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Ivan Godard
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 Posted: Fri Jun 10, 2005 10:21 am Post subject: language suggestion: "using" to resolve ambiguous overriding |
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Code like:
struct base { virtual void foo() = 0; };
struct d1 : public virtual base { virtual void foo() {} };
struct d2 : public virtual base { virtual void foo() {} };
struct der : public d1, public d2 { };
is an error, getting something like:
foo.cc:4: error: no unique final overrider for `virtual void
base::foo()' in `der'
To resolve the ambiguity you have to provide a local override that
forwards to the intended inherited override:
struct der : public d1, public d2 { virtual void foo() {
d1::foo() };
How about letting a using declaration do the job as well:
struct der : public d1, public d2 { using d1::foo; };
It would break no code and is a natural integration of the existing
inheritance and using notions.
Comments?
Ivan
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ta0kira@yahoo.com
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| Posted: Mon Jun 13, 2005 4:16 pm Post subject: Re: language suggestion: "using" to resolve ambiguous overri |
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What happens when either d1 or d2 doesn't extend the virtualness of
foo? A "final overrider" would cover up one/both of them. I think
it's better to have an ambiguity error because even though there is
only one base::foo in der, there is also d1::foo and d2::foo which
under no circumstances should be automatically resolved. In any case,
I don't think it is necessary if you avoid making classes with virtual
functions virtual base classes; wait to make them virtual until your
final non-virtual definition of the functions, or just leave them
abstract until the multiple instances converge.
ta0kira
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Alan McKenney
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| Posted: Mon Jun 13, 2005 7:45 pm Post subject: Re: language suggestion: "using" to resolve ambiguous overri |
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Ivan Godard wrote:
| Quote: | Code like:
struct base { virtual void foo() = 0; };
struct d1 : public virtual base { virtual void foo() {} };
struct d2 : public virtual base { virtual void foo() {} };
struct der : public d1, public d2 { };
is an error, getting something like:
foo.cc:4: error: no unique final overrider for `virtual void
base::foo()' in `der'
To resolve the ambiguity you have to provide a local override that
forwards to the intended inherited override:
struct der : public d1, public d2 { virtual void foo() {
d1::foo() };
How about letting a using declaration do the job as well:
struct der : public d1, public d2 { using d1::foo; };
It would break no code and is a natural integration of the existing
inheritance and using notions.
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"using" refers to names. Functions are not uniquely defined by
their name, but by their name and signature (hence, name-mangling.)
An example where your idea breaks down:
struct base { void f( int ); void f( float ); ... };
struct d1 { void f( int ); void f( float ); ... };
struct d2 { void f( int ); ... };
struct dd { using d2::f; ... };
dd d;
d.f( 2.5 ); // which f? base::f(float)? d1::f(float)? d2::f(int)?
The standard-conforming method for resolving is a little
clunky, but makes it clear which f() you want, and with
a good compiler, should incur no extra cost.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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