To find which library is being used you could run

Copy $ /sbin/ldconfig -p | grep stdc++
    libstdc++.so.6 (libc6) => /usr/lib/libstdc++.so.6

The list of compatible versions for libstdc++ version 3.4.0 and above is provided by

Copy $ strings /usr/lib/libstdc++.so.6 | grep LIBCXX
 GLIBCXX_3.4
 GLIBCXX_3.4.1
 GLIBCXX_3.4.2
 ...

For earlier versions the symbol GLIBCPP is defined.

The date stamp of the library is defined in a macro __GLIBCXX__ or __GLIBCPP__ depending on the version:

Copy// libdatestamp.cxx
#include <cstdio>

int main(int argc, char* argv[]){
#ifdef __GLIBCPP__
    std::printf("GLIBCPP: %d\n",__GLIBCPP__);
#endif
#ifdef __GLIBCXX__
    std::printf("GLIBCXX: %d\n",__GLIBCXX__);
#endif
   return 0;
}

$ g++ libdatestamp.cxx -o libdatestamp
$ ./libdatestamp
GLIBCXX: 20101208

The table of datestamps of libstdc++ versions is listed in the documentation:

What is libstdc++11?

My program compiled but it won't link, and I suspect it's because I have multiple versions of the C++ standard library installed, one for g++ version 7 and one for g++ version 8. I need it to link with libstdc++-8-dev to get C++-17 features.

The reason your program won't link is because you are not passing the correct parameters during the compilation process. The compiler can see the function declarations but the linker cannot find the function implementations.

You need to tell the system which version / implementation to use by explicitly telling the version of the library you want to use and you do this with the -l parameter. In your case, you will probably need to preceed the -l parameter with the -L parameter.

Here is an example:

g++ myfile.cpp -L/usr/lib/gcc/x86_64-linux-gnu/7 -lstdc++

If you have v8 installed, unless the libs are installed somewhere else, you can do:

g++ myfile.cpp -L/usr/lib/gcc/x86_64-linux-gnu/8 -lstdc++

That should do it.

Do the following:

sudo updatedb
locate libstdc

and edit your question and post the output. I will try to build the exact command.

UPDATE: I can see you have:

/usr/lib/gcc/x86_64-linux-gnu/8/libstdc++.a  
/usr/lib/gcc/x86_64-linux-gnu/8/libstdc++.so  
/usr/lib/gcc/x86_64-linux-gnu/8/libstdc++fs.a  

Can you see if this works?

g++ test.cpp -std=c++11 -L/usr/lib/gcc/x86_64-linux-gnu/8 -lstdc++

What is libstdc++.so.6? Is this just a library that gcc ships with?

It’s GCC’s implementation of the C++ Standard Library.

What are all the different GLIBCXX_*? Are these also libraries?

These are version symbols, used to allow the library to remain backward-compatible while still being able to change its API. See What do the multiple GLIBC versions mean in the output of ldd? for details.

Question is why do I have different versions of these libraries in different folders?

The copy in /usr/lib64 is your distribution’s version; the copy in /usr/local/lib64 was installed by something else.

When running gcc how do I tell it to use the 6.0.20 library in /usr/local/lib64 instead of the older one?

You can try building with -L/usr/local/lib64, and running with LD_LIBRARY_PATH=/usr/local/lib64 (if necessary).

Clang does come with its own standard library implementation, it's called libc++. You can use it by adding -stdlib=libc++ to your compile command.

That being said, there are various ways to check Clang/libstdc++ C++ support:

1. Clang has the __has_feature macro (and friends) that can be used to detect language features and language extenstions.
2. Libstdc++ has its own version macros, see the documentation. You'll need to include a libstdc++ header to get these defined though.
3. GCC has its version macros which you already discovered, but those would need to be manually compared to the documentation.

And also, this took me 2 minutes of googling.

As you see in the clean installation the 4.8.1-2ubuntu1~10.04.1 version of the package doesn't exist in precise (I'm wondering from where you got it, since only saucy has that package, maybe a PPA). The best method is removing the package and installing from the repositories. In some cases apt-get will refuse so you must force it to do your will:

sudo apt-get -f install libstdc++6=4.6.3-1ubuntu5

I would check for other packages that might have the same condition.

You look in the manual, specifically at http://gcc.gnu.org/onlinedocs/libstdc++/manual/abi.html which shows the library version numbers for each GCC release.