AP_Compass: add AP::compass() singleton getter

libraries/AP_Compass/AP_Compass.cpp

@@ -462,6 +462,13 @@ Compass::Compass(void) :
     _null_init_done(false),
     _hil_mode(false)
 {
+    if (_singleton != nullptr) {
+#if CONFIG_HAL_BOARD == HAL_BOARD_SITL
+        AP_HAL::panic("Compass must be singleton");
+#endif
+        return;
+    }
+    _singleton = this;
     AP_Param::setup_object_defaults(this, var_info);
     for (uint8_t i=0; i<COMPASS_MAX_BACKEND; i++) {
         _backends[i] = nullptr;
@@ -1324,3 +1331,15 @@ bool Compass::consistent() const
     return true;
 }
 
+
+// singleton instance
+Compass *Compass::_singleton;
+
+namespace AP {
+
+Compass &compass()
+{
+    return *Compass::get_singleton();
+}
+
+}

libraries/AP_Compass/AP_Compass.h

@@ -56,6 +56,11 @@ public:
     Compass(const Compass &other) = delete;
     Compass &operator=(const Compass&) = delete;
 
+    // get singleton instance
+    static Compass *get_singleton() {
+        return _singleton;
+    }
+
     friend class CompassLearn;
 
     /// Initialize the compass device.
@@ -326,6 +331,7 @@ public:
     uint8_t get_filter_range() const { return uint8_t(_filter_range.get()); }
 
 private:
+    static Compass *_singleton;
     /// Register a new compas driver, allocating an instance number
     ///
     /// @return number of compass instances
@@ -466,3 +472,7 @@ private:
     
     AP_Int8 _filter_range;
 };
+
+namespace AP {
+    Compass &compass();
+};