Read data from txt file on c++

edit

In your C++ source code you can make a call to the parameter server, where the path to that file is stored.

In your code, with Node Handle nh write:

std::string fpath;
nh.getParam("~private_param_name", fpath);

And now in your launch file add the line (I believe this is how it works, I've never done it this way, you might need to experiment a bit with this):

<node ...>
    <param name="private_param_name" command="'$(find your_package)/src/your_text_file.txt'"
</node>

Or if you want to run from the command line

rosrun your_package_name your_executable _private_param_name:="path_to_your_package/src/your_text_file.txt"

I don't know how you start your source.cpp ! but in your program you have to add the directory where your file is or at least its name if there are in the same directory.