Message type between publisher and subscriber

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Hi, all! I have a question about the message type between publisher and subscriber with the same topic.

What is the mechanism when publishing a normal type message (std_msg::String) and we can subscribe the message with normal type (std_msg::String) so as the shared_ptr version of it (std_msg::StringConstPtr)?

For example : in the publisher and subscriber tutorial

The publisher publish the normal type while the subscriber subscribe the shared_ptr type.

Thanks!