static overloads of Track::SharedPointer when null check is needed...

... Some optimizing compilers don't let us get away with null check on this
in a nonstatic member function.

src/SelectionState.cpp

@@ -97,7 +97,7 @@ void SelectionState::ChangeSelectionOnShiftClick
 
       // Our track was earlier than the first.  Extend from the last.
       if( !pExtendFrom )
-         pExtendFrom = (*trackRange.rbegin())->SharedPointer();
+         pExtendFrom = Track::SharedPointer( *trackRange.rbegin() );
    }
 
    SelectNone( tracks );

src/Track.h

@@ -238,7 +238,6 @@ class AUDACITY_DLL_API Track /* not final */
    {
       // shared_from_this is injected into class scope by base class
       // std::enable_shared_from_this<Track>
-      if (!this) return {};
       return std::static_pointer_cast<Subclass>( shared_from_this() );
    }
 
@@ -250,10 +249,18 @@ class AUDACITY_DLL_API Track /* not final */
    {
       // shared_from_this is injected into class scope by base class
       // std::enable_shared_from_this<Track>
-      if (!this) return {};
       return std::static_pointer_cast<Subclass>( shared_from_this() );
    }
 
+   // Static overloads of SharedPointer for when the pointer may be null
+   template<typename Subclass = Track>
+   static inline std::shared_ptr<Subclass> SharedPointer( Track *pTrack )
+   { return pTrack ? pTrack->SharedPointer<Subclass>() : nullptr; }
+
+   template<typename Subclass = const Track>
+   static inline std::shared_ptr<Subclass> SharedPointer( const Track *pTrack )
+   { return pTrack ? pTrack->SharedPointer<Subclass>() : nullptr; }
+
    // Find anything registered with TrackList::RegisterPendingChangedTrack and
    // not yet cleared or applied; if no such exists, return this track
    std::shared_ptr<const Track> SubstitutePendingChangedTrack() const;

src/TrackPanel.cpp

@@ -2247,7 +2247,7 @@ void TrackPanel::SetFocusedTrack( Track *t )
    // Make sure we always have the first linked track of a stereo track
    t = *GetTracks()->FindLeader(t);
 
-   auto cell = mAx->SetFocus( t->SharedPointer() ).get();
+   auto cell = mAx->SetFocus( Track::SharedPointer( t ) ).get();
 
    if (cell) {
       AudacityProject::CaptureKeyboard(this);

src/TrackPanelAx.cpp

@@ -64,7 +64,7 @@ std::shared_ptr<Track> TrackPanelAx::GetFocus()
       }
       if (!focusedTrack) {
          focusedTrack =
-            (*mTrackPanel->GetTracks()->Any().first)->SharedPointer();
+            Track::SharedPointer( *mTrackPanel->GetTracks()->Any().first );
          // only call SetFocus if the focus has changed to avoid
          // unnecessary focus events
          if (focusedTrack) 
@@ -99,7 +99,7 @@ std::shared_ptr<Track> TrackPanelAx::SetFocus( std::shared_ptr<Track> track )
 #endif
 
    if( !track )
-      track = (*mTrackPanel->GetTracks()->Any().begin())->SharedPointer();
+      track = Track::SharedPointer( *mTrackPanel->GetTracks()->Any().begin() );
 
    mFocusedTrack = track;
    mNumFocusedTrack = TrackNum(track);

src/tracks/ui/SelectHandle.cpp

@@ -1144,7 +1144,7 @@ void SelectHandle::AdjustSelection
       std::max(0.0, viewInfo.PositionToTime(mouseXCoordinate, trackLeftEdge));
    double origSelend = selend;
 
-   auto pTrack = track->SharedPointer();
+   auto pTrack = Track::SharedPointer( track );
    if (!pTrack)
       pTrack = pProject->GetTracks()->Lock(mpTrack);
 

src/tracks/ui/TimeShiftHandle.cpp

@@ -695,7 +695,7 @@ UIHandle::Result TimeShiftHandle::Drag
    }
 
    // May need a shared_ptr to reassign mCapturedTrack below
-   auto pTrack = track->SharedPointer();
+   auto pTrack = Track::SharedPointer( track );
    if (!pTrack)
       return RefreshCode::RefreshNone;