diff --git a/src/core/atcore.h b/src/core/atcore.h
--- a/src/core/atcore.h
+++ b/src/core/atcore.h
@@ -37,8 +37,6 @@
 class IFirmware;
 class QTime;
 
-struct AtCorePrivate;
-
 /**
  * @brief The AtCore class
  * aims to provides a high level interface for serial based gcode devices<br />
@@ -545,6 +543,7 @@
     /**
      * @brief Hold private data of AtCore.
      */
+    struct AtCorePrivate;
     AtCorePrivate *d;
 
 protected:
diff --git a/src/core/atcore.cpp b/src/core/atcore.cpp
--- a/src/core/atcore.cpp
+++ b/src/core/atcore.cpp
@@ -45,7 +45,7 @@
  * @brief The AtCorePrivate struct
  * Provides a private data set for atcore.
  */
-struct AtCorePrivate {
+struct AtCore::AtCorePrivate {
     IFirmware *firmwarePlugin = nullptr;//!< @param firmwarePlugin: pointer to firmware plugin
     SerialLayer *serial = nullptr;      //!< @param serial: pointer to the serial layer
     QPluginLoader pluginLoader;         //!< @param pluginLoader: QPluginLoader
diff --git a/src/core/ifirmware.h b/src/core/ifirmware.h
--- a/src/core/ifirmware.h
+++ b/src/core/ifirmware.h
@@ -32,7 +32,6 @@
 class Temperature;
 class AtCore;
 
-struct IFirmwarePrivate;
 /**
  * @brief The IFirmware class
  * Base Class for Firmware Plugins
@@ -82,6 +81,7 @@
      */
     AtCore *core() const;
 private:
+    struct IFirmwarePrivate;
     IFirmwarePrivate *d;
 public slots:
     /**
diff --git a/src/core/ifirmware.cpp b/src/core/ifirmware.cpp
--- a/src/core/ifirmware.cpp
+++ b/src/core/ifirmware.cpp
@@ -28,15 +28,15 @@
  * @brief The IFirmwarePrivate struct
  * @param parent: parent of this object
  */
-struct IFirmwarePrivate {
+struct IFirmware::IFirmwarePrivate {
     AtCore *parent;
     /**
      * @brief command finished string
      */
     static const QString _ok;
 };
 
-const QString IFirmwarePrivate::_ok = QStringLiteral("ok");
+const QString IFirmware::IFirmwarePrivate::_ok = QStringLiteral("ok");
 
 IFirmware::IFirmware()
     : d(new IFirmwarePrivate)
diff --git a/src/core/printthread.h b/src/core/printthread.h
--- a/src/core/printthread.h
+++ b/src/core/printthread.h
@@ -27,7 +27,6 @@
 
 #include "atcore.h"
 
-class PrintThreadPrivate;
 /**
  * @brief The PrintThread class
  * A Thread for running a print job
@@ -149,5 +148,6 @@
     /**
      * @brief d: Private storage for the thread
      */
+    class PrintThreadPrivate;
     PrintThreadPrivate *d;
 };
diff --git a/src/core/printthread.cpp b/src/core/printthread.cpp
--- a/src/core/printthread.cpp
+++ b/src/core/printthread.cpp
@@ -32,7 +32,7 @@
 /**
  * @brief The PrintThreadPrivate class
  */
-class PrintThreadPrivate
+class PrintThread::PrintThreadPrivate
 {
 public:
     AtCore *core = nullptr;             //!<@param core: Pointer to AtCore
diff --git a/src/core/seriallayer.h b/src/core/seriallayer.h
--- a/src/core/seriallayer.h
+++ b/src/core/seriallayer.h
@@ -29,7 +29,6 @@
 
 #include "atcore_export.h"
 
-class SerialLayerPrivate;
 /**
  * @brief The SerialLayer class.
  * Provide the low level serial operations
@@ -39,6 +38,7 @@
     Q_OBJECT
 
 private:
+    class SerialLayerPrivate;
     SerialLayerPrivate *d;
 
     /**
diff --git a/src/core/seriallayer.cpp b/src/core/seriallayer.cpp
--- a/src/core/seriallayer.cpp
+++ b/src/core/seriallayer.cpp
@@ -53,7 +53,7 @@
 /**
  * @brief The SerialLayerPrivate class
  */
-class SerialLayerPrivate
+class SerialLayer::SerialLayerPrivate
 {
 public:
     bool _serialOpened;                 //!< @param _serialOpened: is serial port opened
diff --git a/src/core/temperature.h b/src/core/temperature.h
--- a/src/core/temperature.h
+++ b/src/core/temperature.h
@@ -27,7 +27,6 @@
 
 #include "atcore_export.h"
 
-class TemperaturePrivate;
 /**
  * @brief The Temperature class
  *
@@ -125,5 +124,6 @@
     void extruderTargetTemperatureChanged(float temp);
 
 private:
+    class TemperaturePrivate;
     TemperaturePrivate *d;
 };
diff --git a/src/core/temperature.cpp b/src/core/temperature.cpp
--- a/src/core/temperature.cpp
+++ b/src/core/temperature.cpp
@@ -31,7 +31,7 @@
  *
  * Private Data of Temperature
  */
-class TemperaturePrivate
+class Temperature::TemperaturePrivate
 {
 public:
     float extruderTemp;         //!< @param extruderTemp: Extruder current temperature