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C++'s std::string class provides an indexing operator implementation that lets you access individual characters in the string. See std::string::operator[] for details.

In your particular case, you could do something like:

size_t my_index = ...;
char the_byte = msg->data[my_index];

Now the_byte contains the char at position index_of_byte_you_are_interested_in in msg->data (which is of type std::string (wiki/msg)).

As I wrote in my comment on your OP though: std_msgs/String is really not a good message to use for interfacing with CAN wrappers (or for storing byte buffers in, period).

C++'s std::string class provides an indexing operator implementation that lets you access individual characters in the string. See std::string::operator[] for details.

In your particular case, you could do something like:

size_t my_index = ...;
char the_byte = msg->data[my_index];

Now the_byte contains the char at position index_of_byte_you_are_interested_inmy_index in msg->data (which is of type std::string (wiki/msg)).

As I wrote in my comment on your OP though: std_msgs/String is really not a good message to use for interfacing with CAN wrappers (or for storing byte buffers in, period).