Get single parameter (#245)

* added function to get parameter by exact name

* added ros1 style get_parameter for parameter variant

* added ros1 style get_param for non-variant types

* Make the get_parameter functions call a private base function

* Parameter Variant requires name in constructor

* Cleaned up to no longer need private function

* Made exception message more clear

rclcpp/include/rclcpp/node.hpp

@@ -253,6 +253,18 @@ public:
   std::vector<rclcpp::parameter::ParameterVariant>
   get_parameters(const std::vector<std::string> & names) const;
 
+  RCLCPP_PUBLIC
+  const rclcpp::parameter::ParameterVariant
+  get_parameter(const std::string & name) const;
+
+  RCLCPP_PUBLIC
+  bool get_parameter(
+    const std::string & name,
+    rclcpp::parameter::ParameterVariant & parameter) const;
+
+  template<typename ParameterT>
+  bool get_parameter(const std::string & name, ParameterT & parameter) const;
+
   RCLCPP_PUBLIC
   std::vector<rcl_interfaces::msg::ParameterDescriptor>
   describe_parameters(const std::vector<std::string> & names) const;

rclcpp/include/rclcpp/node_impl.hpp

@@ -368,6 +368,16 @@ void Node::register_param_change_callback(CallbackT && callback)
   this->parameters_callback_ = callback;
 }
 
+template<typename ParameterT>
+bool Node::get_parameter(const std::string & name, ParameterT & parameter) const
+{
+  rclcpp::parameter::ParameterVariant parameter_variant(name, parameter);
+  bool result = get_parameter(name, parameter_variant);
+  parameter = parameter_variant.get_value<ParameterT>();
+
+  return result;
+}
+
 }  // namespace node
 }  // namespace rclcpp
 

rclcpp/src/rclcpp/node.cpp

@@ -242,6 +242,31 @@ Node::get_parameters(
   return results;
 }
 
+const rclcpp::parameter::ParameterVariant
+Node::get_parameter(const std::string & name) const
+{
+  rclcpp::parameter::ParameterVariant parameter;
+
+  if (get_parameter(name, parameter)) {
+    return parameter;
+  } else {
+    throw std::out_of_range("Parameter '" + name + "' not set");
+  }
+}
+
+bool Node::get_parameter(const std::string & name,
+  rclcpp::parameter::ParameterVariant & parameter) const
+{
+  std::lock_guard<std::mutex> lock(mutex_);
+
+  if (parameters_.count(name)) {
+    parameter = parameters_.at(name);
+    return true;
+  } else {
+    return false;
+  }
+}
+
 std::vector<rcl_interfaces::msg::ParameterDescriptor>
 Node::describe_parameters(
   const std::vector<std::string> & names) const