When is it necessary to use the flag -stdlib=libstdc++?

When is it necessary to use use the flag -stdlib=libstdc++ for the compiler and linker when compiling with gcc?

Short answer: never

Longer answer: -stdlib is a Clang flag and will not work with any version of GCC ever released. On Mac OS X sometimes the gcc and g++ commands are actually aliases for Clang not GCC, and the version of libstdc++ that Apple ships is ancient (circa 2008) so of course it doesn't support C++11. This means that on OS X when using Clang-pretending-to-be-GCC, you can use -stdlib=libc++ to select Clang's new C++11-compatible library, or you can use -stdlib=libstdc++ to select the pre-C++11 antique version of libstdc++ that belongs in a museum. But on GNU/Linux gcc and g++ really are GCC not Clang, and so the -stdlib option won't work at all.

Does the compiler automatically use libstdc++?

Yes, GCC always uses libstdc++ unless you tell it to use no standard library at all with the -nostdlib option (in which case you either need to avoid using any standard library features, or use -I and -L and -l flags to point it to an alternative set of header and library files).

I am using gcc4.8.2 on Ubuntu 13.10 and I would like to use the c++11 standard. I already pass -std=c++11 to the compiler.

You don't need to do anything else. GCC comes with its own implementation of the C++ standard library (libstdc++) which is developed and tested alongside GCC itself so the version of GCC and the version of libstdc++ are 100% compatible. If you compile with -std=c++11 then that enables the C++11 features in g++ compiler and also the C++11 features in the libstdc++ headers.

The compiler uses the libstdc++ automatically, if you use the g++ frontend, not the gcc frontend.

On Linux: In general, all commonly available linux distributions will use libstdc++ by default, and all modern versions of GCC come with a libstdc++ that supports C++11. If you want to compile c++11 code here, use one of:

• g++ -std=c++11 input.cxx -o a.out (usually GNU compiler)
• g++ -std=gnu++11 input.cxx -o a.out

On OS X before Mavericks: g++ was actually an alias for clang++ and Apple's old version of libstdc++ was the default. You could use libc++ (which included c++11 library support) by passing -stdlib=libc++. If you want to compile c++11 code here, use one of:

• g++ -std=c++11 -stdlib=libc++ input.cxx -o a.out (clang, not GNU compiler!)
• g++ -std=gnu++11 -stdlib=libc++ input.cxx -o a.out (clang, not GNU compiler!)
• clang++ -std=c++11 -stdlib=libc++ input.cxx -o a.out
• clang++ -std=gnu++11 -stdlib=libc++ input.cxx -o a.out

On OS X since Mavericks: libc++ is the default and you should not pass any -stdlib=<...> flag. Since Xcode 10, building against libstdc++ is not supported at all anymore. Existing code built against libstdc++ will keep working because libstdc++.6.dylib is still provided, but compiling new code against libstdc++ is not supported.

• clang++ -std=c++11 input.cxx -o a.out
• clang++ -std=gnu++11 input.cxx -o a.out