Add a scoped holder similar to scoped_ptr for handles that may or may not be pointers

Originally committed to SVN as r6491.

aegisub/libaegisub/include/libaegisub/scoped_ptr.h

@@ -48,4 +48,32 @@ public:
 	~scoped_ptr() { delete ptr; }
 };
 
+/// A generic scoped holder for non-pointer handles
+template<class T, class Del = void(*)(T)>
+class scoped_holder {
+	T value;
+	Del destructor;
+
+	scoped_holder(scoped_holder const&);
+	scoped_holder& operator=(scoped_holder const&);
+public:
+	operator T() const { return value; }
+	T operator->() const { return value; }
+
+	scoped_holder& operator=(T new_value) {
+		if (value)
+			destructor(value);
+		value = new_value;
+		return *this;
+	}
+
+	scoped_holder(T value, Del destructor)
+	: value(value)
+	, destructor(destructor)
+	{
+	}
+
+	~scoped_holder() { if (value) destructor(value); }
+};
+
 }