c++ - C2665 : 'QObject::connect' : none of the 3 overloads could convert all the arguments types

Question

我在 main 中有以下代码

         QProcess process;
        QObject::connect(&process, &QProcess::error, [](QProcess::ProcessError error)
        {
            qDebug() << error;
        }, Qt::QueuedConnection);
        bool launched = process.startDetached("D:\temp.exe");

编译时生成此错误

    D:\main.cpp:5: error: C2665: 'QObject::connect' : none of the 3 overloads could convert all the argument types c:\qt\5.3\msvc2013_64\include\qtcore\qobject.h(205): could be 
'QMetaObject::Connection QObject::connect(const QObject *,const char
    *,const char *,Qt::ConnectionType) const' c:\qt\5.3\msvc2013_64\include\qtcore\qobject.h(201): or 
      'QMetaObject::Connection QObject::connect(const QObject *,const QMetaMethod &,const QObject *,const QMetaMethod &,Qt::ConnectionType)' c:\qt\5.3\msvc2013_64\include\qtcore\qobject.h(198): or      
 'QMetaObject::Connection QObject::connect(const QObject *,const char
    *,const QObject *,const char *,Qt::ConnectionType)' while trying to match the argument list '(QProcess *, overloaded-function, RunGUIMode::<lambda_5d6e7ee926a623cea2a0e4469253d55f>, Qt::ConnectionType)'

有人可以帮助我并告诉我我做错了什么吗？

我想将信号从 QProcess 类连接到我的 lambda

Answer 1

我不应该发布这个答案，但说实话，它不是 same question ，情况比较复杂。

首先，为什么第一个版本不起作用。因为如果不提供 receiver 则无法使用附加参数(连接类型) 。这意味着 next 是错误的。

connect(&process, static_cast<void (QProcess::*)(QProcess::ProcessError)>
        (&QProcess::error),[=](QProcess::ProcessError pError) {
        qWarning() << "error " << pError;
},Qt::QueuedConnection);

但下一个是正确的:

connect(&process, static_cast<void (QProcess::*)(QProcess::ProcessError)>
        (&QProcess::error), this , [=](QProcess::ProcessError pError) {
        qWarning() << "error " << pError;
},Qt::QueuedConnection);

如果您想知道原因，请查看 qobject.h 。我对此文件进行了一些更改，只是为了更准确(不要更改此文件!)。

//first
//connect to a functor
template <typename Func1, typename Func2>
static inline typename QtPrivate::QEnableIf<QtPrivate::FunctionPointer<Func2>::ArgumentCount == -1, QMetaObject::Connection>::Type
connect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal, Func2 slot)
{
    qDebug("There is no here argument for connection, isn't it?");
    return connect(sender, signal, sender, slot, Qt::DirectConnection);
}

//second
//connect to a functor, with a "context" object defining in which event loop is going to be executed
template <typename Func1, typename Func2>
static inline typename QtPrivate::QEnableIf<QtPrivate::FunctionPointer<Func2>::ArgumentCount == -1, QMetaObject::Connection>::Type
connect(const typename QtPrivate::FunctionPointer<Func1>::Object *sender, Func1 signal, const QObject *context, Func2 slot,
        Qt::ConnectionType type = Qt::AutoConnection)
{
    qDebug("This will be called, and as you can see you need specify the context if you want to use connection type.");
    //...

其次，当您运行此代码时，您将得到:

QObject::connect: Cannot queue arguments of type 'QProcess::ProcessError' (Make sure 'QProcess::ProcessError' is registered using qRegisterMetaType().)

所以你需要添加qRegisterMetaType<QProcess::ProcessError>("QProcess::ProcessError");连接之前。

所以最终版本是:

qRegisterMetaType<QProcess::ProcessError>("QProcess::ProcessError");
QProcess process;
connect(&process, static_cast<void (QProcess::*)(QProcess::ProcessError)>
        (&QProcess::error), this , [=](QProcess::ProcessError pError) {
    qWarning() << "error " << pError;
},Qt::QueuedConnection);
process.start("MyProgram");
bool launched = process.startDetached("example");