source: vbox/trunk/include/VBox/settings.h@ 17911

/** @file
 * Settings File Manipulation API.
 */

/*
 * Copyright (C) 2007 Sun Microsystems, Inc.
 *
 * This file is part of VirtualBox Open Source Edition (OSE), as
 * available from http://www.virtualbox.org. This file is free software;
 * you can redistribute it and/or modify it under the terms of the GNU
 * General Public License (GPL) as published by the Free Software
 * Foundation, in version 2 as it comes in the "COPYING" file of the
 * VirtualBox OSE distribution. VirtualBox OSE is distributed in the
 * hope that it will be useful, but WITHOUT ANY WARRANTY of any kind.
 *
 * The contents of this file may alternatively be used under the terms
 * of the Common Development and Distribution License Version 1.0
 * (CDDL) only, as it comes in the "COPYING.CDDL" file of the
 * VirtualBox OSE distribution, in which case the provisions of the
 * CDDL are applicable instead of those of the GPL.
 *
 * You may elect to license modified versions of this file under the
 * terms and conditions of either the GPL or the CDDL or both.
 *
 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa
 * Clara, CA 95054 USA or visit http://www.sun.com if you need
 * additional information or have any questions.
 */

#ifndef ___VBox_settings_h
#define ___VBox_settings_h

#include <iprt/cdefs.h>
#include <iprt/cpputils.h>
#include <iprt/string.h>

#include <list>
#include <memory>
#include <limits>

#include <iprt/time.h>

#include <VBox/com/Guid.h>

#include <VBox/xml.h>

// #include <stdarg.h>

/* these conflict with numeric_digits<>::min and max */
#undef min
#undef max


/** @defgroup   grp_settings    Settings File Manipulation API
 * @{
 *
 * The Settings File Manipulation API allows to maintain a configuration file
 * that contains "name-value" pairs grouped under named keys which are in turn
 * organized in a hierarchical tree-like structure:
 *
 * @code
 * <RootKey>
 *  <Key1 attr1="value" attr2=""/>
 *  <Key2 attr1="value">
 *   <SubKey1>SubKey1_Value</SubKey1>
 *   <SubKey2 attr1="value">SubKey2_Value</SubKey2>
 *   Key2_Value
 *  </Key2>
 * </RootKey>
 * @endcode
 *
 * All strings this API manipulates with are zero-terminated arrays of @c char
 * in UTF-8 encoding. Strings returned by the API are owned by the API unless
 * explicitly stated otherwise. Strings passed to the API are accessed by the
 * API only during the given API call unless explicitly stated otherwise. If
 * necessary, the API will make a copy of the supplied string.
 *
 * Error reporting is perfomed using C++ exceptions. All exceptions thrown by
 * this API are derived from settings::Error. This doesn't cover exceptions
 * that may be thrown by third-party library calls made by this API.
 *
 * All public classes represented by this API that support copy operations
 * (i.e. may be created or assigned from other instsances of the same class),
 * such as Key and Value classes, implement shallow copies and use this mode by
 * default. It means two things:
 *
 * 1. Instances of these classes can be freely copied around and used as return
 *    values. All copies will share the same internal data block (using the
 *    reference counting technique) so that the copy operation is cheap, both
 *    in terms of memory and speed.
 *
 * 2. Since copied instances share the same data, an attempt to change data in
 *    the original will be reflected in all existing copies.
 *
 * Making deep copies or detaching the existing shallow copy from its original
 * is not yet supported.
 *
 * Note that the Settings File API is not thread-safe. It means that if you
 * want to use the same instance of a class from the settings namespace on more
 * than one thread at a time, you will have to provide necessary access
 * serialization yourself.
 *
 * Due to some (not propely studied) libxml2 limitations, the Settings File
 * API is not thread-safe. Therefore, the API caller must provide
 * serialization for threads using this API simultaneously. Note though that
 * if the libxml2 library is (even imlicitly) used on some other thread which
 * doesn't use this API (e.g. third-party code), it may lead to resource
 * conflicts (followed by crashes, memory corruption etc.). A proper solution
 * for these conflicts is to be found.
 *
 * In order to load a settings file the program creates a TreeBackend instance
 * using one of the specific backends (e.g. XmlTreeBackend) and then passes an
 * Input stream object (e.g. File or MemoryBuf) to the TreeBackend::read()
 * method to parse the stream and build the settings tree. On success, the
 * program uses the TreeBackend::rootKey() method to access the root key of
 * the settings tree. The root key provides access to the whole tree of
 * settings through the methods of the Key class which allow to read, change
 * and create new key values. Below is an example that uses the XML backend to
 * load the settings tree, then modifies it and then saves the modifications.
 *
 * @code
    using namespace settings;

    try
    {
        File file (File::ReadWrite, "myfile.xml");
        XmlTreeBackend tree;

