generate_parameter_library

Generate C++ for ROS 2 parameter declaration, getting, and validation using declarative YAML. The generated library contains a C++ struct with specified parameters. Additionally, dynamic parameters and custom validation are made easy.

Killer Features

• Declarative YAML syntax for ROS 2 Parameters converted into C++ struct
• Declaring, Getting, Validating, and Updating handled by generated code
• Dynamic ROS 2 Parameters made easy
• Custom user specified validator functions

Basic Usage

1. Create YAML parameter codegen file
2. Add parameter library generation to project
3. Use generated struct into project source code

Create yaml parameter codegen file

Write a yaml file to declare your parameters and their attributes.

src/turtlesim_parameters.yaml

turtlesim:
  background:
    r: {
      type: int,
      default_value: 0,
      description: "Red color value for the background, 8-bit",
      validation: {
        bounds<>: [0, 255]
      }
    }
    g: {
      type: int,
      default_value: 0,
      description: "Green color value for the background, 8-bit",
      validation: {
        bounds<>: [0, 255]
      }
    }
    b: {
      type: int,
      default_value: 0,
      description: "Blue color value for the background, 8-bit",
      validation: {
        bounds<>: [0, 255]
      }
    }

Add parameter library generation to project

package.xml

<depend>generate_parameter_library</depend>

CMakeLists.txt

find_package(generate_parameter_library REQUIRED)

generate_parameter_library(
  turtlesim_parameters # cmake target name for the parameter library
  src/turtlesim_parameters.yaml # path to input yaml file
)

add_executable(minimal_node src/turtlesim.cpp)
target_link_libraries(minimal_node PRIVATE
  rclcpp::rclcpp
  turtlesim_parameters
)

Use generated struct into project source code

src/turtlesim.cpp

#include <rclcpp/rclcpp.hpp>
#include "turtlesim_parameters.hpp"

int main(int argc, char * argv[])
{
  rclcpp::init(argc, argv);
  auto node = std::make_shared<rclcpp::Node>("turtlesim");
  auto param_listener = std::make_shared<turtlesim::ParamListener>(node);
  auto params = param_listener->get_params();

  auto color = params.background;
  RCLCPP_INFO(node->get_logger(),
    "Background color (r,g,b): %d, %d, %d",
    color.r, color.g, color.b);

  return 0;
}

Use example yaml files in tests

When using parameter library generation it can happen that there are issues when executing tests since parameters are not defined and the library defines them as mandatory. To overcome this it is recommended to define example yaml files for tests and use them as follows:

find_package(ament_cmake_gtest REQUIRED)
add_rostest_with_parameters_gtest(test_turtlesim_parameters test/test_turtlesim_parameters.cpp
  ${CMAKE_CURRENT_SOURCE_DIR}/test/example_turtlesim_parameters.yaml)
target_include_directories(test_turtlesim_parameters PRIVATE include)
target_link_libraries(test_turtlesim_parameters turtlesim_parameters)
ament_target_dependencies(test_turtlesim_parameters rclcpp)

when using gtest, or:

find_package(ament_cmake_gmock REQUIRED)
add_rostest_with_parameters_gmock(test_turtlesim_parameters test/test_turtlesim_parameters.cpp
  ${CMAKE_CURRENT_SOURCE_DIR}/test/example_turtlesim_parameters.yaml)
target_include_directories(test_turtlesim_parameters PRIVATE include)
target_link_libraries(test_turtlesim_parameters turtlesim_parameters)
ament_target_dependencies(test_turtlesim_parameters rclcpp)

when using gmock test library.

🤖 P.S. having this example yaml files will make your users very grateful because they will always have a working example of a configuration for your node.

Detailed Documentation

• Cpp namespace
• Parameter definition
• Built-In Validators
• Custom validator functions
• Nested structures
• Use generated struct in Cpp
• Dynamic Parameters
• Example Project
• Generated code output

Cpp namespace

The root element of the YAML file determines the namespace used in the generated C++ code. We use this to put the Params struct in the same namespace as your C++ code.

cpp_namespace:
# additionally fields  ...

Parameter definition

The YAML syntax can be thought of as a tree since it allows for arbitrary nesting of key-value pairs. For clarity, the last non-nested value is referred to as a leaf. A leaf represents a single parameter and has the following format.

cpp_namespace:
  param_name: {
    type: int,
    default_value: 3,
    read_only: true,
    description: "A read only  integer parameter with a default value of 3",
    validation:
      # validation functions ...
  }

A parameter is a YAML dictionary with the only required key being type.

Field	Description
type	The type (string, double, etc) of the parameter.
default_value	Value for the parameter if the user does not specify a value.
read_only	Can only be set at launch and are not dynamic.
description	Displayed by ros2 param describe.
validation	Dictionary of validation functions and their parameters.

The types of parameters in ros2 map to C++ types.

Parameter Type	C++ Type
string	std::string
double	double
int	int
bool	bool
string_array	std::vector<std::string>
double_array	std::vector<double>
int_array	std::vector<int>
bool_array	std::vector<bool>
string_fixed_XX	FixedSizeString<XX>

Fixed size types are denoted with a suffix _fixed_XX, where XX is the desired size. The corresponding C++ type is a data wrapper class for conveniently accessing the data. Note that any fixed size type will automatically use a size_lt validator. Validators are explained in the next section.

