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Add ParameterDescriptor to 'Using parameters in a class (C++)' (#2865) (
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#3111)

* Rename node name in "Using parameters" C++ tutorial to be consistent with Python tutorial.

Signed-off-by: Tan Chian Fern <chianfern@gmail.com>
Signed-off-by: Chris Lalancette <clalancette@openrobotics.org>
(cherry picked from commit 87bf20a)

Co-authored-by: chianfern <chianfern@gmail.com>
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@mergify @chianfern
mergify[bot] and chianfern authored Oct 18, 2022
1 parent e1dfdb5 commit 2f1e680
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159 changes: 101 additions & 58 deletions source/Tutorials/Beginner-Client-Libraries/Using-Parameters-In-A-Class-CPP.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -70,37 +70,45 @@ Inside the ``ros2_ws/src/cpp_parameters/src`` directory, create a new file calle

.. code-block:: C++

#include <rclcpp/rclcpp.hpp>
#include <chrono>
#include <string>
#include <functional>
#include <string>

#include <rclcpp/rclcpp.hpp>

using namespace std::chrono_literals;

class ParametersClass: public rclcpp::Node
class MinimalParam : public rclcpp::Node
{
public:
ParametersClass()
: Node("parameter_node")
{
this->declare_parameter<std::string>("my_parameter", "world");
timer_ = this->create_wall_timer(
1000ms, std::bind(&ParametersClass::respond, this));
}
void respond()
{
this->get_parameter("my_parameter", parameter_string_);
RCLCPP_INFO(this->get_logger(), "Hello %s", parameter_string_.c_str());
}
private:
std::string parameter_string_;
rclcpp::TimerBase::SharedPtr timer_;
public:
MinimalParam()
: Node("minimal_param_node")
{
this->declare_parameter("my_parameter", "world");

timer_ = this->create_wall_timer(
1000ms, std::bind(&MinimalParam::timer_callback, this));
}

void timer_callback()
{
std::string my_param =
this->get_parameter("my_parameter").get_parameter_value().get<std::string>();

RCLCPP_INFO(this->get_logger(), "Hello %s!", my_param.c_str());

std::vector<rclcpp::Parameter> all_new_parameters{rclcpp::Parameter("my_parameter", "world")};
this->set_parameters(all_new_parameters);
}

private:
rclcpp::TimerBase::SharedPtr timer_;
};

int main(int argc, char** argv)
int main(int argc, char ** argv)
{
rclcpp::init(argc, argv);
rclcpp::spin(std::make_shared<ParametersClass>());
rclcpp::spin(std::make_shared<MinimalParam>());
rclcpp::shutdown();
return 0;
}
Expand All @@ -110,68 +118,103 @@ Inside the ``ros2_ws/src/cpp_parameters/src`` directory, create a new file calle
The ``#include`` statements at the top are the package dependencies.

The next piece of code creates the class and the constructor.
The first line of this constructor creates our parameter.
Our parameter has the name ``my_parameter`` and is assigned the default value ``world``.
Next, ``timer_`` is initialized, which causes the ``respond`` function to be executed once a second.
The first line of this constructor creates a parameter with the name ``my_parameter`` and a default value of ``world``.
The parameter type is inferred from the default value, so in this case it would be set to a string type.
Next the ``timer_`` is initialized with a period of 1000ms, which causes the ``timer_callback`` function to be executed once a second.

.. code-block:: C++

class ParametersClass: public rclcpp::Node
class MinimalParam : public rclcpp::Node
{
public:
ParametersClass()
: Node("parameter_node")
{
this->declare_parameter<std::string>("my_parameter", "world");
timer_ = this->create_wall_timer(
1000ms, std::bind(&ParametersClass::respond, this));
}

The first line of our ``respond`` function gets the parameter ``my_parameter`` from the node, and stores it in ``parameter_string_``.
The ``RCLCPP_INFO`` function ensures the message is logged.
public:
MinimalParam()
: Node("minimal_param_node")
{
this->declare_parameter("my_parameter", "world");

timer_ = this->create_wall_timer(
1000ms, std::bind(&MinimalParam::timer_callback, this));
}

The first line of our ``timer_callback`` function gets the parameter ``my_parameter`` from the node, and stores it in ``my_param``.
Next the ``RCLCPP_INFO`` function ensures the message is logged.
The ``set_parameters`` function then sets the parameter ``my_parameter`` back to the default string value ``world``.
In the case that the user changed the parameter externally, this ensures it is always reset back to the original.

