在Qt中发送信号时的const-ref

Question

This is a thing that I never quite got with const-ref and I really hope that someone could explain it to me.

When calling a function inside of another function, I get that const-ref is the best way when passing stack objects that I don't plan to tamper with. For example:

void someInnerFunction(const QString& text) {
    qDebug() << text;
}

void someFunction() {
    QString test = "lala";
    ....
    someInnerFunction(test);
}

So far so good, I guess. But what about signals? Isn't there any risk that comes with passing a reference? Even though it's const . It feels like I've been reading all the docs about const-ref but I still find a bit risky since I understand it as "sending a reference to an object and keeping it const ". What if the object it's referring to goes out of scope?

For example:

void someFunction() {
    connect(this, SIGNAL(someSignal(const QString&)), this, SLOT(someSlot(const QString&)));

    QString test = "lala";
    emit someSignal(test);

    // doesnt test go out of scope here? and since im not using queued connection the QString object doesnt get copied. 
}

void someSlot(const QString& test) {
    qDebug() << test; // will this work?
}

What is really happening here? I frequently use const-ref on function calls where I just want to access the object but not change it. But what about signals? Most signals seems to have const-ref parm in the Qt doc, but how does it work?

Answer 1

According to this answer , Qt just replaces const references with copies.

EDIT: Apparently not always the case... I just did a basic test program with one thread, and the reference was passed correctly. Its const-ness remained intact, as well. Anyways, yes, you do need to be wary of the variable going out of scope, plus you can't send references across threads this way. If you do, only copies will be passed.

To answer the questions in the comments of your example, yes, it will work regardless of whether it's a direct or queued connection. If it's a direct connection, it will work because someSlot() will be executed before someFunction() finishes; if it's a queued connection, it will work because test will be copied instead of passed by reference.

Answer 2

这是一个很好的演示，展示了Qt信号/插槽如何管理复制： http ： //www.embeddeduse.com/2013/06/29/copied-or-not-copied-arguments-signals-slots/

Answer 3

在Qt中，当发出连接到插槽或插槽的信号时，它等同于同步函数调用...除非你已经将信号和插槽配置为使用排队连接，那么它是异步调用，你应该是传递堆栈数据时要小心，并且应该传递副本，就像将数据传递给另一个线程一样。