Can I use C++ stl in avr-gcc?
It is very well possible to use C++ on the AVR. Arduino has been doing it for ages. Unfortunately, avr-g++ does not ship with a C++ standard library (the successor to the old STL), so you have to rely on C++ core language features. C++, just like C, has many features that don't map well to small microcontrollers; you need to find out which ones. For starters, exceptions, RTTI (typeid), new/delete, and anything that requires those (especially standard library containers like vector, string and queue) don't really work here. Typical microcontroller programs use ring buffers and fixed arrays instead.

On the other hand, classes (including virtual functions), templates/Metaprogramming, constexpr, proper constants, avoiding macros, function overloads and RAII can be put to good use without sacrificing efficiency.

To compile C++ code, substitute calls to avr-gcc with calls to avr-g++. Make sure to use a recent version, since C++ support is in active development.