How to check for a null pointer in qt4?

if (m_ser_1 == NULL) // c++ has no null
        // no match for ‘operator==’ (operand types are ‘QSerialPort’ and ‘long int’)
	    {
	    	MessageBoxRed("m_ser_1 == None. must have at least one board");
	        return;
	    }

	    if (m_ser_2 == nullptr)
        // no match for ‘operator==’ (operand types are ‘QSerialPort’ and ‘std::nullptr_t’)
     	MessageBoxRed("m_ser_2 == None. may or may not have only one board");
	    if (m_ser_3 == null)
         // ‘null’ was not declared in this scope
	        MessageBoxRed("m_ser_3 == None. may or may not have only one board");