Specifying C++17 stops C++14 features

m3g1dd

Environment

Qt 5.11.3
macOS 10.13.6 High Sierra
Xcode version 9.4.1

Clang++ is:

$ /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++ -v
Apple LLVM version 9.1.0 (clang-902.0.39.2)

Code

C++ code is:

template<class T, class O = T>
using IteratorOnly = std::enable_if_t<
    !std::is_same_v<typename std::iterator_traits<T>::value_type, void>, O
>;

C++14 added type trait *_t versions and C++17 added *_v versions. So, compiling the code with C++17 is expected to work.

Observation

Putting CONFIG += c++14 inside QMake project file, throws one error:

error: no template named 'is_same_v' in namespace 'std'; did you mean 'is_same'?

Putting CONFIG += c++17 inside QMake project file, throws two errors:

error: no template named 'enable_if_t' in namespace 'std'; did you mean 'enable_if'?

error: no template named 'is_same_v' in namespace 'std'; did you mean 'is_same'?

Problem

I cannot figure out the observation cause. Is it due to a bad installation of the standard library/compiler? Any other possibility?

Note

I'm able to run the same code on these without any problem:

Windows 10, Qt 5.12.9, MS Visual Studio 2017
Linux openSUSE Leap 15.1, Qt 5.12.7, GCC 7.5.0

m3g1dd

Finally, I used C++14 on QMake project file:

CONFIG += c++14
QMAKE_CXXFLAGS += -std=c++14

Then this error is received:

error: no template named 'is_same_v' in namespace 'std'; did you mean 'is_same'?

I bypass the error by this:

#ifdef __APPLE__

// On our current macOS build environment:
// Qt 5.11.3
// macOS 10.13.6 High Sierra
// Xcode version 9.4.1
//
// C++14 works for 'std::enable_if_t' but not `std::is_same_v`
// C++17 works for none of them!
//
// A workarond is to self-define `is_same_v` according to this:
// https://en.cppreference.com/w/cpp/types/is_same

template< class T, class U >
inline constexpr bool is_same_v = std::is_same<T, U>::value;

template<class T, class O = T>
using IteratorOnly = std::enable_if_t<
    !is_same_v<typename std::iterator_traits<T>::value_type, void>, O
>;

#else // __APPLE__

template<class T, class O = T>
using IteratorOnly = std::enable_if_t<
    !std::is_same_v<typename std::iterator_traits<T>::value_type, void>, O
>;

#endif // __APPLE__

m3g1dd

Another observation

CONFIG += c++14 inside QMake project file, invokes clang++ with std=gnu++1y flag:

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++ -c -pipe -stdlib=libc++ -g -std=gnu++1y  -arch x86_64 -isysroot ...

CONFIG += c++17 inside QMake project file, invokes clang++ with std=gnu++11 flag:

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++ -c -pipe -stdlib=libc++ -g -std=gnu++11  -arch x86_64 -isysroot ...

m3g1dd

Yet another observation

Putting QMAKE_CXXFLAGS += -std=c++14 inside QMake project file, passes both -std=c++14 and -std=gnu++11 to clang++:

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++ -c -pipe -stdlib=libc++ -std=c++14 -g -std=gnu++11  -arch x86_64 -isysroot ...

Putting QMAKE_CXXFLAGS += -std=c++17 inside QMake project file, passes both -std=c++17 and -std=gnu++11 flags to clang++:

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++ -c -pipe -stdlib=libc++ -std=c++17 -g -std=gnu++11  -arch x86_64 -isysroot ...

Christian Ehrlicher

Upgrade your Qt version to a version which adds the proper flags or add them correctly by your own. 5.11 is too old to support the correct -std=c++17 flag.

m3g1dd

@Christian-Ehrlicher Is there any reference on how to add my own proper flags to Qt 5.11? I have never done it before, thanks!

Christian Ehrlicher

Maybe in the qmake documentation. I don't use qmake since ages so I can't really help here.
Maybe you can remove flags with QMAKE_CXXFLAGS -= -std=c++11 ?

m3g1dd

Finally, I used C++14 on QMake project file:

CONFIG += c++14
QMAKE_CXXFLAGS += -std=c++14

Then this error is received:

error: no template named 'is_same_v' in namespace 'std'; did you mean 'is_same'?

I bypass the error by this:

#ifdef __APPLE__

// On our current macOS build environment:
// Qt 5.11.3
// macOS 10.13.6 High Sierra
// Xcode version 9.4.1
//
// C++14 works for 'std::enable_if_t' but not `std::is_same_v`
// C++17 works for none of them!
//
// A workarond is to self-define `is_same_v` according to this:
// https://en.cppreference.com/w/cpp/types/is_same

template< class T, class U >
inline constexpr bool is_same_v = std::is_same<T, U>::value;

template<class T, class O = T>
using IteratorOnly = std::enable_if_t<
    !is_same_v<typename std::iterator_traits<T>::value_type, void>, O
>;

#else // __APPLE__

template<class T, class O = T>
using IteratorOnly = std::enable_if_t<
    !std::is_same_v<typename std::iterator_traits<T>::value_type, void>, O
>;

#endif // __APPLE__