source: trunk/src/xmlpatterns/expr/qevaluationcache_p.h@ 317

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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_EvaluationCache_H
#define Patternist_EvaluationCache_H

#include "qcachingiterator_p.h"
#include "qcommonsequencetypes_p.h"
#include "qnodebuilder_p.h"
#include "qoperandsiterator_p.h"
#include "qsinglecontainer_p.h"
#include "qvariabledeclaration_p.h"

QT_BEGIN_HEADER

QT_BEGIN_NAMESPACE

namespace QPatternist
{
    /**
     * @short Evaluates to the same result as its operand, but ensures the
     * operand is evaluated once even if this Expression is evaluated several
     * times.
     *
     * EvaluationCache does this in a pipelined way, by delivering items from
     * its cache, which is stored in the DynamicContext. If the cache has less
     * items than what the caller requests, EvaluationCache continues to
     * deliver but this time from the source, which it also populates into the
     * cache.
     *
     * EvaluationCache is used as an optimization in order to avoid running
     * expensive code paths multiple times, but also is sometimes a necessity:
     * for instance, when objects must be unique, such as potentially in the
     * case of node identity.
     *
     * EvaluationCache is in particular used for variables, whose sole purpose
     * is to store it once(at least conceptually) and then use it in multiple
     * places.
     *
     * In some cases an EvaluationCache isn't necessary. For instance, when a
     * variable is only referenced once. In those cases EvaluationCache removes
     * itself as an optimization; implemented in compress().
     *
     * @author Frans Englich <[email protected]>
     * @ingroup Patternist_expressions
     */
    template<bool IsForGlobal>
    class EvaluationCache : public SingleContainer
    {
    public:
        EvaluationCache(const Expression::Ptr &operand,
                        const VariableDeclaration::Ptr &varDecl,
                        const VariableSlotID slot);

        virtual Item evaluateSingleton(const DynamicContext::Ptr &context) const;
        virtual Item::Iterator::Ptr evaluateSequence(const DynamicContext::Ptr &context) const;
        virtual Expression::Ptr compress(const StaticContext::Ptr &context);

        virtual SequenceType::Ptr staticType() const;

        /**
         * The first operand must be exactly one @c xs:string.
         */
        virtual SequenceType::List expectedOperandTypes() const;

        virtual ExpressionVisitorResult::Ptr accept(const ExpressionVisitor::Ptr &visitor) const;
        virtual Properties properties() const;
        virtual Expression::Ptr typeCheck(const StaticContext::Ptr &context,
                                          const SequenceType::Ptr &reqType);
        virtual const SourceLocationReflection *actualReflection() const;

        inline VariableSlotID slot() const
        {
            return m_varSlot;
        }

    private:
        static DynamicContext::Ptr topFocusContext(const DynamicContext::Ptr &context);
        const VariableDeclaration::Ptr  m_declaration;