Step 4: Adding the Pin Class
############################

..	include::	/header.inc

Let's start off with our project by building the lowly pin. We need a way to
attach wires to parts, and just pointing a wire to a part will not do. We need
to know what wire we are working with, and there will be several wires
connected to each part. 

A Pin is part of a component, and it is a place to attach a wire. It is also a
convenient place to record the value of a signal, either one arriving at a part
from an attached wire, or one leaving a part, headed out on another attached
wire. The Pin object does not do much. In fact, it does nothing at all. It just
sits there holding a value until some other gadget wants that value. Both wires
and components can write to the pin, or read from the pin.

Signal Values
*************

Computers know nothing about "data types". To the machine every signal moving
is just a pile of binary bits. For our current project, the most bits we will
want to move at one time is 16, which happens to be the size of an address in
this machine. The best C++ data type to use for this kind of signal is a
``uint16_t`` (unsigned 16-bit integer). This data type is defined in the
``cstdint`` header. 

Pin Class Specification
***********************

Let's add the attributes and methods needed for our Pins.

..	note::

        Although we are calling this gadget a "pin" it is really a set of pins,
        over which a multi-bit signal can move all at once (in parallel). The
        "wire" we attach to a pin will be moving all of the required bits as well. The
        proper term for that "wire" is a "bus" (it carries a bunch of bits!)

Here is the new header for the ``Pin`` class:

..	literalinclude::	code/cpu-factory5/include/Pin.h
	:linenos:
	:caption: include/Pin.h

Testing the Pin Class
*********************

Testing this class is pretty easy. All we need to add to our test set is code
that sets a pin value and reads it back. The test code is pretty simple. 

.. literalinclude::  code/cpu-factory5/tests/test_pin.cpp
   :linenos:
   :caption:   tests/test_pin.cpp

Pin Implementation
******************

The code required to make this test pass is all the code we need for this
class. 


.. literalinclude::  code/cpu-factory5/lib/Pin.cpp
   :linenos:
   :caption:   lib/Pin.cpp

At this point, you should run your tests again, and verify that the new Pin
class functions properly! 

The more of this testing you do, the more satisfying it becomes. I like it when
my code is working properly! I can worry about something else, not this code!

.. note::

   As the code gets more complex, so will the tests. Testing cannot fix all of
   your problems in programming, but it sure can limit where you spend you time
   to the really tough parts of your code!