        // load the tree, parse it and validate using the XML schema
        tree.read (aFile, "myfile.xsd", XmlTreeBackend::Read_AddDefaults);

        // get the root key
        Key root = tree.key();
        printf ("root=%s\n", root.name());

        // enumerate all child keys of the root key named Foo
        Key::list children = root.keys ("Foo");
        for (Key::list::const_iterator it = children.begin();
             it != children.end();
             ++ it)
        {
            // get the "level" attribute
            int level = (*it).value <int> ("level");
            if (level > 5)
            {
                // if so, create a "Bar" key if it doesn't exist yet
                Key bar = (*it).createKey ("Bar");
                // set the "date" attribute
                RTTIMESPEC now;
                RTTimeNow (&now);
                bar.setValue <RTTIMESPEC> ("date", now);
            }
            else if (level < 2)
            {
                // if its below 2, delete the whole "Foo" key
                (*it).zap();
            }
        }

        // save the tree on success (the second try is to distinguish between
        // stream load and save errors)
        try
        {
            aTree.write (aFile);
        }
        catch (const EIPRTFailure &err)
        {
            // this is an expected exception that may happen in case of stream
            // read or write errors
            printf ("Could not save the settings file '%s' (%Rrc)");
                    file.uri(), err.rc());

            return FAILURE;
        }

        return SUCCESS;
    }
    catch (const EIPRTFailure &err)
    {
        // this is an expected exception that may happen in case of stream
        // read or write errors
        printf ("Could not load the settings file '%s' (%Rrc)");
                file.uri(), err.rc());
    }
    catch (const XmlTreeBackend::Error &err)
    {
        // this is an XmlTreeBackend specific exception that may
        // happen in case of XML parse or validation errors
        printf ("Could not load the settings file '%s'.\n%s"),
                file.uri(), err.what() ? err.what() : "Unknown error");
    }
    catch (const std::exception &err)
    {
        // the rest is unexpected (e.g. should not happen unless you
        // specifically wish so for some reason and therefore allow for a
        // situation that may throw one of these from within the try block
        // above)
        AssertMsgFailed ("Unexpected exception '%s' (%s)\n",
                         typeid (err).name(), err.what());
    }
    catch (...)
    {
        // this is even more unexpected, and no any useful info here
        AssertMsgFailed ("Unexpected exception\n");
    }

    return FAILURE;
 * @endcode
 *
 * Note that you can get a raw (string) value of the attribute using the
 * Key::stringValue() method but often it's simpler and better to use the
 * templated Key::value<>() method that can convert the string to a value of
 * the given type for you (and throw exceptions when the converison is not
 * possible). Similarly, the Key::setStringValue() method is used to set a raw
 * string value and there is a templated Key::setValue<>() method to set a
 * typed value which will implicitly convert it to a string.
 *
 * Currently, types supported by Key::value<>() and Key::setValue<>() include
 * all C and IPRT integer types, bool and RTTIMESPEC (represented as isoDate
 * in XML). You can always add support for your own types by creating
 * additional specializations of the FromString<>() and ToString<>() templates
 * in the settings namespace (see the real examples in this header).
 *
 * See individual funciton, class and method descriptions to get more details
 * on the Settings File Manipulation API.
 */

/*
 * Shut up MSVC complaining that auto_ptr[_ref] template instantiations (as a
 * result of private data member declarations of some classes below) need to
 * be exported too to in order to be accessible by clients.
 *
 * The alternative is to instantiate a template before the data member
 * declaration with the VBOXXML_CLASS prefix, but the standard disables
 * explicit instantiations in a foreign namespace. In other words, a declaration
 * like:
 *
 *   template class VBOXXML_CLASS std::auto_ptr <Data>;
 *
 * right before the member declaration makes MSVC happy too, but this is not a
 * valid C++ construct (and G++ spits it out). So, for now we just disable the
 * warning and will come back to this problem one day later.
 *
 * We also disable another warning (4275) saying that a DLL-exported class
 * inherits form a non-DLL-exported one (e.g. settings::ENoMemory ->
 * std::bad_alloc). I can't get how it can harm yet.
 */
#if defined(_MSC_VER)
#pragma warning (disable:4251)
#pragma warning (disable:4275)
#endif

/* Forwards */
typedef struct _xmlParserInput xmlParserInput;
typedef xmlParserInput *xmlParserInputPtr;
typedef struct _xmlParserCtxt xmlParserCtxt;
typedef xmlParserCtxt *xmlParserCtxtPtr;
typedef struct _xmlError xmlError;
typedef xmlError *xmlErrorPtr;


/**
 * Settings File Manipulation API namespace.
 */
namespace settings
{

// Exceptions (on top of vboxxml exceptions)
//////////////////////////////////////////////////////////////////////////////

class VBOXXML_CLASS ENoKey : public xml::LogicError
{
public:

