How to set qmake to C++14 with recent MinGW? - c++

Question

I know there are many duplicates.
This is my Test.pro:
CONFIG += c++14
SOURCES += main.cpp
and my main.cpp:
int main(){}
According to the many duplicates this should give me C++14. However, when I build the project with Qt Creator 4.2.0 with Qt 5.8.0-1 and MinGW gcc 5.3.0-1 installed via the maintenance tool I get
g++ -c -pipe -fno-keep-inline-dllexport -g -std=gnu++1y -frtti -Wall -Wextra -fexceptions -mthreads -DUNICODE -DQT_QML_DEBUG -DQT_GUI_LIB -DQT_CORE_LIB -DQT_NEEDS_QMAIN -I..\Test -I. -IC:\Qt\5.8\mingw53_32\include -IC:\Qt\5.8\mingw53_32\include\QtGui -IC:\Qt\5.8\mingw53_32\include\QtANGLE -IC:\Qt\5.8\mingw53_32\include\QtCore -Idebug -IC:\Qt\5.8\mingw53_32\mkspecs\win32-g++ -o debug\main.o ..\Test\main.cpp
which is not the -std=c++14 I expect.
I tried all kinds of tricks from other questions such as
QMAKE_CXXFLAGS_CXX14 = -std=c++14
CONFIG += c++14
QMAKE_CXXFLAGS += -std=c++14
SOURCES += main.cpp
which results in
g++ -c -pipe -fno-keep-inline-dllexport -std=c++14 -g -std=gnu++1y -frtti -Wall -Wextra -fexceptions -mthreads -DUNICODE -DQT_QML_DEBUG -DQT_GUI_LIB -DQT_CORE_LIB -DQT_NEEDS_QMAIN -I..\Test -I. -IC:\Qt\5.8\mingw53_32\include -IC:\Qt\5.8\mingw53_32\include\QtGui -IC:\Qt\5.8\mingw53_32\include\QtANGLE -IC:\Qt\5.8\mingw53_32\include\QtCore -Idebug -IC:\Qt\5.8\mingw53_32\mkspecs\win32-g++ -o debug\main.o ..\Test\main.cpp
where the second option overwrites the first, meaning it is still in gnu++1y-mode or just
QMAKE_CXXFLAGS += -std=c++14
SOURCES += main.cpp
which also results in
g++ -c -pipe -fno-keep-inline-dllexport -std=c++14 -g -std=gnu++11 -frtti -Wall -Wextra -fexceptions -mthreads -DUNICODE -DQT_QML_DEBUG -DQT_GUI_LIB -DQT_CORE_LIB -DQT_NEEDS_QMAIN -I..\Test -I. -IC:\Qt\5.8\mingw53_32\include -IC:\Qt\5.8\mingw53_32\include\QtGui -IC:\Qt\5.8\mingw53_32\include\QtANGLE -IC:\Qt\5.8\mingw53_32\include\QtCore -Idebug -IC:\Qt\5.8\mingw53_32\mkspecs\win32-g++ -o debug\main.o ..\Test\main.cpp
I deleted the build directory and the Test.pro.user file to force a build from scratch, nothing gave me C++14.
How do I tell qmake to use C++14?

Answer 1

The version of Qt that you're using doesn't explicitly support the compiler you're using. You can do either one of the following:
Set both QMAKE_CXXFLAGS_CXX14 and QMAKE_CXXFLAGS_GNUCXX14 in your project:
win32-g++ {
QMAKE_CXXFLAGS_CXX14 = -std=c++14
QMAKE_CXXFLAGS_GNUCXX14 = -std=c++14
}
Edit the default values of those two variables as above, in mkspecs/win32-g++/qmake.conf within your Qt installation folder.
Add a new mkspec copied from win32-g++, targeting your compiler, and build your Qt using it. All the project that use that Qt will then behave correctly w.r.t. C++14 support.