emitting and catching signals

[srohit24]

Hi

I have two classes called receive and write.

What i need is , a signal is emitted in the receive class. this signal has to be caught in the write class and proceed as instructed.

I am emitting a signal called finished_receiving after i have parsed the data I get from a website.
I want to connect this signal to a proceed function in the write class.

What all should i specify in the connect? how should I declare the sender and receiver objects?

[faldzip]

Read about signals and slots in Qt Assistant or just click here as it is the link to online documentations, exactly the same which is in Assistant.

[srohit24]

I have read that. I am able to emit signals and catch them in a single class.

I am facing problems when i need to emit and catch them in 2 different classes.

A bit of help in the direction of the solution will be greatly helpful.

[PaceyIV]

Your class should be instantiate somewhere. There you should add
connect (classA, SIGNAL(signalA()), classB, SLOT(slotB()))
same as in the same class.

[srohit24]

The two classes are classA and classB. The main class is declared as mainclass.

I am emitting a signal in classA. I have written the code for connect in mainclass's constructor
I have created a instance of the classA in the mainclass. The classA action isnt related to the mainclass action.

this is my connect signal

connect(classA_object,SIGNAL(receiving_finished()), classB_object, SLOT(write_data()));

connect(classA_object,SIGNAL(receiving_finished()), classB_object, SLOT(write_data()));

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Its not giving me any errors but its not calling the write_data function either.

what am I doing wrong?

[spirit]

are you sure that receiving_finished signal is emitted? put qDebug in code where you emits this signal.

[nightghost]

It would be easier to detect the problem if you would provide more code.

[srohit24]

void api_request::parse_read_alldata()
{
.......
emit receiving_finished();
}//this is the classA function where i am emitting the signal
 
connect(classA_object,SIGNAL(receiving_finished()), classB_object, SLOT(write_data()));
//This is present in the mainclass's constructor which has a instance of classB_object but no classA instance.
 
void write_data()
{
    qDebug() << "in write data";
}//this is the actual function that needs to be called and is present in classB.

void api_request::parse_read_alldata()
{
.......
emit receiving_finished();
}//this is the classA function where i am emitting the signal

connect(classA_object,SIGNAL(receiving_finished()), classB_object, SLOT(write_data()));
//This is present in the mainclass's constructor which has a instance of classB_object but no classA instance.

void write_data()
{
    qDebug() << "in write data";
}//this is the actual function that needs to be called and is present in classB.

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[nish]

connect(classA_object,SIGNAL(receiving_finished()), classB_object, SLOT(write_data()));
//This is present in the mainclass's constructor which has a instance of classB_object but no classA instance.

connect(classA_object,SIGNAL(receiving_finished()), classB_object, SLOT(write_data()));
//This is present in the mainclass's constructor which has a instance of classB_object but no classA instance.

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you say the "no classA instance." is present in mainclass so how are you able to connect..
i am afraid that you need to give some more code... why not just provide a simple minimal compilable program reproducing the problem?

[srohit24]

Should i place the connect command in classB's constructor to connect the signals as classB has classA's instance?

EDIT

I placed the connect code in the classB's constructor.The app compiled successfully but i got this in qDebug

connect(classA_object,SIGNAL(receiving_finished()), this, SLOT(write_data()));
 
QObject::connect: Cannot connect (null)::receiving_finished() to classB::write_data()

connect(classA_object,SIGNAL(receiving_finished()), this, SLOT(write_data()));

QObject::connect: Cannot connect (null)::receiving_finished() to classB::write_data()

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[spirit]

classA_object is not created, i.e. classA_object == 0.
EDIT: take a debugger in your hands and debug your programm step by step.

[srohit24]

ok. but I have created a class object and I am using it make a function call.

how can that be null?

[spirit]

this message says another

QObject::connect: Cannot connect (null)::receiving_finished() to classB::write_data()