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I have recently updated gcc and g++ to version 7.2. I would like to try out std::experimental::any and std::variant in particular, and I am using Qt 5.9.1 in QtCreator.

So far I have written this in the project file:

CONFIG += c++17

And I have added the correct headers in the correct places:

#include <variant>
#include <experimental/any>

Any works fine, no problems there. However, when I include the variant header file, I get this error:

/usr/include/c++/7/bits/c++17_warning.h:32: error: #error This file requires compiler and library support for the ISO C++ 2017 standard. This support must be enabled with the -std=c++17 or -std=gnu++17 compiler options.

#error This file requires compiler and library support \ ^~~~~

I have tried a variety of things in the project file, here is the full list:

CONFIG += c++17

&

CONFIG += c++1z

&

QMAKE_CXXFLAGS += -std=c++17

&

QMAKE_CXXFLAGS += -std=c++1z

&

CONFIG += c++17
QMAKE_CXXFLAGS += -std=c++17

&

CONFIG += c++1z
QMAKE_CXXFLAGS += -std=c++1z

&

CONFIG += c++11
CONFIG += c++14
CONFIG += c++17

That's every stab in the dark I could think of. What am I missing? And why does experimental::any compile when variant doesn't?

I know I shouldn't use CONFIG += c++xx and QMAKE_CXXFLAGS together in this way, but I thought I'd give it a go as nothing else works. For bonus points, I'm also wondering, should I add the CONFIG calls for 14 and 11 when I already CONFIG for 17?

EDIT:

Here is the compiler output with most of my filenames scrubbed out:

18:04:10: Running steps for project AIQt...
18:04:10: Configuration unchanged, skipping qmake step.
18:04:10: Starting: "/usr/bin/make" 
/home/pete/Qt/5.9.1/gcc_64/bin/qmake -o Makefile ../AIQt/AIQt.pro -spec linux-g++ CONFIG+=debug CONFIG+=qml_debug
WARNING: Failure to find: ../src/stdafx.h
WARNING: Failure to find: ../src/Csound/csd.h
g++ -c -pipe -g -Wall -W -D_REENTRANT -fPIC -DQT_DEPRECATED_WARNINGS -DQT_QML_DEBUG -DQT_DATAVISUALIZATION_LIB -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I../AIQt -I. -I../src -I../src/AIBase -I../src/Maths -I../src/Random -isystem /usr/local/include/csound -I../../../../Qt/5.9.1/gcc_64/include -I../../../../Qt/5.9.1/gcc_64/include/QtDataVisualization -I../../../../Qt/5.9.1/gcc_64/include/QtWidgets -I../../../../Qt/5.9.1/gcc_64/include/QtGui -I../../../../Qt/5.9.1/gcc_64/include/QtCore -I. -isystem /usr/include/libdrm -I. -I../../../../Qt/5.9.1/gcc_64/mkspecs/linux-g++ -o main.o ../AIQt/main.cpp
In file included from /usr/include/c++/7/variant:35:0,
                 from ..###,
                 from ..###,
                 from ..###,
                 from ..###,
                 from ..###,
                 from ..###,
                 from ..###,
                 from ..###:
/usr/include/c++/7/bits/c++17_warning.h:32:2: error: #error This file requires compiler and library support for the ISO C++ 2017 standard. This support must be enabled with the -std=c++17 or -std=gnu++17 compiler options.
 #error This file requires compiler and library support \
  ^~~~~
In file included from ..###,
                 from ..###
                 from ..###,
                 from ..###,
                 from ..###,
                 from ..###,
                 from ..###:
../src/AIBase/Geno.h:70:18: error: ‘variant’ in namespace ‘std’ does not name a type
             std::variant m_valueVariant;
                  ^~~~~~~
In file included from ..###,
                 from ..###,
                 from ..###,
                 from ..###,
                 from ..###,
                 from ..###:
../src/AIBase/Pheno.h:22:13: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
             const double getGenoValue(size_t genoIndex) const;
             ^~~~~
../src/AIBase/Pheno.h:24:13: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
             const UserRating getRating() const;
             ^~~~~
In file included from ..###,
                 from ..###:
../AIRadioQt/GraphDialog.h:16:15: warning: declaration ‘struct ar::ai::ClusterList’ does not declare anything
 class ar::ai::ClusterList;
               ^~~~~~~~~~~
make: *** [main.o] Error 1
18:04:13: The process "/usr/bin/make" exited with code 2.
Error while building/deploying project AIQt (kit: Qt 5.9.1 GCC 64bit)
The kit Qt 5.9.1 GCC 64bit has configuration issues which might be the root cause for this problem.
When executing step "Make"
18:04:13: Elapsed time: 00:03.

