diff --git a/autotests/epubextractortest.cpp b/autotests/epubextractortest.cpp
--- a/autotests/epubextractortest.cpp
+++ b/autotests/epubextractortest.cpp
@@ -42,7 +42,7 @@
     plugin->extract(&result);
 
     QCOMPARE(result.types().size(), 1);
-    QCOMPARE(result.types().first(), Type::Document);
+    QCOMPARE(result.types().constFirst(), Type::Document);
 
     // We're doing a contains instead of an exact check cause the epub file contains
     // a ton of css and other garbage.
diff --git a/autotests/exiv2extractortest.cpp b/autotests/exiv2extractortest.cpp
--- a/autotests/exiv2extractortest.cpp
+++ b/autotests/exiv2extractortest.cpp
@@ -42,7 +42,7 @@
     plugin->extract(&result);
 
     QCOMPARE(result.types().size(), 1);
-    QCOMPARE(result.types().first(), Type::Image);
+    QCOMPARE(result.types().constFirst(), Type::Image);
 
     using namespace KFileMetaData::Property;
     double lat = 41.4114341666667;
diff --git a/autotests/popplerextractortest.cpp b/autotests/popplerextractortest.cpp
--- a/autotests/popplerextractortest.cpp
+++ b/autotests/popplerextractortest.cpp
@@ -42,7 +42,7 @@
     plugin->extract(&result);
 
     QCOMPARE(result.types().size(), 1);
-    QCOMPARE(result.types().first(), Type::Document);
+    QCOMPARE(result.types().constFirst(), Type::Document);
 
     QCOMPARE(result.text(), QStringLiteral("This is a sample PDF file for KFileMetaData. "));
     QCOMPARE(result.properties().value(Property::Author), QVariant(QStringLiteral("Happy Man")));
diff --git a/autotests/taglibextractortest.cpp b/autotests/taglibextractortest.cpp
--- a/autotests/taglibextractortest.cpp
+++ b/autotests/taglibextractortest.cpp
@@ -43,7 +43,7 @@
     plugin->extract(&resultOpus);
 
     QCOMPARE(resultOpus.types().size(), 1);
-    QCOMPARE(resultOpus.types().first(), Type::Audio);
+    QCOMPARE(resultOpus.types().constFirst(), Type::Audio);
 
     QCOMPARE(resultOpus.properties().value(Property::Title), QVariant(QStringLiteral("Title")));
     QCOMPARE(resultOpus.properties().value(Property::Artist), QVariant(QStringLiteral("Artist")));
@@ -61,7 +61,7 @@
     plugin->extract(&resultFlac);
 
     QCOMPARE(resultFlac.types().size(), 1);
-    QCOMPARE(resultFlac.types().first(), Type::Audio);
+    QCOMPARE(resultFlac.types().constFirst(), Type::Audio);
 
     QCOMPARE(resultFlac.properties().value(Property::Title), QVariant(QStringLiteral("Title")));
     QCOMPARE(resultFlac.properties().value(Property::Artist), QVariant(QStringLiteral("Artist")));
@@ -79,7 +79,7 @@
     plugin->extract(&resultOgg);
 
     QCOMPARE(resultOgg.types().size(), 1);
-    QCOMPARE(resultOgg.types().first(), Type::Audio);
+    QCOMPARE(resultOgg.types().constFirst(), Type::Audio);
 
     QCOMPARE(resultOgg.properties().value(Property::Title), QVariant(QStringLiteral("Title")));
     QCOMPARE(resultOgg.properties().value(Property::Artist), QVariant(QStringLiteral("Artist")));
@@ -97,7 +97,7 @@
     plugin->extract(&resultMp3);
 
     QCOMPARE(resultMp3.types().size(), 1);
-    QCOMPARE(resultMp3.types().first(), Type::Audio);
+    QCOMPARE(resultMp3.types().constFirst(), Type::Audio);
 
     QCOMPARE(resultMp3.properties().value(Property::Title), QVariant(QStringLiteral("Title")));
     QCOMPARE(resultMp3.properties().value(Property::Artist), QVariant(QStringLiteral("Artist")));
@@ -115,7 +115,7 @@
     plugin->extract(&resultMpc);
 
     QCOMPARE(resultMpc.types().size(), 1);
-    QCOMPARE(resultMpc.types().first(), Type::Audio);
+    QCOMPARE(resultMpc.types().constFirst(), Type::Audio);
 
     QCOMPARE(resultMpc.properties().value(Property::Title), QVariant(QStringLiteral("Title")));
     QCOMPARE(resultMpc.properties().value(Property::Artist), QVariant(QStringLiteral("Artist")));
@@ -133,7 +133,7 @@
     plugin->extract(&resultMp4);
 
