Why does my code compile with -fno-exceptions in Qt Creator when I try to use exceptions?

Question

In the project .pro file I've specified:

QMAKE_CXXFLAGS += -fno-exceptions  

Yet I'm able to throw exceptions in my app. Any thoughts on that?

Example: This shouldn't work, but it works

#include <QApplication>
#include <QtDebug>

int main(int c, char**v)
{
    QApplication app(c,v);
    try
    {
        throw 1;
    }
    catch(int i)
    {

    }
    return app.exec();
}

Answer 1

You do not turn off exceptions by setting QMAKE_CXXFLAGS because this options is handled by CONFIG. You should use

CONFIG-=exceptions

to turn them off.

See the g++ args when you have neither QMAKE_CXXFLAGS nor CONFIG settings changed:

g++ -c -O2 -frtti -fexceptions -mthreads -Wall <...> main.cpp

Now, let's set QMAKE_CXXFLAGS: get

g++ -c -fno-exceptions -O2 -frtti -fexceptions -mthreads -Wall <...> main.cpp

Ooops, we get our -fno-exceptions is overridden by CONFIG's -fexceptions. Now, let's set CONFIG:

g++ -c -O2 -frtti -Wall -fno-exceptions <...> main.cpp
mingw32-make.exe[1]: Leaving directory `G:/proj/ingeritance'
main.cpp: In function 'int qMain(int, char**)':
main.cpp:22:15: error: exception handling disabled, use -fexceptions to enable
mingw32-make.exe[1]: *** [release/main.o] Error 1
mingw32-make.exe: *** [release] Error 2

Oh! compilation error!

Answer 2

I'm a bit confused by the question.

If you have (or link to) code that throws exceptions, you can't just magically make them go away by building your code with -fno-exceptions. The flag affects the generation of code necessary to support try and catch, more than the code that actually throws exceptions.

See GCC documentation for details. The documentation says:

In sum, valid C++ code with exception handling is transformed into a dialect without exception handling.

So, it seems more as if the compiler more or less causes the exception-handling (and throwing) code to "go away", rather than detect its use and flag an error. The latter seems to be what you're expecting, but that expectation is, then, simply wrong.