Unit: refactor upkeep handling to use a visitor pattern

src/scripting/lua_unit.cpp

@@ -300,13 +300,14 @@ static int impl_unit_get(lua_State *L)
 
 	if(strcmp(m, "upkeep") == 0) {
 		unit::upkeep_t upkeep = u.upkeep_raw();
-		if(boost::get<unit::upkeep_full>(&upkeep) != nullptr){
-			lua_pushstring(L, "full");
-		} else if(boost::get<unit::upkeep_loyal>(&upkeep) != nullptr){
-			lua_pushstring(L, "loyal");
+
+		if(int* v = boost::get<int>(&upkeep)) {
+			lua_push(L, *v);
 		} else {
-			lua_push(L, boost::get<int>(upkeep));
+			const std::string type = boost::apply_visitor(unit::upkeep_type_visitor(), upkeep);
+			lua_pushstring(L, type.c_str());
 		}
+
 		return 1;
 	}
 	if(strcmp(m, "advancements") == 0) {

src/units/unit.cpp

@@ -1467,15 +1467,8 @@ int unit::upkeep() const
 	if(can_recruit()) {
 		return 0;
 	}
-	else if(boost::get<upkeep_full>(&upkeep_) != nullptr) {
-		return level();
-	}
-	else if(boost::get<upkeep_loyal>(&upkeep_) != nullptr) {
-		return 0;
-	}
-	else {
-		return boost::get<int>(upkeep_);
-	}
+
+	return boost::apply_visitor(upkeep_value_visitor(*this), upkeep_);
 }
 
 bool unit::loyal() const
@@ -1894,7 +1887,7 @@ void unit::apply_builtin_effect(std::string apply_to, const config& effect)
 		if(increase.empty() == false) {
 			max_experience_ = utils::apply_modifier(max_experience_, increase, 1);
 		}
-	} else if(apply_to == "loyal") {
+	} else if(apply_to == upkeep_loyal::type()) {
 		upkeep_ = upkeep_loyal();;
 	} else if(apply_to == "status") {
 		const std::string& add = effect["add"];
@@ -2429,28 +2422,21 @@ void unit::parse_upkeep(const config::attribute_value& upkeep)
 	if(upkeep_int != -99) {
 		upkeep_ = upkeep_int;
 	}
-	else if(upkeep == "loyal" || upkeep == "free") {
+	else if(upkeep == upkeep_loyal::type() || upkeep == "free") {
 		upkeep_ = upkeep_loyal();
 	}
-	else if(upkeep == "full") {
+	else if(upkeep == upkeep_full::type()) {
 		upkeep_ = upkeep_full();
 	}
 	else {
 		WRN_UT << "Fund invalid upkeep=\"" << upkeep <<  "\" in a unit\n";
 		upkeep_ = upkeep_full();
 	}
 }
+
 void unit::write_upkeep(config::attribute_value& upkeep) const
 {
-	if(boost::get<upkeep_full>(&upkeep_) != nullptr) {
-		upkeep = "full";
-	}
-	else if(boost::get<upkeep_loyal>(&upkeep_) != nullptr) {
-		upkeep = "loyal";
-	}
-	else {
-		upkeep = boost::get<int>(upkeep_);
-	}
+	upkeep = boost::apply_visitor(upkeep_type_visitor(), upkeep_);
 }
 
 // Filters unimportant stats from the unit config and returns a checksum of

src/units/unit.hpp

@@ -740,10 +740,58 @@ class unit
 	 */
 	int upkeep() const;
 
-	// TODO: look for a cleaner solution than these structs
-	struct upkeep_full {};
-	struct upkeep_loyal {};
-	using upkeep_t =  boost::variant<upkeep_full, upkeep_loyal, int>;
+	struct upkeep_full
+	{
+		static std::string type() { static std::string v = "full"; return v; }
+	};
+
+	struct upkeep_loyal
+	{
+		static std::string type() { static std::string v = "loyal"; return v; }
+	};
+
+	/** Visitor helper class to fetch the appropriate upkeep value. */
+	class upkeep_value_visitor : public boost::static_visitor<int>
+	{
+	public:
+		explicit upkeep_value_visitor(const unit& unit) : u_(unit) {}
+
+		int operator()(const upkeep_full&) const
+		{
+			return u_.level();
+		}
+
+		int operator()(const upkeep_loyal&) const
+		{
+			return 0;
+		}
+
+		int operator()(int v) const
+		{
+			return v;
+		}
+
+	private:
+		const unit& u_;
+	};
+
+	/** Visitor helper struct to fetch the upkeep type flag if applicable, or the the value otherwise. */
+	struct upkeep_type_visitor : public boost::static_visitor<std::string>
+	{
+		template<typename T>
+		typename std::enable_if<!std::is_same<int, T>::value, std::string>::type
+		operator()(T& v) const
+		{
+			return v.type();
+		}
+
+		std::string operator()(int v) const
+		{
+			return std::to_string(v);
+		}
+	};
+
+	using upkeep_t = boost::variant<upkeep_full, upkeep_loyal, int>;
 
 	/**
 	 * Gets the raw variant controlling the upkeep value.
@@ -755,7 +803,7 @@ class unit
 		return upkeep_;
 	}
 
-	/** Sets the upkeep value to a specific numeric value. */
+	/** Sets the upkeep value to a specific value value. Does not necessarily need to be numeric */
 	void set_upkeep(upkeep_t v)
 	{
 		upkeep_ = v;