1. 说明

param就是个中央注册表，可以进行参数的增删改查

2. roscpp中的使用

roscpp提供了两套，一套是放在ros::param namespace下，另一套是在ros::NodeHandle下，这两套API的操作基本一样。
推荐使用ros::param的方式，因为ros::param给人感觉，增删改善参数是静态的，实例无关的，这更像参数的本质。

2.1 ros::param 方式

#include<ros/ros.h>

int main(int argc, char **argv)
{
    ros::init(argc, argv, "param_demo");
    ros::NodeHandle nh;
    int parameter1, parameter2, parameter3, parameter4, parameter5;
    
    // get param
    bool ifget1 = ros::param::get("param1", parameter1);
    
    
    // set param
    parameter4 = 4;
    ros::param::set("param4", parameter4);
    
    
    // check param
    bool ifparam6 = ros::param::has("param6");
    if(ifparam6) { 
        ROS_INFO("Param6 exists"); 
    }
    else { 
        ROS_INFO("Param6 doesn't exist"); 
    }
    

    // delete param
    bool ifdeleted6 = ros::param::del("param6");
    if(ifdeleted6) { 
        ROS_INFO("Param6 deleted"); 
    } 
    else { 
        ROS_INFO("Param6 not deleted"); 
    }
    
    // get all param names.     New in ROS indigo
    std::vector<std::string> keys;
    ros::param::search(keys);
    
    return 0;
}

2.2 ros::NodeHandle 方式

#include<ros/ros.h>

int main(int argc, char **argv)
{
    ros::init(argc, argv, "param_demo");
    ros::NodeHandle nh;
    int parameter1, parameter2, parameter3, parameter4, parameter5;
    
    // get param 1
    bool ifget2 = nh.getParam("param2",parameter2);
    
    // get param 2.  33333 is default value. When no param3, parameter3=33333.
    nh.param("param3", parameter3, 33333);


    // set param
    parameter5 = 5;
    nh.setParam("param5",parameter5);


    // check param
    bool ifparam5 = nh.hasParam("param5");
    if(ifparam5) {
        ROS_INFO("Param5 exists");
    }
    else {
        ROS_INFO("Param5 doesn't exist");
    }
    

    // delete param
    bool ifdeleted5 = nh.deleteParam("param5");
    if(ifdeleted5) {
        ROS_INFO("Param5 deleted");
    }
    else {
        ROS_INFO("Param5 not deleted");
    }
    
    // get all param names.     New in ROS indigo
    std::vector<std::string> keys;
    nh.getParamNames(keys);
    
    return 0;
}

2.3 命名空间

有时会看到 ros::NodeHandle n; 和 ros::NodeHandle nh("~"); 两种用法。

ros::NodeHandle n;是全局命名空间。
ros::NodeHandle nh("~");是局部命名空间

如下一个lauch文件，有两个serial参数，一个全局的，一个局部

<launch>
    <!-- global serial -->
    <param name="serial" value="5" />
    <node name="name_demo" pkg="name_demo" type="name_demo" output="screen">
        <!-- local serial -->
        <param name="serial" value="10" />
    </node>
</launch>

使用两种命名空间句柄获取全局和局部参数的用法如下：

#include <ros/ros.h>

int main(int argc, char* argv[])
{
    int serial_number = -1;
    ros::init(argc, argv, "name_demo");

    ros::NodeHandle nh_global;
    ros::NodeHandle nh_local("~");

    // global namespace
    nh_global.getParam("serial", serial_number);                // get global serial    
    nh_global.getParam("name_demo/serial", serial_number);      // get local serial. add "name_demo/"

    
    // local namespace
    nh_local.getParam("serial", serial_number);                 // get local serial
    nh_local.getParam("/serial", serial_number);                // get global serial. add "/"

    ros::spin();
    return 0;
}

3. rospy中的使用

3.1 get param

rospy.get_param()

parameter1 = rospy.get_param("/param1")
parameter2 = rospy.get_param("/param2", default=222)

3.2 set param

rospy.set_param()

rospy.set_param('/param2',2)

3.3 check param

rospy.has_param()

ifparam3 = rospy.has_param('/param3')
if(ifparam3):
    rospy.loginfo('/param3 exists')
else:
    rospy.loginfo('/param3 does not exist')

3.4 delete param

rospy.delete_param()

rospy.delete_param('/param2')

3.5 get all param names

rospy.get_param_names()

params = rospy.get_param_names()
rospy.loginfo('param list: %s', params)

3.6 search param

rospy.search_param()

param = rospy.search_param('param_name')
v = rospy.get_param(param)

当有多个名为param_name参数时，找到离当前node最近的参数名称。
假设当前node在/foo/bar层级, rospy.search_param()会按如下顺序找

/foo/bar/param_name
/foo/param_name
/param_name

4. 在 launch 文件中载入参数

代码修改参数，必须重新编译。
launch文件可以方便的修改参数。

param_demo.launch

<launch>
    <!--param-->
    <param name="param1" value="1" />
    <param name="param2" value="2" />

    <!--rosparam-->
    <rosparam>   
        param3: 3
        param4: 4
        param5: 5
    </rosparam>
    
    <node pkg="param_demo" type="param_demo" name="param_demo" output="screen" />
</launch>

5. 参考

roscpp/Overview/Parameter Server
http://wiki.ros.org/roscpp/Overview/Parameter%20Server

rospy/Overview/Parameter Server
http://wiki.ros.org/rospy/Overview/Parameter%20Server