Hi what does #include <std_msgs/String.h> mean?

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Hi @sajid1122, so the line #include "std_msgs/String.h includes the std_msgs/String message, which resides in the std_msgs ros package. This is a header that is generated automatically from the String.msgfile in that package. This is the way how ros works. If you want to understand this more deeply, you can refer these links - http://wiki.ros.org/std_msgs. This is the std_msgs package which consists of all the common message types which you can use like int, float, string, boolean etc.

You can refer to this link to understand what exactly a ros msg is - http://wiki.ros.org/msg. Regarding the use of '/' in the include statement, that is because message types in ros are referred to using package resource names meaning we need to specify the ros package that contains these messages followed by a '/' and the message type.

For example, the file geometry_msgs/msg/Twist.msg is commonly referred to as geometry_msgs/Twist. Similar is the case for std_msgs/String but by writing #include "std_msgs/String.h, the string message gets imported in our code like I mentioned earlier.

I suggest reading the documentation given in the links I have provided which can help you in understanding better. Hope this helps you in understanding the meaning of the statement.