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POerror: 'void Base::output()' is protected within this context
 

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  1. POerror: 'void Base::output()' is protected within this context
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    <p>I'm confused about the errors generated by the <a href="http://codepad.org/dGA5KLYs" rel="nofollow noreferrer">following code</a>. In Derived::doStuff, I can access Base::output directly by calling it.</p> <p>Why can't I create a pointer to <code>output()</code> in the same context that I can call <code>output()</code>?</p> <p>(I thought protected / private governed whether you could use a name in a specific context, but apparently that is incomplete?) </p> <p>Is my fix of writing <code>callback(this, &amp;Derived::output);</code> instead of <code>callback(this, Base::output)</code> the correct solution?</p> <pre><code>#include &lt;iostream&gt; using std::cout; using std::endl; template &lt;typename T, typename U&gt; void callback(T obj, U func) { ((obj)-&gt;*(func))(); } class Base { protected: void output() { cout &lt;&lt; "Base::output" &lt;&lt; endl; } }; class Derived : public Base { public: void doStuff() { // call it directly: output(); Base::output(); // create a pointer to it: // void (Base::*basePointer)() = &amp;Base::output; // error: 'void Base::output()' is protected within this context void (Derived::*derivedPointer)() = &amp;Derived::output; // call a function passing the pointer: // callback(this, &amp;Base::output); // error: 'void Base::output()' is protected within this context callback(this, &amp;Derived::output); } }; int main() { Derived d; d.doStuff(); } </code></pre> <p>Edit: I'd love to know where this is in the stardard, but mostly I'm just trying to wrap my head around the concept. I think my problem is that <code>callback</code> doesn't have access to protected members of <code>Derived</code>, but it is able to call <code>Derived::output</code> if you pass it a pointer. How is a protected member of <code>Derived</code> that comes from <code>Derived</code> different from a protected member of <code>Derived</code> that comes from <code>Base</code>?</p>
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    1. CO+1, I am pretty sure that I read the answer recently but can't find it... let's hope some local guru / standardista will be able to enlighten us.
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