Question

I know it doesn’t make sense to actually handle an exception thrown in a different thread, but is there some way I can get notified that at least an exception occurred? E.g. something like

#include <QtConcurrentRun>

#include <iostream>
#include <stdexcept>

void MyFunction()
{
//  std::cout << "MyFunction()" << std::endl;
  throw std::runtime_error("Test exception.");
}

int main()
{
  try
  {
    QtConcurrent::run(MyFunction);
  }
  catch(...)
  {
    std::cout << "Exception caught!" << std::endl;
  }

}

exits quietly, even though an exception occurred. This is sometimes very confusing when the exception comes from deep down in the call stack somewhere.

————EDIT————-

I tried to write a wrapper like UmNyobe suggested, but I must be doing something wrong with the function pointers?

#include <QtConcurrentRun>
#include <QFutureWatcher>
#include <QObject>

#include <iostream>
#include <stdexcept>

void MyFunction()
{
//  std::cout << "MyFunction()" << std::endl;
  throw std::runtime_error("Test exception.");
}

template<typename TFirstParam, typename... TParams>
bool ExceptionWrapper(TFirstParam firstParam, TParams&& ...params)
{
  // Here 'firstParam' should be a function pointer, and 'params' are the arguments
  // that should be passed to the function
  try
  {
    firstParam(params...);
  }
  catch(...)
  {
    std::cout << "Exception caught!" << std::endl;
    return false; // failure
  }

  return true; // success
}

struct MyClass : public QObject
{
  Q_OBJECT

  MyClass()
  {
    connect(&this->FutureWatcher, SIGNAL(finished()), this, SLOT(slot_finished()));
  }

  void DoSomething()
  {
    void (*myFunctionPointer)() = MyFunction;
    bool (*functionPointer)(decltype(myFunctionPointer)) = ExceptionWrapper;

    QFuture<bool> future = QtConcurrent::run(functionPointer);
    this->FutureWatcher.setFuture(future);
  }

  QFutureWatcher<void> FutureWatcher;

  void slot_finished()
  {
    std::cout << "Finished" << std::endl;
    if(!this->FutureWatcher.result())
    {
      std::cout << "There was an error!" << std::endl;
    }
  }
};

#include "ExceptionWrapper.moc"

int main()
{
  MyClass myClass = new MyClass;
  myClass->DoSomething();
}

The error I get is on this line:

QFuture<bool> future = QtConcurrent::run(functionPointer);

error: no matching function for call to 'run(bool (*&)(void (*)()))'

Answer 1

I know it doesn’t make sense to actually handle an exception thrown in a different thread, but is there some way I can get notified that at least an exception occurred?

You can handle it by using the future returned from QtConcurrent::run. See this page for details. When you collect on the future, any unhandled exceptions will be rethrown. You can make a simple wrapper class to capture an exception and examine it in the receiving thread.

#include <QtGui>
#include <iostream>
#include <stdexcept>

class MyException : public QtConcurrent::Exception
{
public:
    MyException(std::exception& err) : e(err) {}
    void raise() const { throw *this; }
    Exception* clone() const { return new MyException(*this); }
    std::exception error() const { return e; }
private:
    std::exception e;
};

// first concurrent function
int addFive(int n)
{
    try
    {
        throw std::runtime_error("kablammo!");
        //throw -1;
        return n + 5;
    }
    catch (std::exception& e)
    {
        throw MyException(e);
    }

}

// second concurrent function    
void myVoidFunction()
{
    try
    {
        throw std::runtime_error("oops!");
        //throw -1;
    }
    catch (std::exception& e)
    {
        throw MyException(e);
    }
}

int main(int argc, char* argv[])
{
    QApplication app(argc, argv);

    QFuture<int> f1 = QtConcurrent::run(addFive, 50);
    try
    {
        int r = f1.result();
        std::cout << "result = " << r << std::endl;
    }
    catch (MyException& me)
    {
        std::cout << me.error().what() << std::endl;
    }
    catch (QtConcurrent::UnhandledException&)
    {
        std::cout << "unhandled exception in addFive\n";
    }

    QFuture<void> f2 = QtConcurrent::run(myVoidFunction);
    try
    {
        // result() not available for QFuture<void>, use waitForFinished() to
        // block until it's done.
        f2.waitForFinished();
        std::cout << "myVoidFunction finished\n";
    }
    catch (MyException& me)
    {
        std::cout << me.error().what() << std::endl;
    }
    catch (QtConcurrent::UnhandledException&)
    {
        std::cout << "unhandled exception in myVoidFunction\n";
    }

    QWidget w;
    w.show();

    return app.exec();
}