"no unique final overrider" error

[Matt Kersen]

Any idea why the code for the following inheritance hierarchy will
compile and run (using gcc 3.2):

A::func()
/ \
/ \
/ \
B::func() C
\ /
\ /
\ /
D

#include <iostream>
using namespace std;

class A
{
public:
virtual void func() { cout << "A::f()" << endl; }
};

class B : public virtual A
{
public:
virtual void func() { cout << "B::f()" << endl; }
};

class C : public virtual A
{ };

class D : public virtual B, public virtual C
{ };

int main()
{
D d;
d.func();
return 0;
}

but the code for the following inheritance hierarchy gives the
following error message:

A::func()
/ \
/ \
/ \
B::func() C::func()
| |
| |
| |
D::func() E
\ /
\ /
\ /
F

test2.cpp:32: no unique final overrider for `virtual void A::func()'
in `F'
test2.cpp: In function `int main()':
test2.cpp:37: request for member `func' is ambiguous
test2.cpp:19: candidates are: virtual void C::func()
test2.cpp:25: virtual void D::func()

#include <iostream>
using namespace std;

class A
{
public:
virtual void func() { cout << "A::func()" << endl; }
};

class B : public virtual A
{
public:
virtual void func() { cout << "B::func()" << endl; }
};

class C : public virtual A
{
public:
virtual void func() { cout << "C::func()" << endl; }
};

class D : public virtual B
{
public:
virtual void func() { cout << "D::func()" << endl; }
};

class E : public virtual C
{ };

class F : public virtual D, public virtual E
{ };

int main()
{
F f;
f.func();
return 0;
}

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[LLeweLLyn]

matthew...@baesystems.com (Matt Kersen) writes:

> Any idea why the code for the following inheritance hierarchy will
> compile and run (using gcc 3.2):
>
> A::func()
> / \
> / \
> / \
> B::func() C
> \ /
> \ /
> \ /
> D

10.2/6:
When virtual base classes are used, a hidden declaration can be
reached along a path through the sub-object lattice that does not
pass through the hiding declaration. This is not an ambiguity. The
identical use with non-virtual base classes is an ambiguity; in
that case there is no unique instance of the name that hides all
the others.

An example like yours, but more complex, follows.

[code snipped]

> but the code for the following inheritance hierarchy gives the
> following error message:
>
> A::func()
> / \
> / \
> / \
> B::func() C::func()
> | |
> | |
> | |
> D::func() E
> \ /
> \ /
> \ /
> F

[code snipped]

Here, D::func() hides B::func(), but does not hide C::func() - so
there is no unique inheritance of the name that hides all the
others.

[Shane Beasley]

matthew...@baesystems.com (Matt Kersen) wrote in message
news:<b498fd45.03052...@posting.google.com>...

> Any idea why the code for the following inheritance hierarchy will
> compile and run (using gcc 3.2):
>
> A::func()
> / \
> / \
> / \
> B::func() C
> \ /
> \ /
> \ /
> D

Only one branch of the diagram overrides A::func(); there's only one
valid override as of class D.

> but the code for the following inheritance hierarchy gives the
> following error message:
>
> A::func()
> / \
> / \
> / \
> B::func() C::func()
> | |
> | |
> | |
> D::func() E
> \ /
> \ /
> \ /
> F
>
> test2.cpp:32: no unique final overrider for `virtual void A::func()'
> in `F'
> test2.cpp: In function `int main()':
> test2.cpp:37: request for member `func' is ambiguous
> test2.cpp:19: candidates are: virtual void C::func()
> test2.cpp:25: virtual void D::func()

Look at the error, then look at the diagram. Both branches offer their
own override -- the left offers D::func() and the right offers
C::func(). As g++ puts it, there is "no unique final overrider."

You don't even need this complex scenario -- you could have done the
same with the first one:

A::func()
/ \
/ \
/ \
B::func() C::func()

\ /
\ /
\ /
D

To fix, you need to provide an override in the bottom-most class of
the diagram to tell the compiler which override to call. For instance,
to select C::func() in the second diagram:

void F::func () { C::func(); }

- Shane