Require a BaseParameters type at compile time via enable_if

include/server/api/parameters_parser.hpp

@@ -4,6 +4,10 @@
 #include "engine/api/route_parameters.hpp"
 #include "engine/api/table_parameters.hpp"
 
+#include <boost/optional/optional.hpp>
+
+#include <type_traits>
+
 namespace osrm
 {
 namespace server
@@ -11,25 +15,28 @@ namespace server
 namespace api
 {
 
-// Starts parsing and iter and modifies it until iter == end or parsing failed
-template<typename ParameterT>
-boost::optional<ParameterT> parseParameters(std::string::iterator& iter, std::string::iterator end);
-
-// copy on purpose because we need mutability
-template<typename ParameterT>
-inline boost::optional<ParameterT> parseParameters(std::string options_string)
+namespace detail
 {
-    auto iter = options_string.begin();
-    return parseParameters<ParameterT>(iter, options_string.end());
+template <typename T> using is_parameter_t = std::is_base_of<engine::api::BaseParameters, T>;
+} // ns detail
+
+// Starts parsing and iter and modifies it until iter == end or parsing failed
+template <typename ParameterT,
+          typename std::enable_if<detail::is_parameter_t<ParameterT>::value, int>::type = 0>
+boost::optional<ParameterT> parseParameters(std::string::iterator &iter, std::string::iterator end);
+
+// Copy on purpose because we need mutability
+template <typename ParameterT,
+          typename std::enable_if<detail::is_parameter_t<ParameterT>::value, int>::type = 0>
+boost::optional<ParameterT> parseParameters(std::string options_string)
+{
+    const auto first = options_string.begin();
+    const auto last = options_string.end();
+    return parseParameters<ParameterT>(first, last);
 }
 
-template<>
-boost::optional<engine::api::RouteParameters> parseParameters(std::string::iterator& iter, std::string::iterator end);
-template<>
-boost::optional<engine::api::TableParameters> parseParameters(std::string::iterator& iter, std::string::iterator end);
-
-}
-}
-}
+} // ns api
+} // ns server
+} // ns osrm
 
 #endif