NOTE: The 6.5 Kernel reports being built as x86_64-linux-gnu-gcc-12 (Ubuntu 12.3.0-1ubuntu1~22.04) and this should be OK. This is because the kernel is not built on your system during install. See https://askubuntu.com/a/1171939/231142 for more info. During any builds it might give you a warning about not matching the installed version since they are not named the same. Installed version reports as gcc-12 (Ubuntu 12.3.0-1ubuntu1~22.04)

gcc-12 should already be installed, but if it isn't, you can install it or reinstall it by running:

sudo apt install --reinstall gcc-12

Then all you should have to do is update the link for gcc to go to the gcc-12 binary:

sudo ln -s -f /usr/bin/gcc-12 /usr/bin/gcc

You can set it back to 11 as well by doing the following:

sudo ln -s -f /usr/bin/gcc-11 /usr/bin/gcc

You can check it by running gcc --version.

terrance@terrance-ubuntu:~$ gcc --version
gcc (Ubuntu 12.3.0-1ubuntu1~22.04) 12.3.0
Copyright (C) 2022 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

This is a common pattern in linux. When there are multiple versions of the same program installed, though the executables are all present in the /usr/bin/ directory, only one of them is "visible" as that program. For example, if you install gcc-9 and gcc-10, both executables are present as /usr/bin/gcc-9 and /usr/bin/gcc-10 but only one of them is visible as gcc. This happens by symlinking a preferred version to the same directory as /usr/bin/gcc. In ubuntu 20.04, the preferred version is gcc-9 and so, gcc-9 is symlinked as gcc.

You can check this by running the following command.

$ which gcc | xargs file

The output will be

/usr/bin/gcc: symbolic link to gcc-9

There are a few things you can do to use gcc-10 as your c compiler.

1. Directly call the gcc-10 executable. Instead of using gcc <code.c>, call gcc-10 <code.c>.
2. You can manually symlink gcc-10 as the preferred gcc. Assuming you did not modify the system paths, the following command can be used.

# ln -s /usr/bin/gcc-10 /usr/local/bin/gcc

This works because, by default, the executables in /usr/local/bin/ take precedence over /usr/bin/.

3. If you are using bash, you can create an alias for gcc as gcc-10. Add the following line to your .bashrc.

alias gcc="gcc-10"

Remember to relaunch bash or source ~/.bashrc.

4. Using update-alternatives (Thanks to @ted-lyngmo for pointing it out). Debian based distributions supply a separate program, that can make symlinking easier / more functional. Read more using man update-alternatives. To use gcc-10 as the preferred gcc, use the following command.

# update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-10 60

The above command says, /usr/bin/gcc is the link needed and the name is gcc, the target executable is /usr/bin/gcc-10 and it has a priority of 60.

This links gcc to /etc/alternatives/gcc, which itself is a symlink to /usr/bin/gcc-10. If a higher priority program is added to update-alternatives, /etc/alternatives/gcc points to the higher priority program.

If you don't have any specific reason, I would also recommend to upgrade to a newer ubuntu version, so that the default gcc is a newer one.

I read on the gcc website that version 9.4 is more up-to-date than some 10.x versions.

With newer gcc versions, new features are added. Support for newer c/c++ standards are also added. Eg. You can read the changes for gcc-10 here. But people still need gcc-9 because some programs only build with gcc-9. So, GNU maintains gcc-9 (and much older versions) for a long time. Bugs are fixed, and newer releases are made. This can happen after the release of a newer gcc version. So, it is very much possible that a version of gcc-9 is newer than a version of gcc-10.

In the absence of specific requirements to the contrary, I tend to use whichever version of gcc is supplied by my (reasonably up-to-date) Linux distribution. This policy has worked pretty well for me so far.

The symlink to the 4.8.2 directory is nothing to worry about, it's normal for the libstdc++ headers on Red Hat Enterprise Linux (and therefore CentOS) to be arranged like that.

gcc --version will tell you the version of the gcc executable in your path.

rpm -q libstdc++-devel will tell you the version of the package that owns the C++ standard library headers.

rpm -ql libstdc++-devel will list the files installed by that package, which will include the files under /usr/include/c++/4.8.2

rpm --verify libstdc++-devel will check that you haven't messed up the C++ headers by replacing them with something else.

The error is more concerning, that implies you have messed something up. My guess would be it's in the from [...omitted by myself as it is irrelevant] part, which may actually be very relevant. std::locale should be declared in <bits/locale_classes.h> which is included before <bits/locale_facets_nonio.h>, so if it wasn't declared my guess is that you have some header that defines _LOCALE_CLASSES_H and prevents the standard library header from being read. Do not define include guards that start with underscores, they are reserved names.

Your Linux Mint comes pre-installed with a GCC package. So first I would recommend you to check if the package is already present in your system by typing the following command in terminal.

apt-cache search gcc

In case you're not having any such package then use the following command in terminal firstly you've add the following repository:

sudo add-apt-repository ppa:ubuntu-toolchain-r/test

then use the next command:

sudo apt-get update
sudo apt-get install g++-4.7 c++-4.7

There is always basic thing we should learn; take it as prerequisite before linux.. Learn googling... try to do more hard search ...