accessing-ros-parameters.md

Accessing ROS parameters

The following code demonstrates how to access a ROS 2 parameter using the SyncParameterClient class:

#include "rclcpp/node.hpp"
#include "rclcpp/parameter_client.hpp"

int main(int argc, char** argv)
{
    rclcpp::init(argc, argv);
    rclcpp::node::Node::SharedPtr node = rclcpp::node::Node::make_shared("test_node");

    rclcpp::parameter_client::SyncParameterClient client(node);

    double dblVal = client.get_parameter("my_double_val");
    int value = client.get_parameter("my_value", 1);
    if (client.has_parameter("optional_param")) {
        std::string opt = client.get_parameter("optional_param", std::string(""));
    }
}

First, you must initialize ROS and create a node handle:

rclcpp::init(argc, argv);
rclcpp::node::Node::SharedPtr node = rclcpp::node::Node::make_shared("test_node");

Then, using the node, you can create a SyncParameterClient:

rclcpp::parameter_client::SyncParameterClient client(node);

Now you can get the values of specific parameters:

double dblVal = client.get_parameter("my_double_val");

This line attempts to get the value of a parameter named "my_double_val". A runtime error is thrown in the event that the parameter does not exist, otherwise the value is stored in the 'dblValue' variable.

int value = client.get_parameter("my_value", 1);

This will get the value of a parameter named "my_value" and store it in the variable 'value'. If the parameter does not exist, the default value (1) will be returned.

if (client.has_parameter("optional_param")) {
    std::string opt = client.get_parameter("optional_param", std::string(""));
}

This code first checks that the parameter exists, and then grabs the value of the "optional_param" value.