add visitor helper class

alib2common/src/core/visitor.hpp

+/*
+ * UnboundedRegExpElement.h
+ *
+ *  Created on: Nov 23, 2013
+ *      Author: Jan Travnicek
+ */
+
+#ifndef VISITOR_HPP_
+#define VISITOR_HPP_
+
+#include <variant>
+
+namespace std {
+
+template < class ... Visitables >
+class VisitorContextBaseBuilder;
+
+template < class Visitable >
+class VisitorContextBaseBuilder < Visitable > {
+public:
+	virtual void visit ( const Visitable & ) = 0;
+};
+
+template < class Visitable, class ... Visitables >
+class VisitorContextBaseBuilder < Visitable, Visitables ... > : public VisitorContextBaseBuilder < Visitables ... > {
+public:
+	using VisitorContextBaseBuilder < Visitables ... >::visit;
+
+	virtual void visit ( const Visitable & ) = 0;
+};
+
+template < class ... Visitables >
+class VisitorContextBase : public VisitorContextBaseBuilder < Visitables ... > {
+};
+
+// -----------------------------------------------------------------
+
+template < class ReturnType, class Visitor, class ... Params >
+class VisitorContextAux {
+	typename std::aligned_storage < SizeOf < ReturnType >::size, AlignOf < ReturnType >::align >::type result;
+
+	std::tuple < Params ... > params;
+
+public:
+	VisitorContextAux ( Params && ... params ) : params ( std::forward < Params > ( params ) ... ) {
+	}
+
+	template < class Inherit, size_t ... Indexes, class ReturnType2 = ReturnType >
+	void call ( const Inherit & inherit, std::index_sequence < Indexes ... >, typename std::enable_if < !std::is_void < ReturnType2 >::value && std::is_same < ReturnType, ReturnType2 >::value >::type * = 0 ) {
+		new ( & result )ReturnType ( Visitor::visit ( inherit, std::forward < Params > ( std::get < Indexes > ( params ) ) ... ) );
+	}
+
+	template < class Inherit, size_t ... Indexes, class ReturnType2 = ReturnType >
+	void call ( const Inherit & inherit, std::index_sequence < Indexes ... >, typename std::enable_if < std::is_void < ReturnType2 >::value && std::is_same < ReturnType, ReturnType2 >::value >::type * = 0 ) {
+		ReturnType ( Visitor::visit ( inherit, std::forward < Params > ( std::get < Indexes > ( params ) ) ... ) );
+	}
+
+	template < class ReturnType2 = ReturnType >
+	ReturnType getResult ( typename std::enable_if < !std::is_void < ReturnType2 >::value && std::is_same < ReturnType, ReturnType2 >::value >::type * = 0 ) {
+		ReturnType res = reinterpret_cast < ReturnType && > ( result );
+
+		 // reinterpret_cast < ReturnType * > ( & result )->~ReturnType ( ); // not needed to delete; data gets moved from on previous line
+		return res;
+	}
+
+	template < class ReturnType2 = ReturnType >
+	void getResult ( typename std::enable_if < std::is_void < ReturnType2 >::value && std::is_same < ReturnType, ReturnType2 >::value >::type * = 0 ) {
+	}
+
+};
+
+} /* namespace std */
+
+#endif /* VISITOR_HPP_ */