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.
However: the 'using' declaration can also be used to change access constraints (not only for functions but also for attributes). For example:
class C{
public:
int a;
void g(){ cout << "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 << d.a << endl; //error: a is inaccessible
C *cp = &d;
cout << cp->a << endl; //works
d.g(); //error: g is inaccessible
cp->g(); //works
return 0;
}
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.
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?
//EDIT:
so from the discussion and answers below my conclusion would be:
- it's allowed to limit access constraints in derived classes with public inheritance
- there are useful examples where it could be used
- 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)
- a cleaner language design should not allow such use
- compiler could at least issue some kind of warning
With regard to your declaration without using
: These are called "access declarations", and are deprecated. Here is the text from the Standard, from 11.3/1
:
The access of a member of a base class can be changed in the derived class by mentioning its qualified-id in the derived class declaration. Such mention is called an access declaration. The effect of an access declaration
qualified-id
;
is defined to be equivalent to the declarationusing
qualified-id
;
[Footnote: Access declarations are deprecated; member using-declarations (7.3.3) provide a better means of doing the same things. In earlier versions of the C++ language, access declarations were more limited; they were generalized and made equivalent to using-declarations - end footnote]
I would say that most often it's not good to change public members to private or protected members in the derived class, because this will violate the substitution principle: You know a base class has some functions, and if you cast to a derived class then you expect those functions to be callable too, because the derived class is-a base. And like you already mentioned, this invariant is already enforced anyway by the language allowing to convert (which working implicitly!) to a base class reference, or qualifying the function name, and then calling the (then public) function.
If you want to forbid someone calling a set of functions of the base, then i think this hints that containment (or in rare cases, private inheritance) is a better idea.