BASE: Remove bad casts between incompatible Plugin types

Previously, a C-style cast was used to convert a
Common::Array<Plugin *>, populated with pointers to StaticPlugin
and DynamicPlugin instances, to a
Common::Array<PluginSubclass<T> *>, but PluginSubclass<T> is a
*sibling* class to StaticPlugin/DynamicPlugin, so this cast was
invalid and the results undefined. The methods for retrieving
subclasses of plugins can't be easily changed to just generate an
array of temporary wrapper objects that expose an identical API
which dereferences to the preferred PluginObject subclass because
pointers to these objects are retained by other parts of ScummVM,
so the wrappers would needed to be persisted or they would need to
just re-expose the underlying Plugin object again. This indicated
that a way to solve this problem is to have the callers receive
Plugin objects and get the PluginObject from the Plugin by
explicitly stating their desired type, in a similar manner to
std::get(std::variant), so that the pattern used by this patch to
solve the problem.

Closes gh-1051.

1 files changed, 9 insertions, 19 deletions

diff --git a/base/plugins.h b/base/plugins.h
index 2793ff3ffd..ce45f7102a 100644
--- a/base/plugins.h
+++ b/base/plugins.h
@@ -190,6 +190,15 @@ public:
 	PluginType getType() const;
 	const char *getName() const;
 
+	template <class T>
+	T &get() const {
+		T *pluginObject = dynamic_cast<T *>(_pluginObject);
+		if (!pluginObject) {
+			error("Invalid cast of plugin %s", getName());
+		}
+		return *pluginObject;
+	}
+
 	/**
 	 * The getFileName() function gets the name of the plugin file for those
 	 * plugins that have files (ie. not static). It doesn't require the plugin
@@ -202,25 +211,6 @@ public:
 typedef Common::Array<Plugin *> PluginList;
 
 /**
- * Convenience template to make it easier defining normal Plugin
- * subclasses. Namely, the PluginSubclass will manage PluginObjects
- * of a type specified via the PO_t template parameter.
- */
-template<class PO_t>
-class PluginSubclass : public Plugin {
-public:
-	PO_t &operator*() const {
-		return *(PO_t *)_pluginObject;
-	}
-
-	PO_t *operator->() const {
-		return (PO_t *)_pluginObject;
-	}
-
-	typedef Common::Array<PluginSubclass *> List;
-};
-
-/**
  * Abstract base class for Plugin factories. Subclasses of this
  * are responsible for creating plugin objects, e.g. by loading
  * loadable modules from storage media; by creating "fake" plugins