Packages roscpp and std_msgs

edit

by using std_msgs::String type, I define an msg variable and publish it via in the node. In this case, which dependency provide std_msgs::String type for us ?

std_msgs::String is defined in a header part of the std_msgs package.

Someone told me that all headers defined for messages are provided by roscpp. Dependency of std_msgs package takes role in creating a custom message. If you do not create user-defined message, you do not make std_msgs a dependency for your package. All the headers for the messages are given to you by roscpp.

Is this correct ?

No. This is not correct.

For your example already we require std_msgs as a dependency, as that package provides the String message type.

It also provides a number of other message types, see the index.

roscpp itself (ie: the package) only provides a few services and messages, but those are not typically used as parts of other (ie: custom) messages. See the index again.