error: expected unqualified-id before ‘(’ token when using emit event of socket.io-client-cpp

[HoaiNam10TH]

when I use:

_io->socket()->emit("join conversation", data);

there is an error, use the following method: error: expected unqualified-id before '(' token
I have attached a photo of the error:

I don't understand where I went wrong. please help me.

[jsulm]

@HoaiNam10TH Why are you trying to emit a signal on another object? Objects should emit their signals by themselfs.
And please post code as text not pictures.

[JonB]

@HoaiNam10TH
Further to @jsulm, and his correct statement that objects should (generally) only do their own signal emission.
In Qt you can only go:

emit signal();

In fact, emit is #defined to be empty/no expansion --- it is only "syntactic sugar" to help you identify where signals are emitted. There is no function named emit() anywhere, you will never be able to go:

something->emit(...);

If you really want to do this the correct syntax will be:

emit object->signal(...);

[HoaiNam10TH]

@JonB Thank you, I understand

[HoaiNam10TH]

@jsulm @JonB
I still can't do it, here is my code, please help me.
Widget.h

#include <sio_client.h>   
class Widget : public QWidget
{
    Q_OBJECT
public:
    Widget(QWidget *parent = nullptr);
    ~Widget();    
signals:
    void joinConversation(std::string userId);
private slots:
private:
    std::unique_ptr<sio::client> _io;
}; 

Widget.cpp

connect(this,&Widget::joinConversation,[=](std::string userId){
	_io->socket()->emit("join conversation", userId); // **error**
});

void Widget::on_listWidgetContact_itemClicked(QListWidgetItem *item)
{    
    QJsonObject userJson = item->data(Qt::UserRole).toJsonObject();    
    QByteArray bytes = userJson["_id"].toString().toUtf8();
    std::string userId(bytes.data(),bytes.length());
    emit joinConversation(userId);
}

[J.Hilk]

@HoaiNam10TH said in error: expected unqualified-id before ‘(’ token when using emit event of socket.io-client-cpp:

connect(this,&Widget::joinConversation,[=](std::string userId){
_io->socket()->emit("join conversation", userId); // error
});

well, yes.
Like @JonB previously stated, the "emit" is in the wrong place, you're also missing the actual signal/function name in your call

[HoaiNam10TH]

@J-Hilk Can you teach me how to fix this error?

[J.Hilk]

@HoaiNam10TH I don't know what your signal that you try to call is named.

replace emit:
_io->socket()->emit("join conversation", userId);

with the signal name:
void joinConversation(std::string userId);

[Pl45m4]

@HoaiNam10TH said in error: expected unqualified-id before ‘(’ token when using emit event of socket.io-client-cpp:

connect(this,&Widget::joinConversation,[=](std::string userId){
_io->socket()->emit("join conversation", userId); // error
});

So you are just forwarding your signal from Widget to _io->socket?
If socket has a public signal called joinConversation you can simply call it, which is nothing else what emit does, since emit itself does bascially nothing :)

_io->socket()->joinConversation(userId);

But when used like this, it's hard to see that joinConversation is a signal and not the fuunction that's executed after the call. Therefore the emit keyword. Besides all this, "calling" signals from other places is not the best practise and somehow bad design, like everybody else above already said

[HoaiNam10TH]

@Pl45m4 this a a example with their code. It's run good. But my project is failed.

void MainWindow::SendBtnClicked()
{
    QLineEdit* messageEdit = this->findChild<QLineEdit*>("messageEdit");
    QString text = messageEdit->text();
    if(text.length()>0)
    {
        QByteArray bytes = text.toUtf8();
        std::string msg(bytes.data(),bytes.length());
        _io->socket()->emit("new message",msg);
        text.append(" : You");
        QListWidgetItem *item = new QListWidgetItem(text);
        item->setTextAlignment(Qt::AlignRight);
        Q_EMIT RequestAddListItem(item);
        messageEdit->clear();
    }
}

_io->socket()->emit("new message",msg);

[Pl45m4]

@HoaiNam10TH said in error: expected unqualified-id before ‘(’ token when using emit event of socket.io-client-cpp:

this a a example with their code.

Their code? Whose code is this?

_io->socket()->emit("new message",msg);

There could be a function called emit() which has nothing to do with Qt, but sets up a connection or something else...
Using the emit (from QObject) in a way like this should never work.

If you really want to emit the signal in _io->socket, then use what @J-Hilk suggested and what I wrote here

@Pl45m4 said in error: expected unqualified-id before ‘(’ token when using emit event of socket.io-client-cpp:

_io->socket()->joinConversation(userId);

If possible show how _io->socket()->emit("new message",msg); is defined...

[HoaiNam10TH]

@Pl45m4 Follow this github page, it has an example of socket.io with cpp (examples folder). Their project runs fine on my machine:
_io->socket()->emit("new message",msg);
emit("new message",msg) it is a function (owned by socket.io-client-cpp) to invoke method on nodejs server. It is not emit of Qt.

[JonB]

@HoaiNam10TH said in error: expected unqualified-id before ‘(’ token when using emit event of socket.io-client-cpp:

emit("new message",msg) it is a function (owned by socket.io-client-cpp) to invoke method on nodejs server. It is not emit of Qt.

Then this is the problem/cause of the error message. Like I said earlier, Qt header files go

#define emit

Just like that. That will make your _io->socket()->emit("new message",msg); become _io->socket()->("new message",msg); and hence error: expected unqualified-id before ‘(’ token.

Do not include Qt header files before the _io->socket()->emit("new message",msg); line. If you cannot do that you may be able to go #undef emit around that _io->socket()->emit(...) line.

[Christian Ehrlicher]

@JonB Or use QT_NO_KEYWORDS :)

[JonB]

@Christian-Ehrlicher
Indeed, but if your own code being compiled is using emit before Qt signals you still have to deal with that. One way or the other it needs sorting.

[Christian Ehrlicher]

@JonB said in error: expected unqualified-id before ‘(’ token when using emit event of socket.io-client-cpp:

but if your own code being compiled is using emit before Qt signals you still have to deal with that

Yes, either remove it or use Q_EMIT instead.

[JonB]

@Christian-Ehrlicher Ah. putting Q_EMIT instead of emit sounds like I good idea. Does Qt source do this?

[Christian Ehrlicher]

@JonB said in error: expected unqualified-id before ‘(’ token when using emit event of socket.io-client-cpp:

Does Qt source do this?

Qt source code does not use 'signals' or 'slots' or 'emit' in public headers - only the Q_foo keywords.

[SGaist]

@HoaiNam10TH said in error: expected unqualified-id before ‘(’ token when using emit event of socket.io-client-cpp:

@Pl45m4 Follow this github page, it has an example of socket.io with cpp (examples folder). Their project runs fine on my machine:
_io->socket()->emit("new message",msg);
emit("new message",msg) it is a function (owned by socket.io-client-cpp) to invoke method on nodejs server. It is not emit of Qt.

Hi,

The Qt example in the repository you link contains a README that explains that you need the no_keyword configuration option and why.

[HoaiNam10TH]

@SGaist
That's right. Thanks everyone