I'm following this manual:
https://sourceware.org/gdb/onlinedocs/gdb/Compiling-and-Injecting-Code.html
$ gdb ./test
(gdb) break main
(gdb) run
(gdb) compile code std::cout << "Hello world\n";
No compiler support for language c++.
(gdb) compile code print("hello world")
No compiler support for language c++.
Does this mean that g++ is not supported? Or I need to configure GDB in some special way?
This is very likely that the debugger you are using is too old.
It sounds too me that prior to 7.12 (maybe even later), user cannot redefine the compile-gcc symbol. As a result, there seems to be no way to specify a compiler for C++.. Nevertheless, the issue has been reported and addressed.
I guess the best solution is to update gdb to a newer version, by compiling it from the official repository.
Another solution would be to pre-compile the code manually and inject it manually within gdb, as explained here. Sounds fun but not sure this is the easy way!