ANSWER:

As mentioned by nwp, I just had to clean it and rebuild.

Another poster also commented that CONFIG += c++17 doesn't appear to be supported yet, so it is necessary to use QMAKE_CXXFLAGS += -std=c++17. He quickly deleted his comment though, so I am unable to thank him personally for going to the effort of checking the docs for me.

Keith Thompson
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Iron Attorney
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    In the bottom there should be a tab "Compile Output". What flags starting with "-std=" are there? Note that there may be multiple of them and only the last one counts. – nwp Oct 06 '17 at 17:33
  • That's a good point, I didn't think to check. It mentions no -std= flags. Do I need to add the CONFIG line in a specific place? Compiler output is added to question... – Iron Attorney Oct 06 '17 at 17:42
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    Rightclick on your project and select "clean" and then "Run qmake". For reasons I will never understand changing the .pro file doesn't always update the makefile properly. – nwp Oct 06 '17 at 17:48
  • Hot damn! As long as I'm using QMAKE_CXXFLAGS += -std=c++17, and not the CONFIG equivalent, it works after I cleaned and rebuild. Thanks man! Someone did briefly write a comment about the docs not showing support for CONFIG += c++17. He deleted his comment, but if he's reading this, that was a helpful thing to say. Feel free to write that comment again and I'll upvote it, haha. But yes, rebuilding was the answer, thanlk very much nwp. – Iron Attorney Oct 06 '17 at 17:52
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    Don't include the answer in the question. Instead, post an answer and accept it. – Keith Thompson Oct 06 '17 at 18:25
  • Right you are, I'll post it when I get home. – Iron Attorney Oct 08 '17 at 15:52
  • ClangCodeModel still has a lot of errors parsing C++17 headers. Hopefully they get clang 5.0 included in the build soon. – nw. Oct 17 '17 at 16:30
  • Another paranoic way is to delete all *.o *.exe Makefile* etc, check compiler (left bottom corner) and run `qmake`, rebuild project. It is better to clean do delete make-files IMHO ... – Ulrich Von Rekkenin May 24 '18 at 05:42

4 Answers4

43

CONFIG += c++17 can be used with Qt 5.12 and later.

For Qt 5.11 and earlier, it is not a recognized QMake flag and you have to get your hands a bit dirty.

Adding QMAKE_CXXFLAGS += -std=c++17 does the job for GCC & Clang; for MSVC you will probably need to specify /std:c++17 or /std:c++latest.

minexew
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    This is true, but it was the clean and build again that actually fixed my issue, so anyone reading this, remember to do that too – Iron Attorney Mar 14 '18 at 15:43
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    QtCreator 4.6 and Qt 5.10 with both `CONFIG += c++17` and `QMAKE_CXXFLAGS += -std=c++17`, works here. – Alexander May 14 '18 at 10:15
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    @Alexander I can't use `` when I add those. Are you sure it works for things that only exist in C++17? – kayleeFrye_onDeck Jun 28 '18 at 02:28
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    @Alexander maybe it's build-tools specific. For MSVC, I had to change the second one to `QMAKE_CXXFLAGS += /std:c++17` -- I got the correctly formatted MSVC C++17 flag by inspecting the build output of a VS2017 C++ project that was explicitly set to c++17. – kayleeFrye_onDeck Jun 28 '18 at 02:33
  • Needed to use `` with msvc and adding `CONFIG += c++17` worked for me on Qt 5.12.1 and QtCreator 4.8.1 – Kasheen Feb 21 '19 at 22:57
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    `CONFIG += c++17` doesn't get the job done for me with Qt 5.12.1 on Windows 10 with MinGW 7.3.0 64-bit. However, `QMAKE_CXXFLAGS += -std=c++17` still gets the job done. – Joel Bodenmann Mar 05 '19 at 11:11
12

Edit 3/2019: You can use CONFIG += c++17 since Qt 5.12.

