only add virtual to superclass function

src/Adafruit_AHRS.h

@@ -1,3 +1,8 @@
+#ifndef __ADAFRUIT_AHRS_H_
+#define __ADAFRUIT_AHRS_H_
+
 #include <Adafruit_AHRS_Madgwick.h>
 #include <Adafruit_AHRS_Mahony.h>
 #include <Adafruit_AHRS_NXPFusion.h>
+
+#endif // __ADAFRUIT_AHRS_H_

src/Adafruit_AHRS_FusionInterface.h

@@ -0,0 +1,41 @@
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2020 Ha Thach (tinyusb.org) for Adafruit Industries
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#ifndef ADAFRUIT_AHRS_FUSIONINTERFACE_H_
+#define ADAFRUIT_AHRS_FUSIONINTERFACE_H_
+
+// Interface for Fusion Algorithm
+class Adafruit_AHRS_FusionInterface {
+public:
+  virtual void begin(float sampleFrequency) = 0;
+  virtual void update(float gx, float gy, float gz, float ax, float ay,
+                      float az, float mx, float my, float mz) = 0;
+
+  virtual float getRoll() = 0;
+  virtual float getPitch() = 0;
+  virtual float getYaw() = 0;
+  virtual void getQuaternion(float *w, float *x, float *y, float *z) = 0;
+};
+
+#endif /* ADAFRUIT_AHRS_FUSIONINTERFACE_H_ */

src/Adafruit_AHRS_Madgwick.h

@@ -16,11 +16,13 @@
 //=============================================================================================
 #ifndef __Adafruit_Madgwick_h__
 #define __Adafruit_Madgwick_h__
+
+#include "Adafruit_AHRS_FusionInterface.h"
 #include <math.h>
 
 //--------------------------------------------------------------------------------------------
 // Variable declaration
-class Adafruit_Madgwick {
+class Adafruit_Madgwick : public Adafruit_AHRS_FusionInterface {
 private:
   static float invSqrt(float x);
   float beta; // algorithm gain

src/Adafruit_AHRS_Mahony.h

@@ -12,12 +12,14 @@
 //=============================================================================================
 #ifndef __Adafruit_Mahony_h__
 #define __Adafruit_Mahony_h__
+
+#include "Adafruit_AHRS_FusionInterface.h"
 #include <math.h>
 
 //--------------------------------------------------------------------------------------------
 // Variable declaration
 
-class Adafruit_Mahony {
+class Adafruit_Mahony : public Adafruit_AHRS_FusionInterface {
 private:
   float twoKp; // 2 * proportional gain (Kp)
   float twoKi; // 2 * integral gain (Ki)

src/Adafruit_AHRS_NXPFusion.h

@@ -1,3 +1,4 @@
+#include "Adafruit_AHRS_FusionInterface.h"
 #include <Arduino.h>
 
 // This is a modification of
@@ -31,7 +32,7 @@
 //
 
 // changed class name to avoid collision
-class Adafruit_NXPSensorFusion {
+class Adafruit_NXPSensorFusion : public Adafruit_AHRS_FusionInterface {
 public:
   void begin(float sampleRate = 100.0f);
   void update(float gx, float gy, float gz, float ax, float ay, float az,