Is C++17 std::shared_mutex not available yet?

c++ mutex libstdc++ c++17 gcc5
alextoind picture alextoind · Mar 25, 2016 · Viewed 10.2k times · Source

Looking at C++ compiler support, it appears that the untimed version of std::shared_mutex is available in GCC 5.0+. However, even with gcc version 5.3.0 20151204 (Ubuntu 5.3.0-3ubuntu1~14.04), and compiling with -std=c++1z, a simple initialization of a shared mutex ends up with:

error: ‘shared_mutex’ in namespace ‘std’ does not name a type
        std::shared_mutex mutex_;

And no, I have already included the proper header: #include <shared_mutex>.

It can't locate the proper header, because it does not seem to exist. Actually, the linker uses the library locate at /usr/include/c++/5/shared_mutex, which contains only the implementation of the std::shared_timed_mutex (like the C++14 standard).

I have installed gcc-5 and g++-5 by adding the repository at ppa:ubuntu-toolchain-r/test and using update-alternatives to properly set up their bins.

Is there something I can do to compile my code correctly using the newest C++17 standard? And probably is a stupid question to ask, but is it too early to start using -std=c++1z even if it should be already supported? Because it is supported, right?

Answer

Jonathan Wakely picture Jonathan Wakely · Feb 17, 2017

The confusion on cppreference was probably because std::shared_mutex really was added to GCC 5.0, in revision 200134. But that was the early incarnation of that type based on a C++1y draft. In fact it was the timed shared mutex, which was called std::shared_mutex at the time.

Before the final C++14 standard was published std::shared_mutex was renamed to std::shared_timed_mutex, and so before the GCC 5.1 release (which is the first release in the 5.x series) the type in libstdc++ was renamed, see revision 207964.

So although at one point during the GCC 5.x pre-release phase there was a std::shared_mutex type, it wasn't the C++17 untimed one, and it got renamed before appearing in any official release of GCC.

Then, during the development of the GCC 6.x release series the C++1z untimed shared mutex got added, reusing the std::shared_mutex name. That's the commit T.C. linked to in the comments above, revision 224158.

So the C++17 untimed shared_mutex was never in any GCC 5.x version. For a brief period before the first 5.x release there was a timed one called std::shared_mutex, but in all proper 5.x releases it's called std::shared_timed_mutex.

The first release to ship the C++17 untimed one was 6.1 in April 2016, so with any GCC release after that you can use std::shared_mutex (as long as you enable C++17 in the compiler, e.g. with the -std=gnu++17 or -std=c++17 flag).

GCC 5 was released in 2015, so expecting to be able to use C++17 with that version is a bit unrealistic. GCC 6.x and 7.x have pretty good C++1z support (but only based on the current drafts at the time of release, of course).

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