How to use template types as slot and signal parameters in multiple threads?

Question

Can I use a template type in any way as a slot or signal argument? As an example, I'm trying to define the following:

void exampleSignal(std::map<non_template_type_1,non_template_type_2> arg);
void exampleSlot(std::map<non_template_type_1,non_template_type_2> arg);

This results in the following during runtime:

QObject::connect: Cannot queue arguments of type 
    'std::map<non_template_type_1,non_template_type_2>'
(Make sure 'std::map<non_template_type_1,non_template_type_2>' 
    is registered using qRegisterMetaType().)

Trying to register std::map<non_template_type_1,non_template_type_2> with Q_DECLARE_METATYPE() results in compilation failure and apparently is not supported.

As a workaround, I'm using QVariantMap instead of std::map. But I really would like to know the correct way to solve this problem; one where it is not possible to modify the template classes.

Edit: I forgot to mention that the signal and slot are emitted and received in different threads. Apparently the runtime error doesn't occur in single-thread scenarios.

score 4 · Answer 1 · answered Oct 14 '14 at 15:03

4

This works for me:

qRegisterMetaType< std::vector<float> >( "std::vector<float>" );
qRegisterMetaType< std::vector<int>   >( "std::vector<int>"   );
qRegisterMetaType< std::map<std::string,int64_t> >( "std::map<std::string,int64_t>" );

answered Oct 14 '14 at 15:03

Diego Sánchez

539
2
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This is what worked for me. I had a `QVector`, and Qt refused to understand my `qRegisterMetaType>();`. It instead insisted on a name without scope of the template type. I.e. I had to say `qRegisterMetaType>("QVector");`, which finally persuaded it to work. – Ruslan May 21 '19 at 17:23

score 2 · Accepted Answer · edited May 23 '17 at 12:29

2

As explained in this thread you can try using a typedef, including the QMetaType header and then using both Q_DECLARE_METATYPE macro and the qRegisterMetaType function (as implied by this thread on a similar issue).

edited May 23 '17 at 12:29

Community

1
1

answered Oct 14 '14 at 15:37

BartoszKP

34,786
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Actually, I confirmed it to work without `Q_DECLARE_METATYPE` so `qRegisterMetaType` seems enough along with a `typedef`. I don't know how safe it is to omit `Q_DECLARE_METATYPE` though. – Ayberk Özgür Oct 14 '14 at 15:47

score 1 · Answer 3 · answered Oct 14 '14 at 15:04

1

There is no problem if you created the class like this and used the Qt moc compiler to create those QMetaObject's automatically:

class MyClass : public QObject
{
    Q_OBJECT
public:
    explicit MyClass(QObject *parent = 0)
        : QObject(parent)
    {
    }
public slots:
    void exampleSlot(std::map<non_template_type_1,non_template_type_2> arg);
signals:
    void exampleSignal(std::map<non_template_type_1,non_template_type_2> arg);
};

Of course you need to include QObject and wherever std::map is located at.

answered Oct 14 '14 at 15:04

msrd0

7,816
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Thank you, indeed this seems possible in a single-thread setting, but I forgot to mention that it is not the case with me. – Ayberk Özgür Oct 14 '14 at 15:08
@AyberkÖzgür So whats the problem? The signal will always be received in Qt's Event Handler Thread – msrd0 Oct 14 '14 at 15:09
It's not. See QueuedConnection in http://qt-project.org/doc/qt-4.8/threads-qobject.html#signals-and-slots-across-threads. – Ayberk Özgür Oct 14 '14 at 15:17

How to use template types as slot and signal parameters in multiple threads?

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