
{
    /**
     * @short A predicate that always selects the first item from its sequence.
     *
     * FirstItemPredicate corresponds exactly to the predicate
     * in the expression <tt>input[1]</tt>.
     *
     * @author Frans Englich <[email protected]>
     * @ingroup Patternist_expressions
     */
    class FirstItemPredicate : public SingleContainer
    {
    public:
        /**
         * Creates a FirstItemPredicate that filters @p source.
         */
        FirstItemPredicate(const Expression::Ptr &source);

        /**
         * @returns the first item, if any, from evaluating the source expression.
         */
        virtual Item evaluateSingleton(const DynamicContext::Ptr &) const;

        /**
         * @returns a list containing one CommonSequenceTypes::ZeroOrMoreItems instance.
         */
        virtual SequenceType::List expectedOperandTypes() const;

        /**
         * @returns a SequenceType where the item type is the same as the source expression
         * and where the cardinality is either Cardinality::zeroOrOne() or Cardinality::exactlyOne(),
         * depending on the source expression.
         */
        virtual SequenceType::Ptr staticType() const;
        virtual ExpressionVisitorResult::Ptr accept(const ExpressionVisitor::Ptr &visitor) const;

        /**
         * Rewrites <tt>expression[1][1]</tt> into <tt>expression[1]</tt>.
         */
        virtual Expression::Ptr compress(const StaticContext::Ptr &context);

        /**
         * @returns always IDFirstItemPredicate.
         */
        virtual ID id() const;
    };
}

QT_END_NAMESPACE

QT_END_HEADER

#endif