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OO Digital Pin class: single class or separate in and out classes

Jan Cumps

I'm experimenting with light-weight C++ for microcontrollers. Looking for your opinion for class(es) that represent GPIO pin behaviour

An output pin can

  • accept a value and set it
  • return the value it is set to
  • maybe go high-z

An input can:

  • return the value at its input

I may represent this

  • very simple as a single class.
    Getting the value would perform the "return" action above. The class would decide if it returns output state or input state internally
    Setting would only succeed if the pin is an output pin, else either throw an exception or silently ignore.
    Switching between the two would be simple
  • as separate classes
    out type pin would set the output as indicated, and return the output state if asked
    in type pin would return the value at its input. It would not provide setters.
    Switching between two modes would mean discarding the original object and creating one of the other kind.
    (should both classes inherit from a common parent that defines the actual pin?)

How would you solve this, as OO designer / abstractor that loves simplicity?

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  • Andrew J said:
    ... Thus separate classes work but a writable pin is a specialisation of a readable pin and thus WriteablePin could be considered to inherit from ReadablePin rather than be viewed as a sibling of some abstract superclass (a WriteablePin is also a ReadablePin and works for polymorphic purposes.)

    MBED has taken one of your suggested approaches. Both In and Out are different classes, with no superclass.

    Andrew J said:
    Once you get into a situation of "If pinIsSettable Then setValue else throw exception" within method code, you've stepped outside OO paradigm (broken encapsulation) which is the situation with your single class.

    That, I think is debatable. And a reason for creating this forum post :). 
    What I have done for now, is: accept a write to an input pin. It will not change its status, but when you switch it from an input to an output pin, it will output that value.

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  • Andrew J said:
    ... Thus separate classes work but a writable pin is a specialisation of a readable pin and thus WriteablePin could be considered to inherit from ReadablePin rather than be viewed as a sibling of some abstract superclass (a WriteablePin is also a ReadablePin and works for polymorphic purposes.)

    MBED has taken one of your suggested approaches. Both In and Out are different classes, with no superclass.

    Andrew J said:
    Once you get into a situation of "If pinIsSettable Then setValue else throw exception" within method code, you've stepped outside OO paradigm (broken encapsulation) which is the situation with your single class.

    That, I think is debatable. And a reason for creating this forum post :). 
    What I have done for now, is: accept a write to an input pin. It will not change its status, but when you switch it from an input to an output pin, it will output that value.

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