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Concurrent Filter and Filter-Reduce

The <QtConcurrentFilter> header provides concurrent Filter and Filter-Reduce. These functions are a part of the Qt Concurrent framework.

The QtConcurrent::filter(), QtConcurrent::filtered() and QtConcurrent::filteredReduced() functions filter items in a sequence such as a QList or a QVector in parallel. QtConcurrent::filter() modifies a sequence in-place, QtConcurrent::filtered() returns a new sequence containing the filtered content, and QtConcurrent::filteredReduced() returns a single result.

Each of the above functions have a blocking variant that returns the final result instead of a QFuture. You use them in the same way as the asynchronous variants.

The following code example is written in c++.

QStringList strings = ...;

// each call blocks until the entire operation is finished
QStringList lowerCaseStrings = QtConcurrent::blockingFiltered(strings, allLowerCase);


QtConcurrent::blockingFilter(strings, allLowerCase);

QSet<QString> dictionary = QtConcurrent::blockingFilteredReduced(strings, allLowerCase, addToDictionary);
Note that the result types above are not QFuture objects, but real result types (in this case, QStringList and QSet<QString>).

Concurrent Filter

QtConcurrent::filtered() takes an input sequence and a filter function. This filter function is then called for each item in the sequence, and a new sequence containing the filtered values is returned.

The filter function must be of the form:

The following code example is written in c++.

bool function(const T &t);
T must match the type stored in the sequence. The function returns true if the item should be kept, false if it should be discarded.

This example shows how to keep strings that are all lower-case from a QStringList:

The following code example is written in c++.

bool allLowerCase(const QString &string)
{
    return string.lowered() == string;
}

QStringList strings = ...;
QFuture<QString> lowerCaseStrings = QtConcurrent::filtered(strings, allLowerCase);
The results of the filter are made available through
QFuture. See the QFuture and QFutureWatcher documentation for more information on how to use QFuture in your applications.

If you want to modify a sequence in-place, use QtConcurrent::filter():

The following code example is written in c++.

QStringList strings = ...;
QFuture<void> future = QtConcurrent::filter(strings, allLowerCase);
Since the sequence is modified in place, QtConcurrent::filter() does not return any results via QFuture. However, you can still use QFuture and QFutureWatcher to monitor the status of the filter.

Concurrent Filter-Reduce

QtConcurrent::filteredReduced() is similar to QtConcurrent::filtered(), but instead of returing a sequence with the filtered results, the results are combined into a single value using a reduce function.

The reduce function must be of the form:

The following code example is written in c++.

V function(T &result, const U &intermediate)
T is the type of the final result, U is the type of items being filtered. Note that the return value and return type of the reduce function are not used.

Call QtConcurrent::filteredReduced() like this:

The following code example is written in c++.

void addToDictionary(QSet<QString> &dictionary, const QString &string)
{
    dictionary.insert(string);
}

QStringList strings = ...;
QFuture<QSet<QString> > dictionary = QtConcurrent::filteredReduced(strings, allLowerCase, addToDictionary);
The reduce function will be called once for each result kept by the filter function, and should merge the intermediate into the result variable. QtConcurrent::filteredReduced() guarantees that only one thread will call reduce at a time, so using a mutex to lock the result variable is not neccesary. The QtConcurrent::ReduceOptions enum provides a way to control the order in which the reduction is done.

Additional API Features

Using Iterators instead of Sequence

Each of the above functions has a variant that takes an iterator range instead of a sequence. You use them in the same way as the sequence variants:

The following code example is written in c++.
QStringList strings = ...;
QFuture<QString> lowerCaseStrings = QtConcurrent::filtered(strings.constBegin(), strings.constEnd(), allLowerCase);

// filter in-place only works on non-const iterators
QFuture<void> future = QtConcurrent::filter(strings.begin(), strings.end(), allLowerCase);

QFuture<QSet<QString> > dictionary = QtConcurrent::filteredReduced(strings.constBegin(), strings.constEnd(), allLowerCase, addToDictionary);

Using Member Functions

QtConcurrent::filter(), QtConcurrent::filtered(), and QtConcurrent::filteredReduced() accept pointers to member functions. The member function class type must match the type stored in the sequence:

The following code example is written in c++.
// keep only images with an alpha channel
QList<QImage> images = ...;
QFuture<void> alphaImages = QtConcurrent::filter(strings, &QImage::hasAlphaChannel);

// keep only gray scale images
QList<QImage> images = ...;
QFuture<QImage> grayscaleImages = QtConcurrent::filtered(images, &QImage::isGrayscale);

// create a set of all printable characters
QList<QChar> characters = ...;
QFuture<QSet<QChar> > set = QtConcurrent::filteredReduced(characters, &QChar::isPrint, &QSet<QChar>::insert);
Note that when using QtConcurrent::filteredReduced(), you can mix the use of normal and member functions freely:

The following code example is written in c++.
// can mix normal functions and member functions with QtConcurrent::filteredReduced()

// create a dictionary of all lower cased strings
extern bool allLowerCase(const QString &string);
QStringList strings = ...;
QFuture<QSet<int> > averageWordLength = QtConcurrent::filteredReduced(strings, allLowerCase, QSet<QString>::insert);

// create a collage of all gray scale images
extern void addToCollage(QImage &collage, const QImage &grayscaleImage);
QList<QImage> images = ...;
QFuture<QImage> collage = QtConcurrent::filteredReduced(images, &QImage::isGrayscale, addToCollage);

Using Function Objects

QtConcurrent::filter(), QtConcurrent::filtered(), and QtConcurrent::filteredReduced() accept function objects, which can be used to add state to a function call. The result_type typedef must define the result type of the function call operator:

The following code example is written in c++.
struct StartsWith
{
    StartsWith(const QString &string)
    : m_string(string) { }

    typedef bool result_type;

    bool operator()(const QString &testString)
    {
        return testString.startsWith(m_string);
    }

    QString m_string;
};

QList<QString> strings = ...;
QFuture<QString> fooString = QtConcurrent::filtered(images, StartsWith(QLatin1String("Foo")));

Using Bound Function Arguments

Note that Qt does not provide support for bound functions. This is provided by 3rd party libraries like Boost or C++ TR1 Library Extensions.

If you want to use a filter function takes more than one argument, you can use boost::bind() or std::tr1::bind() to transform it onto a function that takes one argument.

As an example, we use QString::contains():

The following code example is written in c++.

bool QString::contains(const QRegExp ®exp) const;
QString::contains() takes 2 arguments (including the "this" pointer) and can't be used with QtConcurrent::filtered() directly, because QtConcurrent::filtered() expects a function that takes one argument. To use QString::contains() with QtConcurrent::filtered() we have to provide a value for the regexp argument:

The following code example is written in c++.
boost::bind(&QString::contains, QRegExp("^\\S+$")); // matches strings without whitespace
The return value from boost::bind() is a function object (functor) with the following signature:

The following code example is written in c++.
bool contains(const QString &string)
This matches what QtConcurrent::filtered() expects, and the complete example becomes:

The following code example is written in c++.
QStringList strings = ...;
boost::bind(static_cast<bool(QString::*)(const QRegExp&)>( &QString::contains ), QRegExp("..." ));


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