ros2 publisher for other program

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The ROS 2 publisher executable is only an example to demonstrate how to use API to publish messages. If you want to add a publisher to your program you need to add similar code to your software to do the publishing. As a consequence you will need to link against ROS 2.

You can either use the precompiled binaries (VS 2015/2017/2019 should be binary compatible) or build ROS 2 from source. I would expect that VC6++ won't cut it since the ROS 2 code uses fairly recent C++14 features which that version of VS will likely be unable to parse / compile.