    ENoKey (const char *aMsg = NULL) : xml::LogicError (aMsg) {}
};

class VBOXXML_CLASS ENoValue : public xml::LogicError
{
public:

    ENoValue (const char *aMsg = NULL) : xml::LogicError (aMsg) {}
};

class VBOXXML_CLASS ENoConversion : public xml::RuntimeError
{
public:

    ENoConversion (const char *aMsg = NULL) : RuntimeError (aMsg) {}
};


// Helpers
//////////////////////////////////////////////////////////////////////////////

// string -> type conversions
//////////////////////////////////////////////////////////////////////////////

/** @internal
 * Helper for the FromString() template, doesn't need to be called directly.
 */
DECLEXPORT (uint64_t) FromStringInteger (const char *aValue, bool aSigned,
                                         int aBits, uint64_t aMin, uint64_t aMax);

/**
 * Generic template function to perform a conversion of an UTF-8 string to an
 * arbitrary value of type @a T.
 *
 * This generic template is implenented only for 8-, 16-, 32- and 64- bit
 * signed and unsigned integers where it uses RTStrTo[U]Int64() to perform the
 * conversion. For all other types it throws an ENotImplemented
 * exception. Individual template specializations for known types should do
 * the conversion job.
 *
 * If the conversion is not possible (for example the string format is wrong
 * or meaningless for the given type), this template will throw an
 * ENoConversion exception. All specializations must do the same.
 *
 * If the @a aValue argument is NULL, this method will throw an ENoValue
 * exception. All specializations must do the same.
 *
 * @param aValue Value to convert.
 *
 * @return Result of conversion.
 */
template <typename T>
T FromString (const char *aValue)
{
    if (std::numeric_limits <T>::is_integer)
    {
        bool sign = std::numeric_limits <T>::is_signed;
        int bits = std::numeric_limits <T>::digits + (sign ? 1 : 0);

        return (T) FromStringInteger (aValue, sign, bits,
                                      (uint64_t) std::numeric_limits <T>::min(),
                                      (uint64_t) std::numeric_limits <T>::max());
    }

    throw xml::ENotImplemented (RT_SRC_POS);
}

/**
 * Specialization of FromString for bool.
 *
 * Converts "true", "yes", "on" to true and "false", "no", "off" to false.
 */
template<> DECLEXPORT (bool) FromString <bool> (const char *aValue);

/**
 * Specialization of FromString for RTTIMESPEC.
 *
 * Converts the date in ISO format (<YYYY>-<MM>-<DD>T<hh>:<mm>:<ss>[timezone])
 * to a RTTIMESPEC value. Currently, the timezone must always be Z (UTC).
 */
template<> DECLEXPORT (RTTIMESPEC) FromString <RTTIMESPEC> (const char *aValue);

/**
 * Converts a string of hex digits to memory bytes.
 *
 * @param aValue String to convert.
 * @param aLen   Where to store the length of the returned memory
 *               block (may be NULL).
 *
 * @return Result of conversion (a block of @a aLen bytes).
 */
DECLEXPORT (stdx::char_auto_ptr) FromString (const char *aValue, size_t *aLen);

// type -> string conversions
//////////////////////////////////////////////////////////////////////////////

/** @internal
 * Helper for the ToString() template, doesn't need to be called directly.
 */
DECLEXPORT (stdx::char_auto_ptr)
ToStringInteger (uint64_t aValue, unsigned int aBase,
                 bool aSigned, int aBits);

/**
 * Generic template function to perform a conversion of an arbitrary value to
 * an UTF-8 string.
 *
 * This generic template is implemented only for 8-, 16-, 32- and 64- bit
 * signed and unsigned integers where it uses RTStrFormatNumber() to perform
 * the conversion. For all other types it throws an ENotImplemented
 * exception. Individual template specializations for known types should do
 * the conversion job. If the conversion is not possible (for example the
 * given value doesn't have a string representation), the relevant
 * specialization should throw an ENoConversion exception.
 *
 * If the @a aValue argument's value would convert to a NULL string, this
 * method will throw an ENoValue exception. All specializations must do the
 * same.
 *
 * @param aValue Value to convert.
 * @param aExtra Extra flags to define additional formatting. In case of
 *               integer types, it's the base used for string representation.
 *
 * @return Result of conversion.
 */
template <typename T>
stdx::char_auto_ptr ToString (const T &aValue, unsigned int aExtra = 0)
{
    if (std::numeric_limits <T>::is_integer)
    {
        bool sign = std::numeric_limits <T>::is_signed;
        int bits = std::numeric_limits <T>::digits + (sign ? 1 : 0);

        return ToStringInteger (aValue, aExtra, sign, bits);
    }

    throw xml::ENotImplemented (RT_SRC_POS);
}

/**
 * Specialization of ToString for bool.
 *
 * Converts true to "true" and false to "false". @a aExtra is not used.
 */
template<> DECLEXPORT (stdx::char_auto_ptr)
ToString <bool> (const bool &aValue, unsigned int aExtra);

