exception.cpp proves that catching an exception by ref is more optimal.

cpp/exception.cpp

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+/* Check cf5-opt.vim defs.
+VIM: let g:lcppflags="-std=c++11 -O2 -pthread"
+VIM: let g:wcppflags="/O2 /EHsc /DWIN32"
+VIM: let g:cppflags=g:Iboost.g:Itbb
+VIM: let g:ldflags=g:Lboost.g:Ltbb.g:tbbmalloc.g:tbbmproxy
+VIM: let g:ldlibpath=g:Bboost.g:Btbb
+VIM: let g:argv=""
+VIM-: let g:cf5output=0
+*/
+#include <iostream>
+#include <exception>
+
+/*
+ 	This experiment proves that catching an exception by reference
+	is more effective since it eliminates unneeded copy.
+
+	Also re-throwing an exception throws the original exception but not
+	the exception caught.
+*/
+struct ex
+{
+	mutable int v;
+
+	ex()
+		: v(0)
+	{
+		std::cout << "ex::default" << std::endl;
+	}
+	ex( const ex& e )
+		: v(e.v)
+	{
+		std::cout << "ex::copy" << std::endl;
+	}
+	
+	ex& operator = ( const ex& e )
+	{
+		v = e.v;
+		std::cout << "ex::op =" << std::endl;
+	}
+};
+
+void a ()
+{
+	throw ex();
+}
+
+void b ()
+try {
+	a();
+} catch(ex& e) {
+	e.v |= 1;
+	throw;
+}
+
+void c ()
+try {
+	b();
+} catch(const ex& e) {
+	e.v |= 2;
+	throw;
+}
+
+void d ()
+try {
+	c();
+} catch( ex e) {
+	e.v |= 4;
+	throw;
+}
+
+
+int main ( void )
+{
+	try
+	{
+		d();
+	}
+	catch ( const ex& e )
+	{
+		std::cout << e.v << std::endl;
+	}
+
+	return 0;
+}
+