ROS Microcontroller cpp support?

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rosserial is not tied explicitly to Arduino/AVR, but is instead a serial protocol that could be used with any embedded-scale device.

I believe that it was intended to be easily portable to embedded devices with a C++ compiler.

Something else that may be of interest to you is rosserial_embeddedlinux, which uses the same rosserial protocol to communicate with embedded linux devices. The author writes:

This package contains embedded-linux-specific extensions required to run rosserial on a small embedded linux system such as the VEXPro controller, Chumby alarm clock, WRT54GL router, Raspberry Pi, or many similar devices.

If you are interested in contributing to the longer-term embedded device goals of the ROS community, I encourage you to check out the ROS Embedded SIG.