File base class (more work in progress)

teensy3/FS.h

 	SeekEnd = 2
 };
 
-class File;
-
 class File : public Stream {
 public:
 	constexpr File() : f(nullptr) { }
-	constexpr File(const File &file) : f(file.f) { }
-	virtual ~File();
-	virtual size_t read(void *buf, size_t nbyte);
-	virtual size_t write(const void *buf, size_t size);
-	virtual int available();
-	virtual int read();
-	virtual int peek();
-	virtual void flush();
-	virtual bool seek(uint32_t pos, int mode);
-	virtual uint32_t position() const;
-	virtual uint32_t size() const;
-	virtual void close();
-	virtual operator bool() const;
-	virtual const char* name();
-	virtual bool isDirectory();
-	virtual File openNextFile(uint8_t mode=0);
-	virtual void rewindDirectory(void);
-
-	virtual bool seek(uint32_t pos) {
+	File(File *file) {
+		// "file" must only be a class derived from File
+		// can we use is_same or is_polymorphic with static_assert?
+		// or is_base_of
+		//static_assert(std::is_same<decltype(*file),File>::value,
+			//"File(File *file) constructor only accepts pointers "
+			//"to derived classes, not File itself");
+		f = file;
+		if (f) f->refcount++;
+	}
+	File(const File &file) {
+		//Serial.println("File copy constructor");
+		//static int copycount=0;
+		//if (++copycount > 20) while (1) ;
+		f = file.f;
+		if (f) f->refcount++;
+	}
+	File& operator = (const File &file) {
+		//Serial.println("File assignment");
+		//static int assigncount=0;
+		//if (++assigncount > 20) while (1) ;
+		invalidate();
+		f = file.f;
+		if (f) f->refcount++;
+		return *this;
+	}
+	virtual ~File() {
+		invalidate();
+	}
+#if 1
+	virtual void whoami() { // testing only
+		Serial.printf("  File    this=%x, f=%x\n", (int)this, (int)f);
+		if (f) f->whoami();
+	}
+	unsigned int getRefcount() { // testing only
+		return refcount;
+	}
+#endif
+	virtual size_t read(void *buf, size_t nbyte) {
+		return (f) ? f->read(buf, nbyte) : 0;
+	}
+	virtual size_t write(const void *buf, size_t size) {
+		return (f) ? f->write(buf, size) : 0;
+	}
+	virtual int available() {
+		return (f) ? f->available() : 0;
+	}
+	virtual int read() {
+		return (f) ? f->read() : -1;
+	}
+	virtual int peek() {
+		return (f) ? f->peek() : -1;
+	}
+	virtual void flush() {
+		if (f) f->flush();
+	}
+	virtual bool seek(uint32_t pos, int mode) {
+		return (f) ? f->seek(pos, mode) : false;
+	}
+	virtual uint32_t position() const {
+		return (f) ? f->position() : 0;
+	}
+	virtual uint32_t size() const {
+		return (f) ? f->size() : 0;
+	}
+	virtual void close() {
+		if (f) f->close();
+	}
+	virtual operator bool() const {
+		return (f) ? (bool)*f : false;
+	}
+	virtual const char* name() {
+		return (f) ? f->name() : "";
+	}
+	virtual bool isDirectory() {
+		return (f) ? f->isDirectory() : false;
+	}
+	virtual File openNextFile(uint8_t mode=0) {
+		return (f) ? f->openNextFile(mode) : *this;
+	}
+	virtual void rewindDirectory(void) {
+		if (f) f->rewindDirectory();
+	}
+	bool seek(uint32_t pos) {
 		return seek(pos, SeekSet);
 	}
-	virtual size_t write(uint8_t b) {
+	size_t write(uint8_t b) {
 		return write(&b, 1);
 	}
-	virtual size_t write(const char *str) {
+	size_t write(const char *str) {
 		return write(str, strlen(str));
 	}
-	virtual size_t readBytes(char *buffer, size_t length) {
+	size_t readBytes(char *buffer, size_t length) {
 		return read(buffer, length);
 	}
-	virtual void whoami(); // testing only
-//protected:
-	File *f; // points to actual implementing class, or nullptr
+private:
+	void invalidate() {
+		if (f && --(f->refcount) == 0) delete f;
+	}
+	union {
+		// instances of base File class use this pointer
+		File *f;
+		// instances of derived classes (which actually access media)
+		// use this reference count which is managed by the base class
+		unsigned int refcount;
+	};
 };