Should I Use Libc++ or Libstdc++

Should I use libc++ or libstdc++?

I would use the native library for each OS i.e. libstdc++ on GNU/Linux and libc++ on Mac OS X.

libc++ is not 100% complete on GNU/Linux, and there's no real advantage to using it when libstdc++ is more complete. Also, if you want to link to any other libraries written in C++ they will almost certainly have been built with libstdc++ so you'll need to link with that too to use them.

More info here about the completeness of libc++ on various platforms.

libc++ vs libstdc++ std::is_move_assignable: Which is the most correct?

For anything referenceable, the two implementations do the same thing, since the extraneous const in libc++ is meaningless but also harmless.

(Judging from the diff, it certainly looks like temporary insanity to me :) Seems to be a C&P issue from a (wrong) implementation of is_copy_assignable.)

For anything non-referenceable (i.e., cv void or abominable function types), libstdc++ returns false_type.

In libc++, add_{l,r}value_reference returns it unchanged (this depends on an issue resolution that postdates C++14). Sprinkling a const on top does nothing for AFTs and adds a const for the voidy types.

We then go to is_assignable, which SFINAE-tests the well-formedness of declval<T>() = declval<U>(), for either T == U == some AFT or T == some void type and U = some const-qualified void type. In all cases the expression is ill-formed (in a SFINAE-friendly manner), so we get false_type back.

The two are equivalent.

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

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

Cmake differentiate between libc++ and libstdc++

Something like this should work:

if ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -stdlib=libc++")
endif()

When/why are libstdc++ and libc++ symbols incompatible?

So, is the problem std::allocator<char>?

What? No. It's everything in your example.

The doc you quoted clearly says the goal is compatibility for "low-level features such as exception objects, rtti and memory allocation".

std::set and std::string are not "low-level features such as exception objects, rtti and memory allocation". They are very definitely not compatible between libc++ and libstdc++, which are completely different libraries with completely different implementations.

The compatible pieces are things like std::type_info and std::exception (and the derived exception types in <stdexcept>) because those are part of the basic language runtime. Anything above that, such as containers, strings, algorithms, I/O, locales etc. is not compatible.

how to use libc++ and libstdc++ for different libraries in one project

Do not do this!

Why?

1. depending on the name mangling scheme used it might not even be possible because libc++ is going to have the same symbols as libstdc++
2. if there are no symbol collisions you are probably going to run into undefined behavior when you take objects from one library and mix them with objects/functions from the other.
3. class structures will not be compatible. Internal class structures will be implemented or laid out differently.
4. libstdc++ is no longer available on iOS 12. Your program will not run on it.
5. The correct way to fix your problem is to download/compile updated versions of your libraries. Period.

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