The actual flag is c++1z, not c++17. In short, to get C++17 support, you don't need to modify QMAKE_CXXFLAGS and can instead simply use CONFIG += c++1z.

Discussion on the reason why can be found in this bug report, but it pretty much amounts to "we implemented it as c++1z before C++17 was standardized, and now we won't bother aliasing it."

Note: I realize you just needed a clean and rebuild. I'm answering the underlying question of "what flags do I need to use to enable C++17 support?"

jonspaceharper
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    Neither work with Qt 5.12.4 and LLVM 8.0.0 using clang-cl. I don't know how to use the normal clang cli instead. – purefanatic Jul 12 '19 at 10:34
  • According to https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html, c++1z is deprecated. c++17 is prefered – jjcf89 Dec 15 '20 at 15:25
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    @jjcf89 That's for gcc. These are flags fed to QtCreator that determine what flags get sent to the compiler, be it gcc, MSVC, et cetera. – jonspaceharper Dec 16 '20 at 09:55
  • ``I have Qt 5.11.3 and the "CONFIG += c++1z" worked. Creator version 4.8.2 `` –  Dec 31 '20 at 12:52
3

By Example C++17 C++20 Builds with Qt5 version 5.15.3 (and above) on Linux (or similar on Windows)

Summary: Inside CMakeLists.txt file for an application written in C++17 and using Qt

cmake_minimum_required(VERSION 3.16.0)
project(helloworld VERSION 1.0.0 LANGUAGES CXX)
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

A qt project configuration eg mygame.pro may have c++1z for C++17 or c++2a for C++20 or c++11 for older projects:

CONFIG += c++1z

On the command line of configure eg a C++20 project config, always use a shadow build:

../configure -c++std c++2a
cmake --build .. --parallel

Windows or Linux compiler command arguments examples with or without qt; at time of writing C++20 is c++2a and C++17 is c++1z others to follow.

$ g++ -std=c++2a hello_linux_is_the_worlds_device_future.cpp  
c:\hello> cl /std=c++1z /EHsc hello_windows_is_in_decline.cpp

On windows notice the forward /slash.

More Detail: I have found that building a new C++ project or even building ALL of qt5 open source thus creating my own qt release (which is tough) is possible by C++17 or C++20 (Qt6 requires at least C++17). I cloned and then checked out version qt5 5.15 i.e. ALL of qt5 open source and built it using C++20 - the build takes 23GB ... you don't need to do all that.

Note the config examples below, ... 4 hours of build on a fast PC are super fussy and I managed eventually. Anyway the point is that Qt5.15 was happy to build itself with the C++17 or C++20 settings (in my case using gcc g++-8 or g++-9) by example:

  1. Just take a look at this cmake Get Started (qt6 C++17 example) but 20 works just as well)

  2. Qt5 open source code total build of qt5.15.3 with C++20 configure example:-

    mkdir qt5-build

    cd qt5-build

    ../configure -release -opensource -nomake examples -nomake tests
    -skip qtdocgallery -c++std c++2a -confirm-license -opengl desktop
    -platform linux-g++-64 # put this all on one-line please.

    cmake --build .. --parallel

OR use make -j4 as I did.

Considerable care was taken to write this for my own notes which I share, I am happy to make corrections myself and suggestions are welcome; never ever hack into an authors work and change it! Just as I do not go into a public library and remove pages from books. Examples count more than non-solutions, however examples age faster!

The Software Architect
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For Windows: I'm download qt-opensource-windows-x86-5.11.1, icncluded MinGW32. For MinGW64 I'm download qt-5.5.0-x64-mingw510r0-seh-rev0 and install only compiler. Configure QtCreator, as says here. Create new project and add QMAKE_CXXFLAGS += -std=gnu++1z to .pro file (gcc doc). For test, try compile this simple code:

#include <optional>

std::optional<int> foo() 
{
    return std::nullopt;
}

int main(int argc, char *argv[])
{
    foo();
}
magrif
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