Undo: implement rudimentary undo of dive-notes editing

Implement a first rudimentary dive-editing command. The main code
resides in a base class Command::Edit, which calls virtual functions
to read / set the fields and extract the field name.

Implement an example: editing of dive notes.

This dose not yet update the UI on undo / redo.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

desktop-widgets/command_edit.h

@@ -0,0 +1,58 @@
+// SPDX-License-Identifier: GPL-2.0
+// Note: this header file is used by the undo-machinery and should not be included elsewhere.
+
+#ifndef COMMAND_EDIT_H
+#define COMMAND_EDIT_H
+
+#include "command_base.h"
+
+#include <QVector>
+
+// These are commands that edit individual fields of a set of dives.
+// The implementation is very OO-style. Out-of-fashion and certainly
+// not elegant, but in line with Qt's OO-based design.
+// The actual code is in a common base class "Command::EditBase". To
+// read and set the fields, the base class calls virtual functions of
+// the derived classes.
+//
+// To deal with different data types, the base class is implemented
+// as a template. The template parameter is the type to be read or
+// set. Thus, switch-cascades and union trickery can be avoided.
+
+// We put everything in a namespace, so that we can shorten names without polluting the global namespace
+namespace Command {
+
+template <typename T>
+class EditBase : public Base {
+	T value; // Value to be set
+	T old; // Previous value
+
+	void undo() override;
+	void redo() override;
+	bool workToBeDone() override;
+
+	// Dives to be edited. For historical reasons, the *last* entry was
+	// the active dive when the user initialized the action. This dive
+	// will be made the current dive on redo / undo.
+	std::vector<dive *> dives;
+public:
+	EditBase(const QVector<dive *> &dives, T newValue, T oldValue);
+
+protected:
+	// Get and set functions to be overriden by sub-classes.
+	virtual void set(struct dive *d, T) const = 0;
+	virtual T data(struct dive *d) const = 0;
+	virtual QString fieldName() const = 0;	// Name of the field, used to create the undo menu-entry
+};
+
+class EditNotes : public EditBase<QString> {
+public:
+	using EditBase<QString>::EditBase;	// Use constructor of base class.
+	void set(struct dive *d, QString s) const override;
+	QString data(struct dive *d) const override;
+	QString fieldName() const override;
+};
+
+} // namespace Command
+
+#endif