gcc check standard version

gccstdversion.sh

# Requesting c++20 standard in gcc commandline does not ensure that it is supported or will be used,
# dumping the __cplusplus reveals that gcc completely ignores this request without any warning or error
# Such problems leads to similar error messages:
# main.cpp:1:24: error: ‘bit_cast’ is not a member of ‘std’; did you mean ‘bad_cast’?
#
#
# This shell script prints 
#   #define __cplusplus 202002L
# or 
#   #define __cplusplus 201709L
# depending on the version of gcc
#
# in my case `gcc --version` reports following
#   gcc (Ubuntu 10.4.0-4ubuntu1~22.04) 10.4.0
# vs
#   gcc (Ubuntu 11.3.0-1ubuntu1~22.04.1) 11.3.0
#
# To switch to gcc version 12 and update the gcc/g++ aliases, install more recent version and switch to it:
#   sudo apt-get install g++-12 gcc-12
#   sudo update-alternatives --remove-all g++
#   sudo update-alternatives --remove-all gcc
#   sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-12 50
#   sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-12 50
#
# check alias for g++:
#   sudo update-alternatives --get-selections | grep g++

g++ -std=c++20 -dM -E -x c++ /dev/null | grep -F __cplusplus