Connecting 2 different classes together

[GunkutA]

Hello, I am trying to send a signal from one class to another class. For this, I am using a connect line such as this:

USB *USBInstance = new USB;
  connect(this,SIGNAL(sendChosenComPort(QString)),USBInstance,SLOT(receiveChosenComPort()));

USB *USBInstance = new USB;
  connect(this,SIGNAL(sendChosenComPort(QString)),USBInstance,SLOT(receiveChosenComPort()));

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But after I add these lines, when I run, "The program has unexpectedly finished." error appears.
Then I tried to add

USB* USBInstance ;

USB* USBInstance ;

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to the header file and

USBInstance = new USB()

USBInstance = new USB()

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to cpp. file. But again that gives an error. How can I connect 2 different classes in order to send a signal? Thanks beforehand.
P.S: I am a total newbie to QT, so please explain it accordingly .