我可以使用以下代码(在连接到 myQtSignal 的最终插槽中)获得“悬空引用”吗？

class Test : public QObject
{
    Q_OBJECT

signals:
    void myQtSignal(const FooObject& obj);

public:
    void sendSignal(const FooObject& fooStackObject)
    {
        emit  myQtSignal(fooStackObject);
    }
};

void f()
{
    FooObject fooStackObject;
    Test t;
    t.sendSignal(fooStackObject);
}

int main()
{
    f();
    std::cin.ignore();
    return 0;
}

特别是如果 emit 和 slot 不在同一个线程中执行。

最佳答案

2015 年 4 月 20 日更新

最初我认为传递对堆栈分配对象的引用等同于传递该对象的地址。因此，在没有存储拷贝(或共享指针)的包装器的情况下，排队的插槽连接可能会使用坏数据结束。

但是@BenjaminT 和@cgmb 引起我的注意，Qt 实际上确实对 const 引用参数进行了特殊处理。它将调用复制构造函数并存放复制的对象以用于插槽调用。即使您传递的原始对象在插槽运行时已被销毁，插槽获取的引用也将完全指向不同的对象。

您可以阅读 @cgmb's answer对于机械细节。但这里有一个快速测试:

#include <iostream>
#include <QCoreApplication>
#include <QDebug>
#include <QTimer>

class Param {
public:
    Param () {}
    Param (Param const &) {
        std::cout << "Calling Copy Constructor\n";
    }
};

class Test : public QObject {
    Q_OBJECT

public:
    Test () {
        for (int index = 0; index < 3; index++)
            connect(this, &Test::transmit, this, &Test::receive,
                Qt::QueuedConnection);
    }

    void run() {
        Param p;
        std::cout << "transmitting with " << &p << " as parameter\n";
        emit transmit(p);
        QTimer::singleShot(200, qApp, &QCoreApplication::quit);
    }

signals:
    void transmit(Param const & p);
public slots:
    void receive(Param const & p) {
        std::cout << "receive called with " << &p << " as parameter\n";
    }
};

...还有一个主要的:

#include <QCoreApplication>
#include <QTimer>

#include "param.h"

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    // name "Param" must match type name for references to work (?)
    qRegisterMetaType<Param>("Param"); 

    Test t;

    QTimer::singleShot(200, qApp, QCoreApplication::quit);
    return a.exec();
}

运行这个演示了对于 3 个插槽连接中的每一个，通过复制构造函数制作了一个单独的 Param 拷贝:

Calling Copy Constructor
Calling Copy Constructor
Calling Copy Constructor
receive called with 0x1bbf7c0 as parameter
receive called with 0x1bbf8a0 as parameter
receive called with 0x1bbfa00 as parameter

您可能想知道如果 Qt 只是要进行复制，那么“通过引用传递”有什么好处。但是，它并不总是复制...这取决于连接类型。如果您更改为 Qt::DirectConnection，它不会制作任何拷贝:

transmitting with 0x7ffebf241147 as parameter
receive called with 0x7ffebf241147 as parameter
receive called with 0x7ffebf241147 as parameter
receive called with 0x7ffebf241147 as parameter

如果你切换到按值传递，你实际上会得到更多的中间拷贝，尤其是在 Qt::QueuedConnection 的情况下:

Calling Copy Constructor
Calling Copy Constructor
Calling Copy Constructor
Calling Copy Constructor
Calling Copy Constructor
receive called with 0x7fff15146ecf as parameter
Calling Copy Constructor
receive called with 0x7fff15146ecf as parameter
Calling Copy Constructor
receive called with 0x7fff15146ecf as parameter

但是通过指针传递并没有什么特别的魔力。所以它有原始答案中提到的问题，我将在下面保留。但事实证明，引用处理只是另一种野兽。

ORIGINAL ANSWER

Yes, this can be dangerous if your program is multithreaded. And it's generally poor style even if not. Really you should be passing objects by value over signal and slot connections.

Note that Qt has support for "implicitly shared types", so passing things like a QImage "by value" won't make a copy unless someone writes to the value they receive:

http://qt-project.org/doc/qt-5/implicit-sharing.html

The problem isn't fundamentally anything to do with signals and slots. C++ has all kinds of ways that objects might be deleted while they're referenced somewhere, or even if some of their code is running in the call stack. You can get into this trouble easily in any code where you don't have control over the code and use proper synchronization. Techniques like using QSharedPointer can help.

There are a couple of additional helpful things Qt offers to more gracefully handle deletion scenarios. If there's an object you want to destroy but you are aware that it might be in use at the moment, you can use the QObject::deleteLater() method:

http://qt-project.org/doc/qt-5/qobject.html#deleteLater

That's come in handy for me a couple of times. Another useful thing is the QObject::destroyed() signal:

http://qt-project.org/doc/qt-5/qobject.html#destroyed

关于c++ - 堆栈对象 Qt 信号和参数作为引用，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/8455887/