Re: Derived class hiding a name
On 22 Mai, 13:57, "jord...@gmail.com" <jord...@gmail.com> wrote:
The following code will not compile:
class foo{};
class A{
public:
void f(int a ){a++;};
private:
virtual void f(foo a) = 0;
};
class B : public A{
private:
virtual void f(foo a){};
};
int main(){
B b;
int a=0;
b.f(a);
}
The problem seems to be that all of my functions being named f are
somehow colliding with each other. It seems to me that the
call b.f(a) is unambiguosly pointing to A::f(int), but gcc disagrees.
gcc is right here, v.i.
I can fix this if I namespace the b.f(a) call, but that's a little
ugly. I can also fix it if I put a forwarding function inside B that
calls the proper function inside A, also a little ugly. I could also
mangle my functions' names, but I really feel that's the compiler's
job, not mine, especially since I think I already provided enough
context with a function signature. Also, I'd like to keep the
functions' names as they are, since they reflect an underlying
mathematical structure that really could use the same names.
If a using declaration should be enough to fix this, where should I
place? Or a better fix?
1) Why is this so? This behavior is due to [class.member.lookup]/2:
"[..] A member name f in one subobject B hides a member name f
in a sub-object A if A is a base class sub-object of B. Any
declarations
that are so hidden are eliminated from consideration. Each of these
declarations that was introduced by a using-declaration is considered
to be from each sub-object of C that is of the type containing the
declaration designated by the using-declaration.[..]"
This rule is actually a protection shield that C++ provides for
derived classes, which do not always want uncontrolled name
leaking from it's base classes.
2) How do you solve the problem? You actually recognized the
solution with your last sentence: Just add an in-class-using-
declaration
in the public "domain" of B, i.e.:
class B : public A{
.... // Old code
public:
using A::f;
};
HTH & Greetings from Bremen,
Daniel Kr?gler
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