[wpilibc] Return reference from GetInstance (#3247)

Improves consistency across all classes.

Affects Preferences, LiveWindow, and CameraServer.

Old commands Scheduler::GetInstance() was not updated as this is already
deprecated.

wpilibc/src/main/native/cpp/IterativeRobotBase.cpp

@@ -108,7 +108,7 @@ void IterativeRobotBase::LoopFunc() {
     // Call DisabledInit() if we are now just entering disabled mode from
     // either a different mode or from power-on.
     if (m_lastMode != Mode::kDisabled) {
-      LiveWindow::GetInstance()->SetEnabled(false);
+      LiveWindow::GetInstance().SetEnabled(false);
       Shuffleboard::DisableActuatorWidgets();
       DisabledInit();
       m_watchdog.AddEpoch("DisabledInit()");
@@ -122,7 +122,7 @@ void IterativeRobotBase::LoopFunc() {
     // Call AutonomousInit() if we are now just entering autonomous mode from
     // either a different mode or from power-on.
     if (m_lastMode != Mode::kAutonomous) {
-      LiveWindow::GetInstance()->SetEnabled(false);
+      LiveWindow::GetInstance().SetEnabled(false);
       Shuffleboard::DisableActuatorWidgets();
       AutonomousInit();
       m_watchdog.AddEpoch("AutonomousInit()");
@@ -136,7 +136,7 @@ void IterativeRobotBase::LoopFunc() {
     // Call TeleopInit() if we are now just entering teleop mode from
     // either a different mode or from power-on.
     if (m_lastMode != Mode::kTeleop) {
-      LiveWindow::GetInstance()->SetEnabled(false);
+      LiveWindow::GetInstance().SetEnabled(false);
       Shuffleboard::DisableActuatorWidgets();
       TeleopInit();
       m_watchdog.AddEpoch("TeleopInit()");
@@ -150,7 +150,7 @@ void IterativeRobotBase::LoopFunc() {
     // Call TestInit() if we are now just entering test mode from
     // either a different mode or from power-on.
     if (m_lastMode != Mode::kTest) {
-      LiveWindow::GetInstance()->SetEnabled(true);
+      LiveWindow::GetInstance().SetEnabled(true);
       Shuffleboard::EnableActuatorWidgets();
       TestInit();
       m_watchdog.AddEpoch("TestInit()");
@@ -167,7 +167,7 @@ void IterativeRobotBase::LoopFunc() {
 
   SmartDashboard::UpdateValues();
   m_watchdog.AddEpoch("SmartDashboard::UpdateValues()");
-  LiveWindow::GetInstance()->UpdateValues();
+  LiveWindow::GetInstance().UpdateValues();
   m_watchdog.AddEpoch("LiveWindow::UpdateValues()");
   Shuffleboard::Update();
   m_watchdog.AddEpoch("Shuffleboard::Update()");

wpilibc/src/main/native/cpp/Preferences.cpp

@@ -17,9 +17,9 @@ using namespace frc;
 // The Preferences table name
 static wpi::StringRef kTableName{"Preferences"};
 
-Preferences* Preferences::GetInstance() {
+Preferences& Preferences::GetInstance() {
   static Preferences instance;
-  return &instance;
+  return instance;
 }
 
 std::vector<std::string> Preferences::GetKeys() {
@@ -143,3 +143,5 @@ Preferences::Preferences()
       NT_NOTIFY_NEW | NT_NOTIFY_IMMEDIATE);
   HAL_Report(HALUsageReporting::kResourceType_Preferences, 0);
 }
+
+Preferences::~Preferences() = default;

wpilibc/src/main/native/cpp/livewindow/LiveWindow.cpp

@@ -61,9 +61,9 @@ std::shared_ptr<LiveWindow::Impl::Component> LiveWindow::Impl::GetOrAdd(
   return data;
 }
 
-LiveWindow* LiveWindow::GetInstance() {
+LiveWindow& LiveWindow::GetInstance() {
   static LiveWindow instance;
-  return &instance;
+  return instance;
 }
 
 void LiveWindow::EnableTelemetry(Sendable* sendable) {

wpilibc/src/main/native/cppcs/RobotBase.cpp

@@ -214,7 +214,7 @@ RobotBase::RobotBase() : m_ds(DriverStation::GetInstance()) {
       ->GetEntry("LW Enabled")
       .SetBoolean(false);
 
-  LiveWindow::GetInstance()->SetEnabled(false);
+  LiveWindow::GetInstance().SetEnabled(false);
 }
 
 RobotBase::RobotBase(RobotBase&&) noexcept

wpilibc/src/main/native/include/frc/Preferences.h

@@ -35,9 +35,9 @@ class Preferences : public ErrorBase {
   /**
    * Get the one and only {@link Preferences} object.
    *
-   * @return pointer to the {@link Preferences}
+   * @return reference to the {@link Preferences}
    */
-  static Preferences* GetInstance();
+  static Preferences& GetInstance();
 
   /**
    * Returns a vector of all the keys.
@@ -226,7 +226,7 @@ class Preferences : public ErrorBase {
 
  protected:
   Preferences();
-  ~Preferences() override = default;
+  ~Preferences() override;
 
   Preferences(Preferences&&) = default;
   Preferences& operator=(Preferences&&) = default;

wpilibc/src/main/native/include/frc/livewindow/LiveWindow.h

@@ -29,7 +29,7 @@ class LiveWindow {
    * This is a singleton to guarantee that there is only a single instance
    * regardless of how many times GetInstance is called.
    */
-  static LiveWindow* GetInstance();
+  static LiveWindow& GetInstance();
 
   /**
    * Enable telemetry for a single component.
@@ -69,6 +69,7 @@ class LiveWindow {
 
  private:
   LiveWindow();
+  ~LiveWindow() = default;
 
   struct Impl;
   std::unique_ptr<Impl> m_impl;