emitting and catching signals

[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?