.. code-block:: C++

void respond()
void timer_callback()
{
this->get_parameter("my_parameter", parameter_string_);
RCLCPP_INFO(this->get_logger(), "Hello %s", parameter_string_.c_str());
std::string my_param =
this->get_parameter("my_parameter").get_parameter_value().get<std::string>();

RCLCPP_INFO(this->get_logger(), "Hello %s!", my_param.c_str());

std::vector<rclcpp::Parameter> all_new_parameters{rclcpp::Parameter("my_parameter", "world")};
this->set_parameters(all_new_parameters);
}

Last is the declaration of ``timer_`` and ``parameter_string_``
Last is the declaration of ``timer_``

.. code-block:: C++

private:
std::string parameter_string_;
rclcpp::TimerBase::SharedPtr timer_;

Following our ``ParametersClass`` is our ``main``.
Here ROS 2 is initialized, and ``rclcpp::spin`` starts processing data from the node.
Following our ``MinimalParam`` is our ``main``.
Here ROS 2 is initialized, an instance of the ``MinimalParam`` class is constructed, and ``rclcpp::spin`` starts processing data from the node.

.. code-block:: C++

int main(int argc, char** argv)
int main(int argc, char ** argv)
{
rclcpp::init(argc, argv);
rclcpp::spin(std::make_shared<ParametersClass>());
rclcpp::spin(std::make_shared<MinimalParam>());
rclcpp::shutdown();
return 0;
}

2.1.1 (Optional) Add ParameterDescriptor
""""""""""""""""""""""""""""""""""""""""
Optionally, you can set a descriptor for the parameter.
Descriptors allow you to specify a text description of the parameter and its constraints, like making it read-only, specifying a range, etc.
For that to work, the code in the constructor has to be changed to:

.. code-block:: C++

// ...

class MinimalParam : public rclcpp::Node
{
public:
MinimalParam()
: Node("minimal_param_node")
{
auto param_desc = rcl_interfaces::msg::ParameterDescriptor{};
param_desc.description = "This parameter is mine!";

this->declare_parameter("my_parameter", "world", param_desc);

timer_ = this->create_wall_timer(
1000ms, std::bind(&MinimalParam::timer_callback, this));
}

The rest of the code remains the same.
Once you run the node, you can then run ``ros2 param describe /minimal_param_node my_parameter`` to see the type and description.


2.2 Add executable
~~~~~~~~~~~~~~~~~~

Now open the ``CMakeLists.txt`` file. Below the dependency ``find_package(rclcpp REQUIRED)`` add the following lines of code.

.. code-block:: console
.. code-block:: cmake
add_executable(parameter_node src/cpp_parameters_node.cpp)
ament_target_dependencies(parameter_node rclcpp)
add_executable(minimal_param_node src/cpp_parameters_node.cpp)
ament_target_dependencies(minimal_param_node rclcpp)
install(TARGETS
parameter_node
minimal_param_node
DESTINATION lib/${PROJECT_NAME}
)
Expand Down Expand Up @@ -245,13 +288,13 @@ Now run the node:

.. code-block:: console
ros2 run cpp_parameters parameter_node
ros2 run cpp_parameters minimal_param_node
The terminal should return the following message every second:

.. code-block:: console
[INFO] [parameter_node]: Hello world
[INFO] [minimal_param_node]: Hello world!
Now you can see the default value of your parameter, but you want to be able to set it yourself.
There are two ways to accomplish this.
Expand All @@ -265,7 +308,7 @@ Make sure the node is running:

.. code-block:: console
ros2 run cpp_parameters parameter_node
ros2 run cpp_parameters minimal_param_node
Open another terminal, source the setup files from inside ``ros2_ws`` again, and enter the following line:

Expand All @@ -278,10 +321,10 @@ To change it simply run the following line in the console:

.. code-block:: console
ros2 param set /parameter_node my_parameter earth
ros2 param set /minimal_param_node my_parameter earth
You know it went well if you get the output ``Set parameter successful``.
If you look at the other terminal, you should see the output change to ``[INFO] [parameter_node]: Hello earth``
If you look at the other terminal, you should see the output change to ``[INFO] [minimal_param_node]: Hello earth!``

3.2 Change via a launch file
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Expand All @@ -299,8 +342,8 @@ In there, create a new file called ``cpp_parameters_launch.py``
return LaunchDescription([
Node(
package="cpp_parameters",
executable="parameter_node",
name="custom_parameter_node",
executable="minimal_param_node",
name="custom_minimal_param_node",
output="screen",
emulate_tty=True,
parameters=[
Expand All @@ -309,7 +352,7 @@ In there, create a new file called ``cpp_parameters_launch.py``
)
])
Here you can see that we set ``my_parameter`` to ``earth`` when we launch our node ``parameter_node``.
Here you can see that we set ``my_parameter`` to ``earth`` when we launch our node ``minimal_param_node``.
By adding the two lines below, we ensure our output is printed in our console.

.. code-block:: console
Expand Down Expand Up @@ -381,7 +424,7 @@ The terminal should return the following message every second:

.. code-block:: console
[parameter_node-1] [INFO] [custom_parameter_node]: Hello earth
[INFO] [custom_minimal_param_node]: Hello earth!
Summary
-------
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