Disconnecting lambda functions in Qt5

Question

Is it possible to disconnect a lambda function? And if \"yes\", how?

According to https://qt-project.org/wiki/New_Signal_Slot_Syntax I need to use a QMetaObjec

Answer 1

If you capture conn directly, you're capturing an uninitialised object by copy, which results in undefined behaviour. You need to capture a smart pointer:

std::unique_ptr pconn{new QMetaObject::Connection};
QMetaObject::Connection &conn = *pconn;
conn = QObject::connect(m_sock, &QLocalSocket::readyRead, [this, pconn, &conn](){
    QObject::disconnect(conn);
    // ...
}

Or using a shared pointer, with slightly greater overhead:

auto conn = std::make_shared();
*conn = QObject::connect(m_sock, &QLocalSocket::readyRead, [this, conn](){
    QObject::disconnect(*conn);
    // ...
}

From Qt 5.2 you could instead use a context object:

std::unique_ptr context{new QObject};
QObject* pcontext = context.get();
QObject::connect(m_sock, &QLocalSocket::readyRead, pcontext,
    [this, context = std::move(context)]() mutable {
    context.release();
        // ...
 });