slot return type

[adamatic]

Hello!

I have a basic question, can a slot return a type or the return type has to be always void?

If it can, how could I save the value in a variable?, I mean, if the slot it is call like this:

connect(timer1, SIGNAL(timeOut()),this, SLOT(read()));

And read() return a char*.

Cheers

[spirit]

slots can return a value. from Qt Assistant

Slots
A slot is called when a signal connected to it is emitted. Slots are normal C++ functions and can be called normally; their only special feature is that signals can be connected to them....

[adamatic]

Ok, so if I want to save the value can i do this?

char* var;

connect(timer1, SIGNAL(timeOut()),this, SLOT(var=read()));

[spirit]

no, you can not do this. you need implement a slot and then store value, e.g.

...
connect(timer1, SIGNAL(timeOut()),this, SLOT(readSlot()));
...
void MyObj::readSlot()//slot
{
    const char *var = read();
    ...
}

...
connect(timer1, SIGNAL(timeOut()),this, SLOT(readSlot()));
...
void MyObj::readSlot()//slot
{
    const char *var = read();
    ...
}

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[adamatic]

If I want to use the varible in another function, do I have to have var as global varible? . I mean in the same function than implement the
connect(timer1, SIGNAL(timeOut()),this, SLOT(readSlot()));

[spirit]

make this variable as class member.

[adamatic]

with class member, do you mean define it with the class atributes?

[spirit]

with class member, do you mean define it with the class atributes?

variables which declared in class called as class members, e.g.

class foo {
...
private:
     int m_value;//class member
};

class foo {
...
private:
     int m_value;//class member
};

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[wysota]

Slots can return values but only when they are called as regular methods or using QMetaObject::invokeMethod().

If you need a return value when using the slot function as a slot, then emit a signal from it and catch it back in the original sender.

[adamatic]

Are you saying come back to the function sender? But this is going to execute all the code in the original sender, no?.

I understand that you say something more o less like this:

..............................
connect(this,SIGNAL(comeback()),this,function1());
....................
 
void function1(){
    ....
    connect(timer1, SIGNAL(timeOut()),this, SLOT(readSlot()));
    char *var=read();
    ....
}
readSlot(){
    emit comeback();
}

..............................
connect(this,SIGNAL(comeback()),this,function1());
....................

void function1(){
    ....
    connect(timer1, SIGNAL(timeOut()),this, SLOT(readSlot()));
    char *var=read();
    ....
}
readSlot(){
    emit comeback();
}

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[talk2amulya]

i think wysota is saying this:

connect(A, SIGNAL(b()), A, SLOT(d()));
connect(A, SIGNAL(customSignal(urValue)), A, SLOT(anotherSlot(urValue)));
 
d()
{
.
.
.
emit customSignal(&valueYouWantToReturn);
}

connect(A, SIGNAL(b()), A, SLOT(d()));
connect(A, SIGNAL(customSignal(urValue)), A, SLOT(anotherSlot(urValue)));

d()
{
.
.
.
emit customSignal(&valueYouWantToReturn);
}

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[wysota]

Something like that, yeah...

An example of transmitting a bool value follows...

connect(A, SIGNAL(someSignal()), B, SLOT(execute()));
connect(B, SIGNAL(executeFinished(bool)), A, processExecuteResult(bool)));
//...
bool BClass::execute(){
  bool result = doSomething();
  emit executeFinished(result);
  return result;
}

connect(A, SIGNAL(someSignal()), B, SLOT(execute()));
connect(B, SIGNAL(executeFinished(bool)), A, processExecuteResult(bool)));
//...
bool BClass::execute(){
  bool result = doSomething();
  emit executeFinished(result);
  return result;
}

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Another approach is to use a solution similar to QFuture just for signals and slots or to emit a signal "resultReady" from the slot so that you don't transmit the result itself but only an information that it is available and you can read it from the object owning the original slot. That's more or less what QFuture does only that the result is available not in the object itself (B in our case) but in a separate object (the future).

[adamatic]

Ok, I have got the idea. But i have finally defined the variable as class member to have easier access to it.

But thank you anyway because for sure I will use this knowledge in the future.

[astodolski]

Slots can return values but only when they are called as regular methods or using QMetaObject::invokeMethod().

If you need a return value when using the slot function as a slot, then emit a signal from it and catch it back in the original sender.

I thought that may be the preferred way - seems messy, but I don't know of another way. In my case the original sender lives in a worker object moved to another thread