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    <p>You can only access <code>protected</code> members from your own base class instance... not one provided to you as a parameter. It's all about OO encapsulation really. Without this restriction, the object under construction could invalidate invariants of the <code>baseTest&amp;</code> parameter.</p> <p>Put another way, your <code>SubTest</code> may decide on a use for a <code>protected</code> member that conflicts with the usage made of the same member by another <code>BaseTest</code>-derived class (say <code>SubTest2 : BaseTest</code>). If your <code>SubTest</code> code was allowed to fiddle with the other object's data, it could invalidate the invariants in a <code>SubTest2</code> object, or get some values out that were - in the intended encapsulation - only meant to be exposed to <code>SubTest2</code> and (optionally - see below) <code>SubTest2</code> derivatives.</p> <blockquote> <p>Followup question: Why is it, that in the added copy constructor I can access protected members of another instance?</p> </blockquote> <pre><code>SubTest(const SubTest&amp; x); // can access x._protMember SubTest(const BaseTest&amp; x); // cannot access x._protMember </code></pre> <p>The same insights above explain why this is allowed: the copy constructor gets a <code>SubTest&amp;</code> rather than just any old object derived from <code>BaseTest</code>, and this constructor is clearly within the <code>SubTest</code> abstraction. The <code>SubTest</code> coder is assumed to be conversant with the intended design/encapsulation <code>SubTest</code> provides, and the copy constructor is given access to bypass and enforce post-conditions/invariants on the other <code>SubTest&amp;</code> object too. (You are copying from an object that might itself have been copy-constructed by the very same function, so protecting it when on the "<code>*this</code>" side but not the parameter-by-ref side isn't much protection at all, even ignoring all the sound reasons you may want/need that access).</p> <p>It is possible that a <code>SubTest</code>-derived object will be accidentally passed to the <code>SubTest</code> copy constructor ("slicing"), but even for that scenario the <code>SubTest&amp;</code> class can control whether the further-derived object could have been doing anything unexpected with <code>_protMember</code> - adding a <code>private</code> <code>using BaseTest::_protMember;</code> statement if it wants to "finalise" access to <code>_protMember</code> and forbid any derived classes from using it.</p>
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    1. POWhy can't my object access protected members of another object defined in common base class?
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    1. COThanks for your answer. Could you have a look at my followup question I added (with a function that _can_ access protected members of another instance.
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      1. USRonald McBean
    2. CO@RonaldMcBean: I've added to my answer addressing that. ("that can access protected members of another instance" - but not a `BaseTest`-but-not-`SubTest`-derived instance - there's the rub).
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    3. COThis is a great answer. I came across this issue a few years ago and it took me a long time to work out **why** the compiler behaved that way. What I realized is that the designers of C++ have done a great job in understanding the subtleties of where the responsibilities lie in object orientation.
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