Qt signal/slots and C++ Lambda expression

Question

Why this doesn't work ?

Class inherit from QObject

b is Class child.

bar is Foo child.

void Class::method(Foo& b) {
    Bar* bar = b.getBar();
    QObject::connect(bar, &Bar::s1, [&]{
        auto x = bar->x(); // this line throw an exception read access violation.
    });
}

As first guess I think the bar is no longer exist when the slot is called. to correct it I need to capture by value.

Am I getting it right ?.

CHANGES THAT MAKE IT WORK:

void Class::method(Foo& b) {
    Bar* bar = b.getBar();
    QObject::connect(bar, &Bar::s1, [bar]{
        auto x = bar->x(); // this line throw no more exceptions and work as expected.
    });
}

Probably because `bar` is no more valid when the signal is called. — Jepessen, Aug 12 '17 at 12:08
Note that you can pass a "guard object" to `connect` as the third argument (your lambda then being the fourth), that will destroy the connection to the lambda if the guard object is destroyed. In this case you'd probably want to use `bar` as that guard (requires Qt 5.2 or later). So `QObject::connect(bar, &Bar::s1, bar, [&]{...`. — Jesper Juhl, Aug 12 '17 at 12:08
bar is b child and b is Class child I am sure that it exist. — Houss_gc, Aug 12 '17 at 12:08
@Jesper Juhl it will simplify connection management very much, I will apply it. — Houss_gc, Aug 12 '17 at 12:10

score 2 · Accepted Answer · answered Aug 12 '17 at 12:26

bar is local pointer variable. When you capture by reference it's the same as to capture [&bar], which type it Bar**. After that you try to access to bar in lambda assuming that pointer to Bar is located by captured &bar address. And it is not true because local variable was destroyed. The actual object of type Bar is remains located by the same address, but this address is corrupted when capturing by [&]. So it's correct to change capture to [bar] and thus capturing pointer directly, not the address where this pointer can be found.

Thank you for the clear explanation, this is what I assumed. — Houss_gc, Aug 12 '17 at 12:31

Qt signal/slots and C++ Lambda expression

1 Answers1