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POc++ using declaration, scope and access control
 

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    <p>Typically the 'using' declaration is used to bring into scope some member functions of base classes that would otherwise be hidden. From that point of view it is only a mechanism for making accessible information more convenient to use.<br> However: the 'using' declaration can also be used to change access constraints (not only for functions but also for attributes). For example: </p> <pre><code>class C{ public: int a; void g(){ cout &lt;&lt; "C:g()\n"; } C() : a(0){} }; class D : public C{ private: using C::a; using C::g; public: D() { a = 1; } }; int main(void){ D d; cout &lt;&lt; d.a &lt;&lt; endl; //error: a is inaccessible C *cp = &amp;d; cout &lt;&lt; cp-&gt;a &lt;&lt; endl; //works d.g(); //error: g is inaccessible cp-&gt;g(); //works return 0; } </code></pre> <p>I think this limitation of access in the derived class is actually of no use, because you can always access g() and a from a pointer to the base class. So should't there be at least some kind of compiler warning? Or wouldn't it been even better to forbid such limitation of access by a derived class? The using declaration is not the only possibility to add constraints to access. It could also be done via overriding a base class' function an placing it in a section with more access constraints. Are there some reasonable examples where it is indeed nessecary to limit access in such a way? If not I don't see why it should be allowed. </p> <p>And another thing: at least with g++ the same code compiles well without the word 'using'. That means for the example above: it's possible to write C::a; and C::g; instead of using C::a; using C::g; Is the first only a shortcut for the latter or are there some subtle differences? </p> <p>//EDIT:<br> so from the discussion and answers below my conclusion would be:<br> - it's allowed to limit access constraints in derived classes with public inheritance<br> - there are useful examples where it could be used<br> - it's use might cause problem in combination with templates (e.g. a derived class could not be a valid parameter for some template class/function any more although it's base is)<br> - a cleaner language design should not allow such use<br> - compiler could at least issue some kind of warning</p>
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