[FIXED] Why are Qt's CONFIG flags rtti and rtti_off only processed by qmake for Windows target

Issue

I am working on a cross platform Qt project and recently used dynamic_cast, which worked just fine under Linux. But our CI could not compile the code for Windows with the error message:

error: ‘dynamic_cast’ not permitted with -fno-rtti

I found that in our project CONFIG += rtti_off was set. According to Qt documentation rtti_off means:

RTTI support is disabled. By default, the compiler default is used.

There is nothing written about platform specific behavior of the flag (or at least I did not find it).

grep -ir rtti in Qt's mkspecs gave:

features/win32/rtti.prf:CONFIG -= rtti_off
features/win32/rtti.prf:QMAKE_CFLAGS += $$QMAKE_CFLAGS_RTTI_ON
features/win32/rtti.prf:QMAKE_CXXFLAGS += $$QMAKE_CXXFLAGS_RTTI_ON
features/win32/rtti_off.prf:CONFIG -= rtti
features/win32/rtti_off.prf:QMAKE_CFLAGS += $$QMAKE_CFLAGS_RTTI_OFF
features/win32/rtti_off.prf:QMAKE_CXXFLAGS += $$QMAKE_CXXFLAGS_RTTI_OFF
common/g++-win32.conf:QMAKE_CXXFLAGS_RTTI_ON  = -frtti
common/g++-win32.conf:QMAKE_CXXFLAGS_RTTI_OFF = -fno-rtti
common/winrt_winphone/qmake.conf:CONFIG                  = package_manifest $$CONFIG incremental flat precompile_header autogen_precompile_source debug_and_release debug_and_release_target rtti
common/winrt_winphone/qmake.conf:QMAKE_CXXFLAGS_RTTI_ON  = -GR
common/winrt_winphone/qmake.conf:QMAKE_CXXFLAGS_RTTI_OFF =
common/msvc-desktop.conf:QMAKE_CXXFLAGS_RTTI_ON  = -GR
common/msvc-desktop.conf:QMAKE_CXXFLAGS_RTTI_OFF =

So these flags are obviously only checked for Windows platforms.

My questions

• Why is it done only for Windows?
• Is this documented anywhere?
  • If so, where?
  • If not, why?

Edit

To clarify: I am using mingw32-gcc under Windows and gcc under Linux. Both version have the capability to deactivate RTTI using the flag -fno-rtti. This flag is integrated into the generated Makefile if I run qmake with target platform win32-g++, but not with linux. Hence the questions above.

Solution

This is QTBUG-26595, which was resolved in Qt 5.12.1.

Answered By - orgads

Answer Checked By - Mary Flores (PHPFixing Volunteer)