     QCOMPARE(resultMp4.types().size(), 1);
-    QCOMPARE(resultMp4.types().first(), Type::Audio);
+    QCOMPARE(resultMp4.types().constFirst(), Type::Audio);
 
     QCOMPARE(resultMp4.properties().value(Property::Title), QVariant(QStringLiteral("Title")));
     QCOMPARE(resultMp4.properties().value(Property::Artist), QVariant(QStringLiteral("Artist")));
diff --git a/src/externalextractor.h b/src/externalextractor.h
--- a/src/externalextractor.h
+++ b/src/externalextractor.h
@@ -29,6 +29,9 @@
 
 class ExternalExtractor : public ExtractorPlugin
 {
+
+    Q_OBJECT
+
 public:
     explicit ExternalExtractor(QObject* parent = 0);
     ExternalExtractor(const QString& pluginPath);
@@ -38,7 +41,7 @@
     void extract(ExtractionResult* result) Q_DECL_OVERRIDE;
 
 private:
-    struct ExternalExtractorPrivate;
+    class ExternalExtractorPrivate;
     ExternalExtractorPrivate *d_ptr;
     Q_DECLARE_PRIVATE(ExternalExtractor)
 };
diff --git a/src/externalextractor.cpp b/src/externalextractor.cpp
--- a/src/externalextractor.cpp
+++ b/src/externalextractor.cpp
@@ -38,7 +38,8 @@
 
 using namespace KFileMetaData;
 
-struct ExternalExtractor::ExternalExtractorPrivate {
+class ExternalExtractor::ExternalExtractorPrivate {
+public:
     QString path;
     QStringList writeMimetypes;
     QString mainPath;
@@ -78,7 +79,7 @@
     QJsonObject rootObject = manifestDoc.object();
     QJsonArray mimetypesArray = rootObject.value(QStringLiteral("mimetypes")).toArray();
     QStringList mimetypes;
-    Q_FOREACH(QVariant mimetype, mimetypesArray) {
+    Q_FOREACH(const QVariant &mimetype, mimetypesArray) {
         mimetypes << mimetype.toString();
     }
 
@@ -135,7 +136,7 @@
     QJsonObject rootObject = extractorData.object();
     QJsonObject propertiesObject = rootObject[QStringLiteral("properties")].toObject();
 
-    Q_FOREACH(auto key, propertiesObject.keys()) {
+    Q_FOREACH(const auto &key, propertiesObject.keys()) {
         if (key == QStringLiteral("typeInfo")) {
             TypeInfo info = TypeInfo::fromName(propertiesObject.value(key).toString());
             result->addType(info.type());
diff --git a/src/externalwriter.h b/src/externalwriter.h
--- a/src/externalwriter.h
+++ b/src/externalwriter.h
@@ -29,9 +29,12 @@
 
 class ExternalWriter : public WriterPlugin
 {
+
+    Q_OBJECT
+
 public:
     explicit ExternalWriter(QObject* parent = 0);
-    ExternalWriter(const QString& pluginPath);
+    explicit ExternalWriter(const QString& pluginPath);
     virtual ~ExternalWriter();
 
     void write(const WriteData& data) Q_DECL_OVERRIDE;
@@ -41,7 +44,7 @@
     bool runtimeInstalled() const;
     bool dependenciesSatisfied() const;
 
-    struct ExternalWriterPrivate;
+    class ExternalWriterPrivate;
     ExternalWriterPrivate *d_ptr;
     Q_DECLARE_PRIVATE(ExternalWriter)
 };
diff --git a/src/externalwriter.cpp b/src/externalwriter.cpp
--- a/src/externalwriter.cpp
+++ b/src/externalwriter.cpp
@@ -37,7 +37,8 @@
 
 using namespace KFileMetaData;
 
-struct ExternalWriter::ExternalWriterPrivate {
+class ExternalWriter::ExternalWriterPrivate {
+public:
     QString path;
     QStringList writeMimetypes;
     QString mainPath;
@@ -81,7 +82,7 @@
     QJsonObject rootObject = manifestDoc.object();
     QJsonArray mimetypesArray = rootObject.value(QStringLiteral("mimetypes")).toArray();
     QStringList mimetypes;
-    Q_FOREACH(QVariant mimetype, mimetypesArray) {
+    Q_FOREACH(const QVariant &mimetype, mimetypesArray) {
         mimetypes << mimetype.toString();
     }
 
@@ -105,8 +106,9 @@
     QByteArray errorOutput;
 
     QMap<Property::Property, QVariant> properties = data.getAllProperties();
+    const auto &propertiesKeys = properties.keys();
 
