无法在 QtCreator 中使用 g++ 7.2 使用 c++17 功能

Question

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.

Answer 1

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 .

Answer 2

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?"

Answer 3

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();
}

Answer 4

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 ie 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!