Is the C++ STL fully supported on Arduino?

The STL is not a part of Arduino IDE.

Although efficient by desktop standards, the consensus is it doesn't fit comfortably in an Arduino's limited space. That said, here's someone who seems to have done it:

https://github.com/maniacbug/StandardCplusplus

Check out the forks, they seem more up to date

STL is efficient on the platform it was designed for, which is personal computers and similar-scale devices, where allocating a single byte in the heap consumes a 4k memory page (that's several times as much as ALL Arduino RAM), and where array indexes can be efficiently replaced by pointers (8-bit microcontrollers need at least two commands to deal with a pointer). So no, it's not efficient with Arduino.

Think of quicksort algorithm - it performs very well on large lists, but is easily beaten by a simple sort if you need to sort an array of 5 elements. Being asymptotically efficient doesn't mean being efficient in every case.

The STL is not a part of Arduino IDE.

Another answer is mentioning https://github.com/maniacbug/StandardCplusplus although this library doesn't seem to be maintained anymore.

Maybe a better idea could be to try https://github.com/mike-matera/ArduinoSTL

It's a port of uClibc++ packaged as an Arduino library.