ROS scattered knowledge points and error resolution

One 、 Knowledge point
１．rviz Set the point cloud color
Remove 　use rainbow, Set up min_color and max_color Relevant point clouds can be displayed according to the set color .
２．Ｃ＋＋ Time calculation

#include<ctime>

clock_t startTime,endTime;
startTime = clock();// Timing begins 
for (int i = 0; i < 2147483640; i++)
	{
    
        i++;    }
            endTime = clock();// End of the timing 
     cout << "The run time is: " <<(double)(endTime - startTime) / CLOCKS_PER_SEC << "s" << endl;

3.ros NodeHandle and ros NodeHandle("~")
Private node handles can generally be used to obtain node configuration parameters , from launch Get in file

 ros::init (argc, argv, "ply_load");

ros::NodeHandle nh;

ros::NodeHandle pri_nh("~")

ros::Publisher pcl_pub = nh.advertise<sensor_msgs::PointCloud2> ("pcl_output", 1);// The published topic has no node name namespace , Topic name is /pcl_output
 ros::Publisher pcl_pub = nh.advertise<sensor_msgs::PointCloud2> ("pcl_output", 1);// The topic is called /ply_load/pcl_output

4. explicit keyword
   Role is ： about The constructor of a parameter , Cancel its implicit conversion .
   Implicit conversion ：

class CxString  //  Not used explicit Class declaration for keyword ,  That is, implicitly declaring  
{
      
public:  
    char *_pstr;  
    int _size;  
    CxString(int size)  
    {
      
        _size = size;                // string Default size for  
        _pstr = malloc(size + 1);    //  Distribute string Of memory  
        memset(_pstr, 0, size + 1);  
    }  
}

 When called ：
 CxString string2 = 10;    //  This is a OK Of ,  by CxString Pre allocation 10 Byte size memory  
 CxString string6 = 'c';   //  This is also OK Of ,  It's actually calling theta CxString(int size),  And size be equal to 'c' Of ascii code  

 Because of the implicit conversion ：
CxString string2 = 10;
 Equivalent to ：
CxString tem(10);
CxString string2 = tem;

Added explicit after , This implicit conversion is canceled ：

explicit CxString(int size)  
    {
      
        _size = size;  
        //  Same code as above ,  Omit ... 
    } 
 When called ：
 CxString string2 = 10;    //  That's not good 
 CxString string6 = 'c';   //  It doesn't work 

5.std::iota
Used to generate continuous numbers

// from 0 Start generating a set of numbers to fill the array 
 std::iota(unassigned_objects.begin(), unassigned_objects.end(), 0);

Two 、 Error resolution
2.
sudo apt-get update Report errors

W:  Error checking digital signature . This warehouse has not been updated , So still use the previous index file .GPG  error ：http://packages.ros.org/ros/ubuntu bionic InRelease:  The following signature is invalid ： EXPKEYSIG F42ED6FBAB17C654 Open Robotics <[email protected].org>
E:  Warehouse  “http://ppa.launchpad.net/zarquon42/meshlab/ubuntu bionic Release”  No,  Release  file .
N:  Cannot safely update with this source , So the source is disabled by default .

resolvent ：
Note that the last number corresponds to the above

sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv F42ED6FBAB17C654

E:  Warehouse  “http://ppa.launchpad.net/zarquon42/meshlab/ubuntu bionic Release”  No,  Release  file .
N:  Cannot safely update with this source , So the source is disabled by default .

resolvent ：

 stay etc/apt/sources.list.d  Delete the corresponding... From the directory ppa
cd etc/apt/sources.list.d
sudo mv zarquon42-ubuntu-meshlab-bionic.list zarquon42-ubuntu-meshlab-bionic.list.bak

3. stay ros There is no source Generally, you will report an error and cannot find the package . such source once bash Just file it , There is also a real lack of a package , Install it.

ResourceNotFound: rgbd_launch
ROS path [0]=/opt/ros/kinetic/share/ros
ROS path [1]=/opt/ros/kinetic/share

sudo apt-get install ros-kinetic-rgbd-launch

4.ROS There is an error when defining subscribers in 　invalid use of non-static member function
initialization subscriber Solution of time problems
The wrong way to write ：

datasub = nh.subscribe("/objects", 10, &track_processor::process);

After the modification ：

datasub = nh.subscribe("/objects", 10, &track_processor::process, this);

And note that the callback function must return a value of void In the form of .