I'm still on Ubuntu 22.04 LTS but needed g++14. The sudo apt-get gcc-14 did not work for me, as it installed clang++14 for some reason (perhaps a misconfiguration on my part). What did work for me was following the instructions I found at https://www.dedicatedcore.com/blog/install-gcc-compiler-ubuntu/

The steps I took:

sudo apt install build-essential
sudo apt install libmpfr-dev libgmp3-dev libmpc-dev -y
wget https://ftp.gnu.org/gnu/gcc/gcc-14.1.0/gcc-14.1.0.tar.gz
tar -xf gcc-14.1.0.tar.gz
cd gcc-14.1.0
./configure -v --build=$(uname -m)-linux-gnu --host=$(uname -m)-linux-gnu --target=$(uname -m)-linux-gnu --prefix=/usr/local/gcc-14.1.0 --enable-checking=release --enable-languages=c,c++ --disable-multilib --program-suffix=-14.1.0
make
sudo make install

And if you would like to make it the default:

sudo update-alternatives --install /usr/bin/g++ g++ /usr/local/gcc-14.1.0/bin/g++-14.1.0 14
sudo update-alternatives --install /usr/bin/gcc gcc /usr/local/gcc-14.1.0/bin/gcc-14.1.0 14

After that, g++ showed I was running version 14.1.0. I was then able to compile my project that included some c++20/23 features that were not in the previous versions of g++ (chrono/format).

The best way to correctly install gcc-4.9 and set it as your default gcc version use:

sudo add-apt-repository ppa:ubuntu-toolchain-r/test
sudo apt-get update
sudo apt-get install gcc-4.9 g++-4.9
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.9 60 --slave /usr/bin/g++ g++ /usr/bin/g++-4.9

The --slave, with g++, will cause g++ to be switched along with gcc, to the same version. But, at this point gcc-4.9 will be your only version configured in update-alternatives, so add 4.8 to update-alternatives, so there actually is an alternative, by using:

sudo apt-get install gcc-4.8 g++-4.8
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.8 60 --slave /usr/bin/g++ g++ /usr/bin/g++-4.8

Then you can check which one that is set, and change back and forth using:

sudo update-alternatives --config gcc

If you have an issue with update-alternatives gcc priority 60 not being higher than previous versions installed you can use the previous update-alternatives --config gcc command to check installed versions and use:

sudo update-alternatives --remove gcc

Or:

sudo update-alternatives --remove-all gcc

NOTE: You could skip installing the PPA Repository and just use /usr/bin/gcc-4.9-base but I prefer using the fresh updated toolchains.

For GCC 5.X or 6, the packages (and correspondingly, the commands) are just called gcc-5, gcc-6, etc. This is due to the change in GCC's version scheme, where 5.1 is the first GCC 5 release, and future 5.X releases are for bug fixes.

gcc-12 is not available in ubuntu 20.04, so we need to compile it from source code, here are the steps which I borrowed from this video:

• Step 1: clone gcc source code and checkout gcc-12 branch

$ git clone https://gcc.gnu.org/git/gcc.git gcc-source
$ cd gcc-source/
$ git branch -a
$ git checkout remotes/origin/releases/gcc-12

• Step 2: make another build dir

Note this is important as running ./configure from within the source directory is not supported as documented here.

$ mkdir ../gcc-12-build
$ cd ../gcc-12-build/
$ ./../gcc-source/configure --prefix=$HOME/install/gcc-12 --enable-languages=c,c++

• Step 3: installing GCC prequisites and run configure again

The missing libraries will be shown in above ./confgiure output, search and install them one by one.

$ apt-cache search MPFR
$ sudo apt-get install libmpfrc++-dev
$ apt-cache search MPC | grep dev
$ sudo apt-get install libmpc-dev
$ apt-cache search GMP | grep dev
$ sudo apt-get install libgmp-dev
$ sudo apt-get install gcc-multilib
$ ./../gcc-source/configure --prefix=$HOME/install/gcc-12 --enable-languages=c,c++

An alternative is to run the download_prerequisites script.

$ cd ../
$ cd gcc-source/
$ ./contrib/download_prerequisites
$ ./../gcc-source/configure --prefix=$HOME/install/gcc-12 --enable-languages=c,c++

• Step 4: compile gcc-12

$ make -j16

Still flex is missing:

$ sudo apt-get install flex
$ ./../gcc-source/configure --prefix=$HOME/install/gcc-12 --enable-languages=c,c++
$ make -j16
$ make install

Another way is to use Ubuntu 22.04 where gcc-12 is available. In Ubuntu 22.04, gcc-12 can be installed with apt:

$ sudo apt install gcc-12

I am currently using Ubuntu 22.04 but frustrated that I can't load the latest {something}

First step: You're using Ubuntu 22.04, which is already the old long-term stable releas of Ubuntu. The immediate first thing you should do before even considering switching distro is updating yours; to 24.04, specifically. If you feel the need to install very recent software more often, you won't benefit from long-term supported distro releases, so the most recent version of Ubuntu (as of now, 24.10, soon 25.04) and biannual updates would serve you better (or one of the even more fast-moving distros, like Fedora, or even rolling release distros).

That instantly makes sudo apt install gcc-14 g++-14 gfortran-14 (depending on whether you need C, C++ and/or Fortran compilers) give you gcc 14.2.0; beautifully installed side-by-side with any other Ubuntu-installed gcc version, selectable by specifying the compiler to be gcc-14 instead of gcc for the programs you want to compile.

I did build a cross-compiler once. It kept crashing my system during the build.

Your system crashing when you tried to build a bleeding-edge compiler from source has nothing to do with the distro being incompatible with that compiler. It honestly sounds like a hardware stability issue!

It's a pretty normal thing to compile a modern compiler on an older platform – after all, someone has to do that to develop that newer compiler, right?

In the case of GCC, it's (last time I checked) sufficient to specify a --prefix=/path/to/prefix --with-local-prefix=/path/to/prefix at the ./configure step, into which the newly compiled compiler suite would be installed when you make; sudo make install. Then, a simple export LD_LIBRARY_PATH=/path/to/prefix/lib64:${LD_LIBRARY_PATH} and export PATH=/path/to/prefix/bin:${PATH} is what it takes to then make use of this compiler.

Other than that, often you can just actually get the newer compilers packaged for your distro, either from additional package repositories, or through backporting them. gcc-15 is, unsurprisingly due to the lack of existence of the 15. release of GCC yet, not available on any Ubuntu version, but chances are not bad it would appear in the backports repository for the current Ubuntu LTS release, i.e., of Ubuntu 24.04.