diff --git a/src/common/davitemslistjob.cpp b/src/common/davitemslistjob.cpp
--- a/src/common/davitemslistjob.cpp
+++ b/src/common/davitemslistjob.cpp
@@ -30,6 +30,8 @@
 #include <KIO/DavJob>
 #include <KIO/Job>
 
+#include <QSet>
+
 using namespace KDAV;
 
 namespace KDAV {
diff --git a/src/common/etagcache.h b/src/common/etagcache.h
--- a/src/common/etagcache.h
+++ b/src/common/etagcache.h
@@ -21,12 +21,14 @@
 
 #include "kpimkdav_export.h"
 
-#include <QMap>
 #include <QObject>
-#include <QSet>
 #include <QStringList>
 
+#include <memory>
+
 namespace KDAV {
+class EtagCachePrivate;
+
 /**
  * @short A helper class to cache etags.
  *
@@ -45,6 +47,7 @@
      * of items found in @p collection.
      */
     explicit EtagCache(QObject *parent = nullptr);
+    ~EtagCache();
 
     /**
      * Sets the ETag for the remote ID. If the remote ID is marked as
@@ -97,8 +100,7 @@
     void setEtagInternal(const QString &remoteId, const QString &etag);
 
 private:
-    QMap<QString, QString> mCache;
-    QSet<QString> mChangedRemoteIds;
+    const std::unique_ptr<EtagCachePrivate> d;
 };
 }
 
diff --git a/src/common/etagcache.cpp b/src/common/etagcache.cpp
--- a/src/common/etagcache.cpp
+++ b/src/common/etagcache.cpp
@@ -18,62 +18,77 @@
 
 #include "etagcache.h"
 
+#include <QMap>
+#include <QSet>
+
 using namespace KDAV;
 
+namespace KDAV {
+class EtagCachePrivate
+{
+public:
+    QMap<QString, QString> mCache;
+    QSet<QString> mChangedRemoteIds;
+};
+}
+
 EtagCache::EtagCache(QObject *parent)
     : QObject(parent)
+    , d(new EtagCachePrivate)
 {
 }
 
+EtagCache::~EtagCache() = default;
+
 void EtagCache::setEtag(const QString &remoteId, const QString &etag)
 {
     setEtagInternal(remoteId, etag);
 
-    if (mChangedRemoteIds.contains(remoteId)) {
-        mChangedRemoteIds.remove(remoteId);
+    if (d->mChangedRemoteIds.contains(remoteId)) {
+        d->mChangedRemoteIds.remove(remoteId);
     }
 }
 
 void EtagCache::setEtagInternal(const QString &remoteId, const QString &etag)
 {
-    mCache[ remoteId ] = etag;
+    d->mCache[ remoteId ] = etag;
 }
 
 bool EtagCache::contains(const QString &remoteId) const
 {
-    return mCache.contains(remoteId);
+    return d->mCache.contains(remoteId);
 }
 
 bool EtagCache::etagChanged(const QString &remoteId, const QString &refEtag) const
 {
     if (!contains(remoteId)) {
         return true;
     }
-    return mCache.value(remoteId) != refEtag;
+    return d->mCache.value(remoteId) != refEtag;
 }
 
 void EtagCache::markAsChanged(const QString &remoteId)
 {
-    mChangedRemoteIds.insert(remoteId);
+    d->mChangedRemoteIds.insert(remoteId);
 }
 
 bool EtagCache::isOutOfDate(const QString &remoteId) const
 {
-    return mChangedRemoteIds.contains(remoteId);
+    return d->mChangedRemoteIds.contains(remoteId);
 }
 
 void EtagCache::removeEtag(const QString &remoteId)
 {
-    mChangedRemoteIds.remove(remoteId);
-    mCache.remove(remoteId);
+    d->mChangedRemoteIds.remove(remoteId);
+    d->mCache.remove(remoteId);
 }
 
 QStringList EtagCache::urls() const
 {
-    return mCache.keys();
+    return d->mCache.keys();
 }
 
 QStringList EtagCache::changedRemoteIds() const
 {
-    return mChangedRemoteIds.toList();
+    return d->mChangedRemoteIds.toList();
 }