/**
 * Specialization of ToString for RTTIMESPEC.
 *
 * Converts the RTTIMESPEC value to the date string in ISO format
 * (<YYYY>-<MM>-<DD>T<hh>:<mm>:<ss>[timezone]). Currently, the timezone will
 * always be Z (UTC).
 *
 * @a aExtra is not used.
 */
template<> DECLEXPORT (stdx::char_auto_ptr)
ToString <RTTIMESPEC> (const RTTIMESPEC &aValue, unsigned int aExtra);

/**
 * Converts memory bytes to a null-terminated string of hex values.
 *
 * @param aData Pointer to the memory block.
 * @param aLen  Length of the memory block.
 *
 * @return Result of conversion.
 */
DECLEXPORT (stdx::char_auto_ptr) ToString (const void *aData, size_t aLen);

#if defined VBOX_MAIN_SETTINGS_ADDONS

/// @todo once string data in Bstr and Utf8Str is auto_ref_ptr, enable the
/// code below

#if 0

/** Specialization of FromString for Bstr. */
template<> com::Bstr FromString <com::Bstr> (const char *aValue);

#endif

/** Specialization of ToString for Bstr. */
template<> stdx::char_auto_ptr
ToString <com::Bstr> (const com::Bstr &aValue, unsigned int aExtra);

/** Specialization of FromString for Guid. */
template<> com::Guid FromString <com::Guid> (const char *aValue);

/** Specialization of ToString for Guid. */
template<> stdx::char_auto_ptr
ToString <com::Guid> (const com::Guid &aValue, unsigned int aExtra);

#endif // VBOX_MAIN_SETTINGS_ADDONS

// the rest
//////////////////////////////////////////////////////////////////////////////

/**
 * The Key class represents a settings key.
 *
 * Every settings key has a name and zero or more uniquely named values
 * (attributes). There is a special attribute with a NULL name that is called
 * a key value.
 *
 * Besides values, settings keys may contain other settings keys. This way,
 * settings keys form a tree-like (or a directory-like) hierarchy of keys. Key
 * names do not need to be unique even if they belong to the same parent key
 * which allows to have an array of keys of the same name.
 *
 * @note Key and Value objects returned by methods of the Key and TreeBackend
 * classes are owned by the given TreeBackend instance and may refer to data
 * that becomes invalid when this TreeBackend instance is destroyed.
 */
class VBOXXML_CLASS Key
{
public:

    typedef std::list <Key> List;

    /**
     * Key backend interface used to perform actual key operations.
     *
     * This interface is implemented by backends that provide specific ways of
     * storing settings keys.
     */
    class VBOXXML_CLASS Backend : public stdx::auto_ref
    {
    public:

        /** Performs the Key::name() function. */
        virtual const char *name() const = 0;

        /** Performs the Key::setName() function. */
        virtual void setName (const char *aName) = 0;

        /** Performs the Key::stringValue() function. */
        virtual const char *value (const char *aName) const = 0;

        /** Performs the Key::setStringValue() function. */
        virtual void setValue (const char *aName, const char *aValue) = 0;

        /** Performs the Key::keys() function. */
        virtual List keys (const char *aName = NULL) const = 0;

        /** Performs the Key::findKey() function. */
        virtual Key findKey (const char *aName) const = 0;

        /** Performs the Key::appendKey() function. */
        virtual Key appendKey (const char *aName) = 0;

        /** Performs the Key::zap() function. */
        virtual void zap() = 0;

        /**
         * Returns an opaque value that uniquely represents the position of
         * this key on the tree which is used to compare two keys. Two or more
         * keys may return the same value only if they actually represent the
         * same key (i.e. they have the same list of parents and children).
         */
        virtual void *position() const = 0;
    };

    /**
     * Creates a new key object. If @a aBackend is @c NULL then a null key is
     * created.
     *
     * Regular API users should never need to call this method with something
     * other than NULL argument (which is the default).
     *
     * @param aBackend      Key backend to use.
     */
    Key (Backend *aBackend = NULL) : m (aBackend) {}

    /**
     * Returns @c true if this key is null.
     */
    bool isNull() const { return m.is_null(); }

    /**
     * Makes this object a null key.
     *
     * Note that as opposed to #zap(), this methid does not delete the key from
     * the list of children of its parent key.
     */
    void setNull() { m = NULL; }

    /**
     * Returns the name of this key.
     * Returns NULL if this object a null (uninitialized) key.
     */
    const char *name() const { return m.is_null() ? NULL : m->name(); }

    /**
     * Sets the name of this key.
     *
     * @param aName New key name.
     */
    void setName (const char *aName) { if (!m.is_null()) m->setName (aName); }

