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POC++ Inherit from unknown intermediate class(es) with known base class
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  1. POC++ Inherit from unknown intermediate class(es) with known base class
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    copied!<p>I have an abstract class, called UsbDevice (mostly pure virtual). It doesn't do much, but the three derived classes do:</p> <pre><code>class WindowsUsbDevice : public UsbDevice class LinuxUsbDevice : public UsbDevice class OsxUsbDevice : public UsbDevice </code></pre> <p>each of these classes doesn't really add any public methods - instead they are just different implementations of the UsbDevice abstract class.</p> <p>Now, I want to create a new class that derives from any one of these classes, let's call it FancyUsbDevice. Lets say it represents a specific device.</p> <p>This new class needs to expose the full UsbDevice interface (implemented by the intermediate class), as well as expose a few fancy high-level functions that only use methods on the UsbDevice interface - no need to expose anything specific to one of those intermediate classes.</p> <p>I can see three possible options, and I would like to know if they would work, and which one is the best.</p> <p>A) don't inherit, instead make FancyUsbDevice HAS-A UsbDevice* instead of be IS-A. (sidestep the problem. I don't like this option, but it feels safe.)</p> <p>B) Assuming I have the rare advantage of only having one of those classes actually build on any specific platform, #ifdef the inheritance string: (sidestep the problem again)</p> <pre><code>#ifdef WIN32 class FancyUsbDevice : public WindowsUsbDevice #endif #ifdef LINUX class FancyUsbDevice : public LinuxUsbDevice #endif </code></pre> <p>...</p> <p>C) Templates! Inherit from the template type:</p> <pre><code>Template&lt;typename T&gt; class FancyUsbDevice : public T </code></pre> <p>construct it as the child type, and cast it to the interface:</p> <pre><code>FancyUsbDevice&lt;WindowsUsbDevice&gt; fancy_device; FancyUsbDevice&lt;UsbDevice&gt; generic_fancy_device = dynamic_cast&lt;FancyUsbDevice&lt;UsbDevice&gt;&gt;(fancy_device); </code></pre> <p>however, I don't think that would work, for at least the reason that the compiler will think that generic_fancy_device is smaller than fancy_device, and all of the member variables will be offset and broken. (I think)</p> <p>Thanks! Sorry for using interface / abstract / pure virtual interchangeably. </p>
 

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