Upgrade the existing scope exit utility

src/gui/dialogs/multiplayer/mp_join_game.cpp

@@ -306,7 +306,9 @@ bool mp_join_game::show_flg_select(int side_num, bool first_time)
 		{
 			gui2::dialogs::faction_select flg_dialog(flg, color, side_num);
 			flg_dialog_ = &flg_dialog;
-			utils::scope_exit se([this]() { flg_dialog_ = nullptr; });
+			ON_SCOPE_EXIT(this) {
+				flg_dialog_ = nullptr;
+			};
 
 			if(!flg_dialog.show() && !first_time) {
 				return true;

src/units/udisplay.cpp

@@ -817,7 +817,9 @@ void unit_recruited(const map_location& loc,const map_location& leader_loc)
 	unit_animator animator;
 
 	{
-		utils::scope_exit se([u] () { u->set_hidden(false); });
+		ON_SCOPE_EXIT(u) {
+			u->set_hidden(false);
+		};
 		u->set_hidden(true);
 
 		if (leader_visible && unit_visible) {

src/utils/scope_exit.hpp

@@ -14,15 +14,29 @@
 #pragma once
 
 #include <functional>
+#include <boost/preprocessor/cat.hpp>
 
 namespace utils {
 
+template<typename F>
 class scope_exit {
-	//TODO: with c++17 we could make this a template class with 'F f_' instead of 'std::function<void ()> f_';
-	std::function<void ()> f_;
+	F f_;
 public:
+	explicit scope_exit(F&& f) : f_(std::forward<F>(f)) {}
+	~scope_exit() { f_(); }
+};
+
+struct scope_exit_syntax_support {
 	template<typename F>
-	explicit scope_exit(F&& f) : f_(f) {}
-	~scope_exit() { if(f_) { f_(); }}
+	scope_exit<F> operator<<(F&& f) {
+		return scope_exit<F>(std::forward<F>(f));
+	}
 };
 } // namespace utils
+
+/**
+ * Run some arbitrary code (a lambda) when the current scope exits
+ * The lambda body follows this header, terminated by a semicolon.
+ * @arg ... Capture clause for the lambda.
+ */
+#define ON_SCOPE_EXIT(...) [[maybe_unused]] const auto& BOOST_PP_CAT(scope_exit, __LINE__) = utils::scope_exit_syntax_support() << [__VA_ARGS__]()