    /**
     * Returns the value of the attribute with the given name as an UTF-8
     * string. Returns @c NULL if there is no attribute with the given name.
     *
     * @param aName     Name of the attribute. NULL may be used to
     *                  get the key value.
     */
    const char *stringValue (const char *aName) const
    {
        return m.is_null() ? NULL : m->value (aName);
    }

    /**
     * Sets the value of the attribute with the given name from an UTF-8
     * string. This method will do a copy of the supplied @a aValue string.
     *
     * @param aName     Name of the attribute. NULL may be used to
     *                  set the key value.
     * @param aValue    New value of the attribute. NULL may be used to
     *                  delete the value instead of setting it.
     */
    void setStringValue (const char *aName, const char *aValue)
    {
        if (!m.is_null()) m->setValue (aName, aValue);
    }

    /**
     * Returns the value of the attribute with the given name as an object of
     * type @a T. Throws ENoValue if there is no attribute with the given
     * name.
     *
     * This function calls #stringValue() to get the string representation of
     * the attribute and then calls the FromString() template to convert this
     * string to a value of the given type.
     *
     * @param aName     Name of the attribute. NULL may be used to
     *                  get the key value.
     */
    template <typename T>
    T value (const char *aName) const
    {
        try
        {
            return FromString <T> (stringValue (aName));
        }
        catch (const ENoValue &)
        {
            throw ENoValue(com::Utf8StrFmt("No such attribute '%s'", aName));
        }
    }

    /**
     * Returns the value of the attribute with the given name as an object of
     * type @a T. Returns the given default value if there is no attribute
     * with the given name.
     *
     * This function calls #stringValue() to get the string representation of
     * the attribute and then calls the FromString() template to convert this
     * string to a value of the given type.
     *
     * @param aName     Name of the attribute. NULL may be used to
     *                  get the key value.
     * @param aDefault  Default value to return for the missing attribute.
     */
    template <typename T>
    T valueOr (const char *aName, const T &aDefault) const
    {
        try
        {
            return FromString <T> (stringValue (aName));
        }
        catch (const ENoValue &)
        {
            return aDefault;
        }
    }

    /**
     * Sets the value of the attribute with the given name from an object of
     * type @a T. This method will do a copy of data represented by @a aValue
     * when necessary.
     *
     * This function converts the given value to a string using the ToString()
     * template and then calls #setStringValue().
     *
     * @param aName     Name of the attribute. NULL may be used to
     *                  set the key value.
     * @param aValue    New value of the attribute.
     * @param aExtra    Extra field used by some types to specify additional
     *                  details for storing the value as a string (such as the
     *                  base for decimal numbers).
     */
    template <typename T>
    void setValue (const char *aName, const T &aValue, unsigned int aExtra = 0)
    {
        try
        {
            stdx::char_auto_ptr value = ToString (aValue, aExtra);
            setStringValue (aName, value.get());
        }
        catch (const ENoValue &)
        {
            throw ENoValue(com::Utf8StrFmt("No value for attribute '%s'", aName));
        }
    }

    /**
     * Sets the value of the attribute with the given name from an object of
     * type @a T. If the value of the @a aValue object equals to the value of
     * the given @a aDefault object, then the attribute with the given name
     * will be deleted instead of setting its value to @a aValue.
     *
     * This function converts the given value to a string using the ToString()
     * template and then calls #setStringValue().
     *
     * @param aName     Name of the attribute. NULL may be used to
     *                  set the key value.
     * @param aValue    New value of the attribute.
     * @param aDefault  Default value to compare @a aValue to.
     * @param aExtra    Extra field used by some types to specify additional
     *                  details for storing the value as a string (such as the
     *                  base for decimal numbers).
     */
    template <typename T>
    void setValueOr (const char *aName, const T &aValue, const T &aDefault,
                     unsigned int aExtra = 0)
    {
        if (aValue == aDefault)
            zapValue (aName);
        else
            setValue <T> (aName, aValue, aExtra);
    }

    /**
     * Deletes the value of the attribute with the given name.
     * Shortcut to <tt>setStringValue(aName, NULL)</tt>.
     */
    void zapValue (const char *aName) { setStringValue (aName, NULL); }

    /**
     * Returns the key value.
     * Shortcut to <tt>stringValue (NULL)</tt>.
     */
    const char *keyStringValue() const { return stringValue (NULL); }

    /**
     * Sets the key value.
     * Shortcut to <tt>setStringValue (NULL, aValue)</tt>.
     */
    void setKeyStringValue (const char *aValue) { setStringValue (NULL, aValue); }

    /**
     * Returns the key value.
     * Shortcut to <tt>value (NULL)</tt>.
     */
    template <typename T>
    T keyValue() const { return value <T> (NULL); }

