RAII for locking of WaveTracks

src/WaveTrack.h

@@ -285,8 +285,29 @@ class AUDACITY_DLL_API WaveTrack final : public Track {
    // doing a copy and paste between projects.
    //
 
-   bool Lock();
-   bool Unlock();
+   bool Lock() const;
+   bool Unlock() const;
+
+   struct WaveTrackLockDeleter {
+      inline void operator () (const WaveTrack *pTrack) { pTrack->Unlock(); }
+   };
+   using LockerBase = std::unique_ptr<
+      const WaveTrack, WaveTrackLockDeleter
+   >;
+
+   // RAII object for locking.
+   struct Locker : private LockerBase
+   {
+      friend LockerBase;
+      Locker (const WaveTrack *pTrack)
+         : LockerBase{ pTrack }
+      { pTrack->Lock(); }
+      Locker(Locker &&that) : LockerBase{std::move(that)} {}
+      Locker &operator= (Locker &&that) {
+         (LockerBase&)(*this) = std::move(that);
+         return *this;
+      }
+   };
 
    bool CloseLock(); //similar to Lock but should be called when the project closes.
    // not balanced by unlocking calls.