Refactor simple dialogs to use Lazy

ext/libclementine-common/core/lazy.h

@@ -29,6 +29,9 @@ class Lazy {
  public:
   explicit Lazy(std::function<T*()> init) : init_(init) {}
 
+  // Convenience constructor that will lazily default construct the object.
+  Lazy() : init_([]() { return new T; }) {}
+
   T* get() const {
     CheckInitialised();
     return ptr_.get();

src/ui/mainwindow.cpp

@@ -2196,10 +2196,6 @@ void MainWindow::PlaylistUndoRedoChanged(QAction* undo, QAction* redo) {
 }
 
 void MainWindow::AddFilesToTranscoder() {
-  if (!transcode_dialog_) {
-    transcode_dialog_.reset(new TranscodeDialog);
-  }
-
   QStringList filenames;
 
   for (const QModelIndex& index :
@@ -2475,24 +2471,14 @@ EditTagDialog* MainWindow::CreateEditTagDialog() {
 }
 
 void MainWindow::ShowAboutDialog() {
-  if (!about_dialog_) {
-    about_dialog_.reset(new About);
-  }
-
   about_dialog_->show();
 }
 
 void MainWindow::ShowTranscodeDialog() {
-  if (!transcode_dialog_) {
-    transcode_dialog_.reset(new TranscodeDialog);
-  }
   transcode_dialog_->show();
 }
 
 void MainWindow::ShowErrorDialog(const QString& message) {
-  if (!error_dialog_) {
-    error_dialog_.reset(new ErrorDialog);
-  }
   error_dialog_->ShowMessage(message);
 }
 

src/ui/mainwindow.h

@@ -297,7 +297,7 @@ signals:
   SystemTrayIcon* tray_icon_;
   OSD* osd_;
   Lazy<EditTagDialog> edit_tag_dialog_;
-  std::unique_ptr<About> about_dialog_;
+  Lazy<About> about_dialog_;
 
   GlobalShortcuts* global_shortcuts_;
 
@@ -318,8 +318,8 @@ signals:
   std::unique_ptr<AddStreamDialog> add_stream_dialog_;
   std::unique_ptr<AlbumCoverManager> cover_manager_;
   std::unique_ptr<Equalizer> equalizer_;
-  std::unique_ptr<TranscodeDialog> transcode_dialog_;
-  std::unique_ptr<ErrorDialog> error_dialog_;
+  Lazy<TranscodeDialog> transcode_dialog_;
+  Lazy<ErrorDialog> error_dialog_;
   Lazy<OrganiseDialog> organise_dialog_;
   std::unique_ptr<QueueManager> queue_manager_;