    /**
     * Returns the key value or the given default if the key value is NULL.
     * Shortcut to <tt>value (NULL)</tt>.
     */
    template <typename T>
    T keyValueOr (const T &aDefault) const { return valueOr <T> (NULL, aDefault); }

    /**
     * Sets the key value.
     * Shortcut to <tt>setValue (NULL, aValue, aExtra)</tt>.
     */
    template <typename T>
    void setKeyValue (const T &aValue, unsigned int aExtra = 0)
    {
        setValue <T> (NULL, aValue, aExtra);
    }

    /**
     * Sets the key value.
     * Shortcut to <tt>setValueOr (NULL, aValue, aDefault)</tt>.
     */
    template <typename T>
    void setKeyValueOr (const T &aValue, const T &aDefault,
                        unsigned int aExtra = 0)
    {
        setValueOr <T> (NULL, aValue, aDefault, aExtra);
    }

    /**
     * Deletes the key value.
     * Shortcut to <tt>zapValue (NULL)</tt>.
     */
    void zapKeyValue () { zapValue (NULL); }

    /**
     * Returns a list of all child keys named @a aName.
     *
     * If @a aname is @c NULL, returns a list of all child keys.
     *
     * @param aName     Child key name to list.
     */
    List keys (const char *aName = NULL) const
    {
        return m.is_null() ? List() : m->keys (aName);
    };

    /**
     * Returns the first child key with the given name.
     *
     * Throws ENoKey if no child key with the given name exists.
     *
     * @param aName     Child key name.
     */
    Key key (const char *aName) const
    {
        Key key = findKey (aName);
        if (key.isNull())
        {
            throw ENoKey(com::Utf8StrFmt("No such key '%s'", aName));
        }
        return key;
    }

    /**
     * Returns the first child key with the given name.
     *
     * As opposed to #key(), this method will not throw an exception if no
     * child key with the given name exists, but return a null key instead.
     *
     * @param aName     Child key name.
     */
    Key findKey (const char *aName) const
    {
        return m.is_null() ? Key() : m->findKey (aName);
    }

    /**
     * Creates a key with the given name as a child of this key and returns it
     * to the caller.
     *
     * If one or more child keys with the given name already exist, no new key
     * is created but the first matching child key is returned.
     *
     * @param aName Name of the child key to create.
     */
    Key createKey (const char *aName)
    {
        Key key = findKey (aName);
        if (key.isNull())
            key = appendKey (aName);
        return key;
    }

    /**
     * Appends a key with the given name to the list of child keys of this key
     * and returns the appended key to the caller.
     *
     * @param aName Name of the child key to create.
     */
    Key appendKey (const char *aName)
    {
        return m.is_null() ? Key() : m->appendKey (aName);
    }

    /**
     * Deletes this key.
     *
     * The deleted key is removed from the list of child keys of its parent
     * key and becomes a null object.
     */
    void zap()
    {
        if (!m.is_null())
        {
            m->zap();
            setNull();
        }
    }

    /**
     * Compares this object with the given object and returns @c true if both
     * represent the same key on the settings tree or if both are null
     * objects.
     *
     * @param that Object to compare this object with.
     */
    bool operator== (const Key &that) const
    {
        return m == that.m ||
               (!m.is_null() && !that.m.is_null() &&
                m->position() == that.m->position());
    }

    /**
     * Counterpart to operator==().
     */
    bool operator!= (const Key &that) const { return !operator== (that); }

private:

    stdx::auto_ref_ptr <Backend> m;

    friend class TreeBackend;
};

/**
 * The TreeBackend class represents a storage backend used to read a settings
 * tree from and write it to a stream.
 *
 * @note All Key objects returned by any of the TreeBackend methods (and by
 * methods of returned Key objects) are owned by the given TreeBackend
 * instance. When this instance is destroyed, all Key objects become invalid
 * and an attempt to access Key data will cause the program crash.
 */
class VBOXXML_CLASS TreeBackend
{
public:

    /**
     * Reads and parses the given input stream.
     *
     * On success, the previous settings tree owned by this backend (if any)
     * is deleted.
     *
     * The optional schema URI argument determines the name of the schema to
     * use for input validation. If the schema URI is NULL then the validation
     * is not performed. Note that you may set a custom input resolver if you
     * want to provide the input stream for the schema file (and for other
     * external entities) instead of letting the backend to read the specified
     * URI directly.
     *
     * This method will set the read/write position to the beginning of the
     * given stream before reading. After the stream has been successfully
     * parsed, the position will be set back to the beginning.
     *
     * @param aInput        Input stream.
     * @param aSchema       Schema URI to use for input stream validation.
     * @param aFlags        Optional bit flags.
     */
    void read (xml::Input &aInput, const char *aSchema = NULL, int aFlags = 0)
    {
        aInput.setPos (0);
        rawRead (aInput, aSchema, aFlags);
        aInput.setPos (0);
    }