-    Q_FOREACH(Property::Property property, properties.keys()) {
+    Q_FOREACH(const Property::Property &property, propertiesKeys) {
         PropertyInfo propertyInfo(property);
         propertiesObject[propertyInfo.name()] = QJsonValue::fromVariant(properties[property]);
     }
diff --git a/src/extractorcollection.cpp b/src/extractorcollection.cpp
--- a/src/extractorcollection.cpp
+++ b/src/extractorcollection.cpp
@@ -39,24 +39,26 @@
 public:
     QHash<QString, Extractor*> m_extractors;
 
+    QList<Extractor*> m_allExtractors;
+
     QList<Extractor*> allExtractors() const;
 };
 
 ExtractorCollection::ExtractorCollection()
     : d(new Private)
 {
-    QList<Extractor*> all = d->allExtractors();
+    d->m_allExtractors = d->allExtractors();
 
-    foreach (Extractor* ex, all) {
+    foreach (Extractor* ex, d->m_allExtractors) {
         foreach (const QString& type, ex->mimetypes()) {
             d->m_extractors.insertMulti(type, ex);
         }
     }
 }
 
 ExtractorCollection::~ExtractorCollection()
 {
-    qDeleteAll(d->m_extractors.values().toSet());
+    qDeleteAll(d->m_allExtractors.begin(), d->m_allExtractors.end());
     delete d;
 }
 
diff --git a/src/writedata.h b/src/writedata.h
--- a/src/writedata.h
+++ b/src/writedata.h
@@ -53,7 +53,7 @@
     QMap<Property::Property, QVariant> getAllProperties() const;
 
 private:
-    struct WriteDataPrivate;
+    class WriteDataPrivate;
     WriteDataPrivate* d_ptr;
     Q_DECLARE_PRIVATE(WriteData)
 };
diff --git a/src/writedata.cpp b/src/writedata.cpp
--- a/src/writedata.cpp
+++ b/src/writedata.cpp
@@ -27,7 +27,8 @@
 
 using namespace KFileMetaData;
 
-struct WriteData::WriteDataPrivate {
+class WriteData::WriteDataPrivate {
+public:
     QString url;
     QString mimetype;
     PropertyMap properties;
@@ -52,7 +53,6 @@
 
 WriteData& WriteData::operator=(const WriteData& rhs)
 {
-    Q_D(WriteData);
     *d_ptr = *rhs.d_ptr;
     return *this;
 }
diff --git a/src/writer.h b/src/writer.h
--- a/src/writer.h
+++ b/src/writer.h
@@ -47,7 +47,7 @@
 
     void operator =(const Writer&);
 
-    struct WriterPrivate;
+    class WriterPrivate;
     WriterPrivate *d_ptr;
     Q_DECLARE_PRIVATE(Writer)
 
diff --git a/src/writer_p.h b/src/writer_p.h
--- a/src/writer_p.h
+++ b/src/writer_p.h
@@ -28,8 +28,9 @@
 
 class WriterPlugin;
 
-struct Writer::WriterPrivate
+class Writer::WriterPrivate
 {
+public:
     WriterPlugin *m_plugin;
 };
 
diff --git a/src/writercollection.h b/src/writercollection.h
--- a/src/writercollection.h
+++ b/src/writercollection.h
@@ -41,7 +41,7 @@
     QList<Writer*> fetchWriters(const QString& mimetype) const;
 
 private:
-    struct WriterCollectionPrivate;
+    class WriterCollectionPrivate;
     WriterCollectionPrivate* d_ptr;
     Q_DECLARE_PRIVATE(WriterCollection)
 };
diff --git a/src/writercollection.cpp b/src/writercollection.cpp
--- a/src/writercollection.cpp
+++ b/src/writercollection.cpp
@@ -38,7 +38,8 @@
 
 using namespace KFileMetaData;
 
-struct WriterCollection::WriterCollectionPrivate {
+class WriterCollection::WriterCollectionPrivate {
+public:
     QHash<QString, Writer*> m_writers;
 
     QList<Writer*> allWriters() const;
@@ -60,7 +61,7 @@
 WriterCollection::~WriterCollection()
 {
     Q_D(WriterCollection);
-    qDeleteAll(d->m_writers.values().toSet());
+    qDeleteAll(d->m_writers.begin(), d->m_writers.end());
     delete d;
 }
 
diff --git a/src/writers/taglibwriter.cpp b/src/writers/taglibwriter.cpp
--- a/src/writers/taglibwriter.cpp
+++ b/src/writers/taglibwriter.cpp
@@ -33,7 +33,6 @@
 void TagLibWriter::write(const WriteData& data)
 {
     const QString fileUrl = data.inputUrl();
-    const QString mimeType = data.inputMimetype();
     const PropertyMap properties = data.getAllProperties();
 
     TagLib::FileRef file(fileUrl.toUtf8().constData(), true);