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+// © 2016 and later: Unicode, Inc. and others.
+// License & terms of use: http://www.unicode.org/copyright.html
+/*
+**********************************************************************
+* Copyright (C) 1999-2010, International Business Machines Corporation and others.
+* All Rights Reserved.
+**********************************************************************
+*   Date        Name        Description
+*   11/17/99    aliu        Creation.
+**********************************************************************
+*/
+#ifndef UNIFILT_H
+#define UNIFILT_H
+
+#include "unicode/utypes.h"
+
+#if U_SHOW_CPLUSPLUS_API
+
+#include "unicode/unifunct.h"
+#include "unicode/unimatch.h"
+
+/**
+ * \file 
+ * \brief C++ API: Unicode Filter
+ */
+
+U_NAMESPACE_BEGIN
+
+/**
+ * U_ETHER is used to represent character values for positions outside
+ * a range.  For example, transliterator uses this to represent
+ * characters outside the range contextStart..contextLimit-1.  This
+ * allows explicit matching by rules and UnicodeSets of text outside a
+ * defined range.
+ * @stable ICU 3.0
+ */
+#define U_ETHER ((char16_t)0xFFFF)
+
+/**
+ *
+ * <code>UnicodeFilter</code> defines a protocol for selecting a
+ * subset of the full range (U+0000 to U+10FFFF) of Unicode characters.
+ * Currently, filters are used in conjunction with classes like
+ * {@link Transliterator} to only process selected characters through a
+ * transformation.
+ *
+ * <p>Note: UnicodeFilter currently stubs out two pure virtual methods
+ * of its base class, UnicodeMatcher.  These methods are toPattern()
+ * and matchesIndexValue().  This is done so that filter classes that
+ * are not actually used as matchers -- specifically, those in the
+ * UnicodeFilterLogic component, and those in tests -- can continue to
+ * work without defining these methods.  As long as a filter is not
+ * used in an RBT during real transliteration, these methods will not
+ * be called.  However, this breaks the UnicodeMatcher base class
+ * protocol, and it is not a correct solution.
+ *
+ * <p>In the future we may revisit the UnicodeMatcher / UnicodeFilter
+ * hierarchy and either redesign it, or simply remove the stubs in
+ * UnicodeFilter and force subclasses to implement the full
+ * UnicodeMatcher protocol.
+ *
+ * @see UnicodeFilterLogic
+ * @stable ICU 2.0
+ */
+class U_COMMON_API UnicodeFilter : public UnicodeFunctor, public UnicodeMatcher {
+
+public:
+    /**
+     * Destructor
+     * @stable ICU 2.0
+     */
+    virtual ~UnicodeFilter();
+
+    /**
+     * Clones this object polymorphically.
+     * The caller owns the result and should delete it when done.
+     * @return clone, or nullptr if an error occurred
+     * @stable ICU 2.4
+     */
+    virtual UnicodeFilter* clone() const override = 0;
+
+    /**
+     * Returns <tt>true</tt> for characters that are in the selected
+     * subset.  In other words, if a character is <b>to be
+     * filtered</b>, then <tt>contains()</tt> returns
+     * <b><tt>false</tt></b>.
+     * @stable ICU 2.0
+     */
+    virtual UBool contains(UChar32 c) const = 0;
+
+    /**
+     * UnicodeFunctor API.  Cast 'this' to a UnicodeMatcher* pointer
+     * and return the pointer.
+     * @stable ICU 2.4
+     */
+    virtual UnicodeMatcher* toMatcher() const override;
+
+    /**
+     * Implement UnicodeMatcher API.
+     * @stable ICU 2.4
+     */
+    virtual UMatchDegree matches(const Replaceable& text,
+                                 int32_t& offset,
+                                 int32_t limit,
+                                 UBool incremental) override;
+
+    /**
+     * UnicodeFunctor API.  Nothing to do.
+     * @stable ICU 2.4
+     */
+    virtual void setData(const TransliterationRuleData*) override;
+
+    /**
+     * ICU "poor man's RTTI", returns a UClassID for this class.
+     *
+     * @stable ICU 2.2
+     */
+    static UClassID U_EXPORT2 getStaticClassID();
+
+protected:
+
+    /*
+     * Since this class has pure virtual functions,
+     * a constructor can't be used.
+     * @stable ICU 2.0
+     */
+/*    UnicodeFilter();*/
+};
+
+/*inline UnicodeFilter::UnicodeFilter() {}*/
+
+U_NAMESPACE_END
+
+#endif /* U_SHOW_CPLUSPLUS_API */
+
+#endif