    /**
     * Reads and parses the given input stream in a raw fashion.
     *
     * This method doesn't set the stream position to the beginnign before and
     * after reading but instead leaves it as is in both cases. It's the
     * caller's responsibility to maintain the correct position.
     *
     * @see read()
     */
    virtual void rawRead (xml::Input &aInput, const char *aSchema = NULL,
                          int aFlags = 0) = 0;

    /**
     * Writes the current settings tree to the given output stream.
     *
     * This method will set the read/write position to the beginning of the
     * given stream before writing. After the settings have been successfully
     * written to the stream, the stream will be truncated at the position
     * following the last byte written by this method anc ghd position will be
     * set back to the beginning.
     *
     * @param aOutput       Output stream.
     */
    void write (xml::Output &aOutput)
    {
        aOutput.setPos (0);
        rawWrite (aOutput);
        aOutput.truncate();
        aOutput.setPos (0);
    }

    /**
     * Writes the current settings tree to the given output stream in a raw
     * fashion.
     *
     * This method doesn't set the stream position to the beginnign before and
     * after reading and doesn't truncate the stream, but instead leaves it as
     * is in both cases. It's the caller's responsibility to maintain the
     * correct position and perform truncation.
     *
     * @see write()
     */
    virtual void rawWrite (xml::Output &aOutput) = 0;

    /**
     * Deletes the current settings tree.
     */
    virtual void reset() = 0;

    /**
     * Returns the root settings key.
     */
    virtual Key &rootKey() const = 0;

protected:

    static Key::Backend *GetKeyBackend (const Key &aKey) { return aKey.m.raw(); }
};

class XmlKeyBackend;

/**
 * The XmlTreeBackend class uses XML markup to store settings trees.
 *
 * @note libxml2 and libxslt libraries used by the XmlTreeBackend are not
 * fully reentrant. To "fix" this, the XmlTreeBackend backend serializes access
 * to such non-reentrant parts using a global mutex so that only one thread can
 * use non-reentrant code at a time. Currently, this relates to the #rawRead()
 * method (and to #read() as a consequence). This means that only one thread can
 * parse an XML stream at a time; other threads trying to parse same or
 * different streams using different XmlTreeBackend and Input instances
 * will have to wait.
 *
 * Keep in mind that the above reentrancy fix does not imply thread-safety: it
 * is still the caller's responsibility to provide serialization if the same
 * XmlTreeBackend instnace (as well as instances of other classes from the
 * settings namespace) needs to be used by more than one thread.
 */
class VBOXXML_CLASS XmlTreeBackend : public TreeBackend
{
public:

    /** Flags for TreeBackend::read(). */
    enum
    {
        /**
         * Sbstitute default values for missing attributes that have defaults
         * in the XML schema. Otherwise, stringValue() will return NULL for
         * such attributes.
         */
        Read_AddDefaults = RT_BIT (0),
    };

    /**
     * The EConversionCycle class represents a conversion cycle detected by the
     * AutoConverter::needsConversion() implementation.
     */
    class VBOXXML_CLASS EConversionCycle : public xml::RuntimeError
    {
    public:

        EConversionCycle (const char *aMsg = NULL) : RuntimeError (aMsg) {}
    };

    /**
     * The InputResolver class represents an interface to provide input streams
     * for external entities given an URL and entity ID.
     */
    class VBOXXML_CLASS InputResolver
    {
    public:

        /**
         * Returns a newly allocated input stream for the given arguments. The
         * caller will delete the returned object when no more necessary.
         *
         * @param aURI  URI of the external entity.
         * @param aID   ID of the external entity (may be NULL).
         *
         * @return      Input stream created using @c new or NULL to indicate
         *              a wrong URI/ID pair.
         *
         * @todo Return by value after implementing the copy semantics for
         * Input subclasses.
         */
        virtual xml::Input *resolveEntity (const char *aURI, const char *aID) = 0;
    };

    /**
     * The AutoConverter class represents an interface to automatically convert
     * old settings trees to a new version when the tree is read from the
     * stream.
     */
    class VBOXXML_CLASS AutoConverter
    {
    public:

