QtConcurrent::run 异常通知

Question

我知道实际处理在不同线程中抛出的异常是没有意义的，但是有什么方法可以让我得到至少发生异常的通知？例如。像

#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;
  }

}

即使发生异常，也会安静地退出。当异常来自调用堆栈深处的某个地方时，这有时会非常令人困惑。

------------编辑----------------

我尝试像 UmNyobe 建议的那样编写一个包装器，但我一定是函数指针有问题？

#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();
}

我得到的错误在这一行：

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

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

Answer 1

我知道实际处理在不同线程中抛出的异常是没有意义的，但是有什么方法可以让我得到至少发生异常的通知？

您可以使用从QtConcurrent::run 返回的未来来处理它。有关详细信息，请参阅this page。当您在未来收集时，将重新抛出任何未处理的异常。您可以创建一个简单的包装类来捕获异常并在接收线程中对其进行检查。

#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();
}

Answer 2

似乎如果抛出异常，关联的QFutureWatcher的isCanceled()返回true：

#include <QApplication>
#include <QtConcurrentRun>
#include <QFutureWatcher>

#include <iostream>
#include <stdexcept>

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

struct MyClass : public QObject
{
  Q_OBJECT

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

  void DoSomething()
  {
    QFuture<void> future = QtConcurrent::run(MyFunction);
    this->FutureWatcher.setFuture(future);
  }

  QFutureWatcher<void> FutureWatcher;

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

#include "Exception.moc"

int main(int argc, char*argv[])
{
  MyClass myClass;
  myClass.DoSomething();

  QApplication app(argc, argv);

  return app.exec();
}

--------- 编辑（戈登弗里曼答案的简化版）---------

即使不使用 QtConcurrent::Exception 子类，似乎也会重新抛出异常？

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

// non-void concurrent function
int addFive(int n)
{
  throw std::runtime_error("addFive throw!");
  return n+5;
}

// void concurrent function
void myVoidFunction()
{
   throw std::runtime_error("myVoidFunction throw!");
}

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 (...)
    {
      std::cout << "exception in addFive." << std::endl;
    }

    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 (...)
    {
       std::cout << "exception in myVoidFunction\n";
    }

    QWidget w;
    w.show();

    return app.exec();
}

Answer 3

QtConcurrent::run 的好处是它接受带有返回值的函数。 我的两分钱：尽早捕获异常...

免责声明：C++ 中的异常我真的很糟糕：D

在不同线程中调用的代码应该返回一个值。对于 void 函数或不捕获异常的现有函数，您可以定义一个包装器（通用或非通用）。比如

 int exceptionwrapper(){
   int exception  = 0;
   try
   {
      myFunction();
   }
   catch(...){
      exception = 1;
      std::cout << "Exception caught!" << std::endl;
   }
   return exception;
 }

后来

 QFuture<int> future = QtConcurrent::run(exemptionwrapper);
 futurewatcher.setFuture(future);

当函数结束时，您只需使用未来观察器就可以在以后检查未来。