问堆栈对象QT信号和参数作为参考?

提问于 2018-01-28 16:44:32

回答 2关注 0查看 240

可以使用以下代码吗？

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;
}

回答 2

逆鳞若光

发布于 2018-01-29 01:48:00

由于信号/插槽连接的工作方式，传递Qt信号的参考并不危险：

如果连接是直接的，则直接直接调用连接的插槽，例如，emit MySignal(my_string)直接返回所有直接连接的插槽。
如果连接，Qt创建一个引用的副本。所以当这个插槽被调用的时候，它有自己的引用传递的变量的有效副本。但是这意味着参数必须是Qt知道的类型才能复制。

http://qt-project.org/doc/qt-5.1/qtcore/qt.html#ConnectionType-enum

微信资讯小编

发布于 2018-01-29 01:19:45

可以看下这个测试：

#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

#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

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

尝试下这个

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

如果转到按值传递：

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