        /**
         * Returns @true if the given tree needs to be converted using the XSLT
         * template identified by #templateUri(), or @false if no conversion is
         * required.
         *
         * The implementation normally checks for the "version" value of the
         * root key to determine if the conversion is necessary. When the
         * @a aOldVersion argument is not NULL, the implementation must return a
         * non-NULL non-empty string representing the old version (before
         * conversion) in it this string is used by XmlTreeBackend::oldVersion()
         * and must be non-NULL to indicate that the conversion has been
         * performed on the tree. The returned string must be allocated using
         * RTStrDup() or such.
         *
         * This method is called again after the successful transformation to
         * let the implementation retry the version check and request another
         * transformation if necessary. This may be used to perform multi-step
         * conversion like this: 1.1 => 1.2, 1.2 => 1.3 (instead of 1.1 => 1.3)
         * which saves from the need to update all previous conversion
         * templates to make each of them convert directly to the recent
         * version.
         *
         * @note Multi-step transformations are performed in a loop that exits
         *       only when this method returns @false. It's up to the
         *       implementation to detect cycling (repeated requests to convert
         *       from the same version) wrong version order, etc. and throw an
         *       EConversionCycle exception to break the loop without returning
         *       @false (which means the transformation succeeded).
         *
         * @param aRoot                 Root settings key.
         * @param aOldVersionString     Where to store old version string
         *                              pointer. May be NULL. Allocated memory is
         *                              freed by the caller using RTStrFree().
         */
        virtual bool needsConversion (const Key &aRoot,
                                      char **aOldVersion) const = 0;

        /**
         * Returns the URI of the XSLT template to perform the conversion.
         * This template will be applied to the tree if #needsConversion()
         * returns @c true for this tree.
         */
        virtual const char *templateUri() const = 0;
    };

    XmlTreeBackend();
    ~XmlTreeBackend();

    /**
     * Sets an external entity resolver used to provide input streams for
     * entities referred to by the XML document being parsed.
     *
     * The given resolver object must exist as long as this instance exists or
     * until a different resolver is set using setInputResolver() or reset
     * using resetInputResolver().
     *
     * @param aResolver Resolver to use.
     */
    void setInputResolver (InputResolver &aResolver);

    /**
     * Resets the entity resolver to the default resolver. The default
     * resolver provides support for 'file:' and 'http:' protocols.
     */
    void resetInputResolver();

    /**
     * Sets a settings tree converter and enables the automatic conversion.
     *
     * The Automatic settings tree conversion is useful for upgrading old
     * settings files to the new version transparently during execution of the
     * #read() method.
     *
     * The automatic conversion takes place after reading the document from the
     * stream but before validating it. The given converter is asked if the
     * conversion is necessary using the AutoConverter::needsConversion() call,
     * and if so, the XSLT template specified by AutoConverter::templateUri() is
     * applied to the settings tree.
     *
     * Note that in order to make the result of the conversion permanent, the
     * settings tree needs to be exlicitly written back to the stream.
     *
     * The given converter object must exist as long as this instance exists or
     * until a different converter is set using setAutoConverter() or reset
     * using resetAutoConverter().
     *
     * @param aConverter    Settings converter to use.
     */
    void setAutoConverter (AutoConverter &aConverter);

    /**
     * Disables the automatic settings conversion previously enabled by
     * setAutoConverter(). By default automatic conversion it is disabled.
     */
    void resetAutoConverter();

    /**
     * Returns a non-NULL string if the automatic settings conversion has been
     * performed during the last successful #read() call. Returns @c NULL if
     * there was no settings conversion.
     *
     * If #read() fails, this method will return the version string set by the
     * previous successful #read() call or @c NULL if there were no #read()
     * calls.
     */
    const char *oldVersion() const;

    void rawRead (xml::Input &aInput, const char *aSchema = NULL, int aFlags = 0);
    void rawWrite (xml::Output &aOutput);
    void reset();
    Key &rootKey() const;

private:

    /* Obscure class data */
    struct Data;
    std::auto_ptr <Data> m;

    /* auto_ptr data doesn't have proper copy semantics */
    DECLARE_CLS_COPY_CTOR_ASSIGN_NOOP (XmlTreeBackend)

    static int ReadCallback (void *aCtxt, char *aBuf, int aLen);
    static int WriteCallback (void *aCtxt, const char *aBuf, int aLen);
    static int CloseCallback (void *aCtxt);

    static void ValidityErrorCallback (void *aCtxt, const char *aMsg, ...);
    static void ValidityWarningCallback (void *aCtxt, const char *aMsg, ...);
    static void StructuredErrorCallback (void *aCtxt, xmlErrorPtr aErr);

    static xmlParserInput *ExternalEntityLoader (const char *aURI,
                                                 const char *aID,
                                                 xmlParserCtxt *aCtxt);

    static XmlTreeBackend *sThat;

    static XmlKeyBackend *GetKeyBackend (const Key &aKey)
    { return (XmlKeyBackend *) TreeBackend::GetKeyBackend (aKey); }
};

} /* namespace settings */


/*
 * VBoxXml
 *
 *
 */


class VBoxXmlBase
{
protected:
    VBoxXmlBase();

    ~VBoxXmlBase();

    xmlParserCtxtPtr m_ctxt;
};

class VBoxXmlFile : public VBoxXmlBase
{
public:
    VBoxXmlFile();
    ~VBoxXmlFile();
};



#if defined(_MSC_VER)
#pragma warning (default:4251)
#endif

/** @} */

#endif /* ___VBox_settings_h */