How to create a single-shot, one-time connection to a lambda?

Violet Giraffe

I use the new Qt5 syntax and lambdas a lot, e. g.:

connect(_textFadeOutAnimation, &QPropertyAnimation::finished, [this, text](){
});

The problem I'm facing right now is I need this slot to only be fired once. Since there's no ConnectionType for that, I need to disconnect inside the slot, apparently. But how do I disconnect a lambda? I can't pass the QMetaObject::Connection inside the lambda for the obvious reason...

Paul Colby

Hi @Violet-Giraffe,

Interesting question.

What about something like (untested):

QMetaObject::Connection * const connection = new QMetaObject::Connection;
*connection = connect(_textFadeOutAnimation, &QPropertyAnimation::finished, [this, text, connection](){
    QObject::disconnect(*connection);
    delete connection;
});

Violet Giraffe

@Paul-Colby, brilliant, that works! Thank you. It didn't occur to me Connection has an assignment operator.

beecksche

This post is deleted!

seyed

There is another way. Create a context object and use it as receiver for the connection. If you delete it, the connected signal will be disconnected.

QObject* ctx = new QObject();
connect(d, &MyClass::mySignal, ctx, [=]() {
    // destroy the context/receiver to disconnect
    ctx->deleteLater();

    // slot routine here...
});

qwasder85

@Violet-Giraffe Mark the thread as 'solved', please.

thomaso

In Qt6 there is a new connection type Qt::SingleShotConnection.
See more in this KDAB blog: https://www.kdab.com/single-shot-connections/