Built-In Validators

Validators are C++ functions that take arguments represented by a key-value pair in yaml. The key is the name of the function. The value is an array of values that are passed in as parameters to the function. If the function does not take any values you write null or [] to for the value.

joint_trajectory_controller:
  command_interfaces: {
    type: string_array,
    description: "Names of command interfaces to claim",
    validation: {
      size_gt<>: [0],
      unique<>: null,
      subset_of<>: [["position", "velocity", "acceleration", "effort",]],
    }
  }

Above are validations for command_interfaces from ros2_controllers. This will require this string_array to have these properties:

• There is at least one value in the array
• All values are unique
• Values are only in the set ["position", "velocity", "acceleration", "effort",]

You will note that some validators have a suffix of <>, this tells the code generator to pass the C++ type of the parameter as a function template. Some of these validators work only on value types, some on string types, and others on array types. The built-in validator functions provided by this package are:

Value validators

Function	Arguments	Description
bounds<>	[lower, upper]	Bounds checking (inclusive)
lower_bounds<>	[lower]	Lower bounds (inclusive)
upper_bounds<>	[upper]	Upper bounds (inclusive)
lt<>	[value]	parameter < value
gt<>	[value]	parameter > value
lt_eq<>	[value]	parameter <= value (upper_bounds)
gt_eq<>	[value]	parameter >= value (lower_bounds)
one_of<>	[[val1, val2, ...]]	Value is one of the specified values

String validators

Function	Arguments	Description
fixed_size<>	[length]	Length string is specified length
size_gt<>	[length]	Length string is greater than specified length
size_lt<>	[length]	Length string is less less specified length
not_empty<>	[]	String parameter is not empty
one_of<>	[[val1, val2, ...]]	String is one of the specified values

Array validators

Function	Arguments	Description
unique<>	[]	Contains no duplicates
subset_of<>	[[val1, val2, ...]]	Every element is one of the list
fixed_size<>	[length]	Number of elements is specified length
size_gt<>	[length]	Number of elements is greater than specified length
size_lt<>	[length]	Number of elements is less less specified length
not_empty<>	[]	Has at-least one element
element_bounds<>	[lower, upper]	Bounds checking each element (inclusive)
lower_element_bounds<>	[lower]	Lower bound for each element (inclusive)
upper_element_bounds<>	[upper]	Upper bound for each element (inclusive)

Custom validator functions

Validators are functions that return a tl::expected<void, std::string> type and accept a rclcpp::Parameter const& as their first argument and any number of arguments after that can be specified in YAML. Validators are C++ functions defined in a header file similar to the example shown below.

Here is an example custom allocator.

#include <rclcpp/rclcpp.hpp>

#include <fmt/core.h>
#include <tl_expected/expected.hpp>

namespace my_project {

tl::expected<void, std::string> integer_equal_value(
    rclcpp::Parameter const& parameter, int expected_value) {
  int param_value = parameter.as_int();
    if (param_value != expected_value) {
        return tl::make_unexpected(fmt::format(
            "Invalid value {} for parameter {}. Expected {}",
            param_value, parameter.get_name(), expected_value);
    }

  return {};
}

}  // namespace my_project

To configure a parameter to be validated with the custom validator function integer_equal_value with an expected_value of 3 you could would this to the YAML.

validation: {
  "my_project::integer_equal_value": [3]
}

Nested structures

After the top level key, every subsequent non-leaf key will generate a nested c++ struct. The struct instance will have the same name as the key.

cpp_name_space:
  nest1:
    nest2:
      param_name: { # this is a leaf
        type: string_array
      }

The generated parameter value can then be access with params.nest1.nest2.param_name

Use generated struct in Cpp

The generated header file is named based on the target library name you passed as the first argument to the cmake function. If you specified it to be turtlesim_parameters you can then include the generated code with #include "turtlesim_parameters.hpp".

#include "turtlesim_parameters.hpp"

In your initialization code, create a ParamListener which will declare and get the parameters. An exception will be thrown if any validation fails or any required parameters were not set. Then call get_params on the listener to get a copy of the Params struct.

auto param_listener = std::make_shared<turtlesim::ParamListener>(node);
auto params = param_listener->get_params();

Dynamic Parameters

If you are using dynamic parameters, you can use the following code to check if any of your parameters have changed and then get a new copy of the Params struct.

if (param_listener->is_old(params_)) {
  params_ = param_listener->get_params();
}

Example Project

See example project for a complete example of how to use the generate_parameter_library.

Generated code output

The generated code is primarily consists of two major components:

1. struct Params that contains values of all parameters and
2. class ParamListener that handles parameter declaration, updating, and validation. The general structure is shown below.

namespace cpp_namespace {

struct Params {
  int param_name = 3;
  struct {
    struct{
      std::string param_name;
      // arbitrary nesting depth...
    } nest2;
  } nest1;
  // for detecting if the parameter struct has been updated
  rclcpp::Time __stamp;
};

class ParamListener {
 public:
  ParamListener(rclcpp::ParameterInterface);
  ParamListener(rclcpp::Node::SharedPtr node)
    : ParameterListener(node->get_parameters_interface()) {}
  ParamListener(rclcpp_lifecycle::LifecycleNode::SharedPtr node)
    : ParameterListener(node->get_parameters_interface()) {}

  // create a copy of current parameter values
  Params get_params() const;

  // returns true if parameters have been updated since last time get_params was called
  bool is_old(Params const& other) const;

  // loop over all parameters: perform validation then update
  rcl_interfaces::msg::SetParametersResult update(const std::vector<rclcpp::Parameter> &parameters);

  // declare all parameters and throw exception if non-optional value is missing or validation fails
  void declare_params(const std::shared_ptr<rclcpp::node_interfaces::NodeParametersInterface>& parameters_interface);

 private:
  Params params_;
};

} // namespace cpp_namespace

The structure of the Params struct and the logic for declaring and updating parameters is generated from a YAML file specification.