Re-introduce a shared pointer class in mbed OS

Author: Donatien Garnier

platform/SharedPtr.h

@@ -0,0 +1,215 @@
+/* mbed Microcontroller Library
+ * Copyright (c) 2006-2018 ARM Limited
+ *
+ * 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 __SHAREDPTR_H__
+#define __SHAREDPTR_H__
+
+#include <stdlib.h>
+
+#include <stdint.h>
+#include <stddef.h>
+
+/** Shared pointer class.
+  *
+  * Similar to std::shared_ptr in C++11.
+  *
+  * Usage: SharedPtr<class> POINTER(new class())
+  *
+  * When POINTER is passed around by value the copy constructor and
+  * destructor counts the number of references to the original object.
+  * If the counter reaches zero, delete is called on the object pointed to.
+  *
+  * To avoid loops, "weak" references should be used by calling the original
+  * pointer directly through POINTER.get().
+  */
+
+template <class T>
+class SharedPtr {
+public:
+    /**
+     * @brief Create empty SharedPtr not pointing to anything.
+     * @details Used for variable declaration.
+     */
+    SharedPtr(): pointer(NULL), counter(NULL) {
+    }
+
+    /**
+     * @brief Create new SharedPtr
+     * @param _pointer Pointer to take control over
+     */
+    SharedPtr(T* _pointer): pointer(_pointer) {
+        // allocate counter on the heap so it can be shared
+        counter = (uint32_t*) malloc(sizeof(uint32_t));
+        *counter = 1;
+    }
+
+    /**
+     * @brief Destructor.
+     * @details Decrement reference counter and delete object if no longer pointed to.
+     */
+    ~SharedPtr() {
+        decrementCounter();
+    }
+
+    /**
+     * @brief Copy constructor.
+     * @details Create new SharePointer from other SharedPtr by
+     *          copying pointer to original object and pointer to counter.
+     * @param source Object being copied from.
+     */
+    SharedPtr(const SharedPtr& source): pointer(source.pointer), counter(source.counter) {
+        // increment reference counter
+        if (counter) {
+            (*counter)++;
+        }
+    }
+
+    /**
+     * @brief Assignment operator.
+     * @details Cleanup previous reference and assign new pointer and counter.
+     * @param source Object being assigned from.
+     * @return Object being assigned.
+     */
+    SharedPtr operator=(const SharedPtr& source) {
+        if (this != &source) {
+            // clean up by decrementing counter
+            decrementCounter();
+
+            // assign new values
+            pointer = source.get();
+            counter = source.getCounter();
+
+            // increment new counter
+            if (counter) {
+                (*counter)++;
+            }
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Raw pointer accessor.
+     * @details Get raw pointer to object pointed to.
+     * @return Pointer.
+     */
+    T* get() const {
+        return pointer;
+    }
+
+    /**
+     * @brief Reference count accessor.
+     * @return Reference count.
+     */
+    uint32_t use_count() const {
+        if (counter) {
+            return *counter;
+        } else {
+            return 0;
+        }
+    }
+
+    /**
+     * @brief Dereference object operator.
+     * @details Override to return the object pointed to.
+     */
+    T& operator*() const {
+        return *pointer;
+    }
+
+    /**
+     * @brief Dereference object member operator.
+     * @details Override to return return member in object pointed to.
+     */
+    T* operator->() const {
+        return pointer;
+    }
+
+    /**
+     * @brief Boolean conversion operator.
+     * @return Whether or not the pointer is NULL.
+     */
+    operator bool() const {
+        return (pointer != 0);
+    }
+
+private:
+    /**
+     * @brief Get pointer to reference counter.
+     * @return Pointer to reference counter.
+     */
+    uint32_t* getCounter() const {
+        return counter;
+    }
+
+    /**
+     * @brief Decrement reference counter.
+     * @details If count reaches zero, free counter and delete object pointed to.
+     */
+    void decrementCounter() {
+        if (counter) {
+            if (*counter == 1) {
+                free(counter);
+                delete pointer;
+            } else {
+                (*counter)--;
+            }
+        }
+    }
+
+private:
+    // pointer to shared object
+    T* pointer;
+
+    // pointer to shared reference counter
+    uint32_t* counter;
+};
+
+/** Non-member relational operators.
+  */
+template <class T, class U>
+bool operator== (const SharedPtr<T>& lhs, const SharedPtr<U>& rhs) {
+    return (lhs.get() == rhs.get());
+}
+
+template <class T, typename U>
+bool operator== (const SharedPtr<T>& lhs, U rhs) {
+    return (lhs.get() == (T*) rhs);
+}
+
+template <class T, typename U>
+bool operator== (U lhs, const SharedPtr<T>& rhs) {
+    return ((T*) lhs == rhs.get());
+}
+
+/** Non-member relational operators.
+  */
+template <class T, class U>
+bool operator!= (const SharedPtr<T>& lhs, const SharedPtr<U>& rhs) {
+    return (lhs.get() != rhs.get());
+}
+
+template <class T, typename U>
+bool operator!= (const SharedPtr<T>& lhs, U rhs) {
+    return (lhs.get() != (T*) rhs);
+}
+
+template <class T, typename U>
+bool operator!= (U lhs, const SharedPtr<T>& rhs) {
+    return ((T*) lhs != rhs.get());
+}
+
+#endif // __SHAREDPTR_H__