Implement polymorphism for DigitalOut

drivers/include/drivers/DigitalOut.h

@@ -18,6 +18,8 @@
 #define MBED_DIGITALOUT_H
 
 #include "platform/platform.h"
+
+#include "interfaces/InterfaceDigitalOut.h"
 #include "hal/gpio_api.h"
 
 namespace mbed {
@@ -46,7 +48,11 @@ namespace mbed {
  * }
  * @endcode
  */
-class DigitalOut {
+class DigitalOut
+#ifdef FEATURE_EXPERIMENTAL_API
+    final : public interface::DigitalOut
+#endif
+{
 
 public:
     /** Create a DigitalOut connected to the specified pin

drivers/include/drivers/interfaces/InterfaceDigitalOut.h

@@ -0,0 +1,70 @@
+/*
+ * Mbed-OS Microcontroller Library
+ * Copyright (c) 2021 Embedded Planet
+ * Copyright (c) 2021 ARM Limited
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License
+ */
+
+#ifndef MBED_INTERFACE_DIGITALOUT_H_
+#define MBED_INTERFACE_DIGITALOUT_H_
+
+#include "PinNames.h"
+
+namespace mbed {
+
+class DigitalOut;
+
+namespace interface {
+
+#ifdef FEATURE_EXPERIMENTAL_API
+
+// TODO - move method doxygen comments to interface once this polymorphism is mainstream
+
+// Pure interface definition for DigitalOut
+struct DigitalOut {
+    virtual ~DigitalOut() = default;
+    virtual void write(int value) = 0;
+    virtual int read() = 0;
+    virtual int is_connected() = 0;
+
+    virtual DigitalOut &operator= (int value)
+    {
+        // Underlying implementation is responsible for thread-safety
+        write(value);
+        return *this;
+    }
+
+    virtual DigitalOut &operator= (DigitalOut &rhs) {
+        // Underlying implementation is responsible for thread-safety
+        write(rhs.read());
+        return *this;
+    }
+
+    virtual operator int()
+    {
+        // Underlying implementation is responsible for thread-safety
+        return read();
+    }
+
+};
+#else
+using DigitalOut = ::mbed::DigitalOut;
+#endif /* FEATURE_EXPERIMENTAL_API */
+
+} // namespace interface
+} // namespace mbed
+
+
+#endif /* MBED_INTERFACE_DIGITALOUT_H_ */