On 7 Mrz., 18:37, AlbertoBarb...@libero.it (Alberto Ganesh Barbati)
wrote:
> Daniel Kr   gler ha scritto:
> > So, if VS2005 compiles this successfully (modulo function definition),
> > it behaves non-conforming in this regard (Even VS2005-SP1
> > does accept this program).
>
> Interesting. How does VS2005 compile this, then?
>
> struct base{};
> struct derived:base{};
>
> template<base*>    void foo() {}
> template<derived*> void foo() {} // overload!
>
> derived d;
>
> int main()
> {
>   foo<&d>();  // should unambigously select foo<derived*>
>
> }
>
> For the OP: this example might show the rationale that you were asking.
> If you allow derived-to-base conversion, foo<derived*> can never be
> called, unless complex "best-match" rules are further introduced to
> disambiguate this scenario. I've never seen this case used in practice,
> so I guess the committee decided that writing down those rules just
> wasn't worth the effort.

Good point. VS2005-SP1 behaves exactly as you predict:

"error C2668: 'foo' : ambiguous call to overloaded function
main.cpp(5): could be 'void foo<& d>(void)'
main.cpp(4): or 'void foo<& d>(void)'
while trying to match the argument list '(void)'"

Greetings from Bremen,

Daniel


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Hello,

What is the reason for the standard to not allow common conversions
like derived-to-base for non-type template-parameters of type pointer
to
object?

struct base{};
struct derived:base{};

template<base*> void foo();

derived d;

int main()
{
  // error: no instance of function template "foo" matches the
  // argument list
  foo<&d>();
}

Thank you in advance,

Joaqu   n M L   pez Mu   oz
Telef   nica, Investigaci   n y Desarrollo


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Did you forgot to define the function body of the foo()? If so, you
will get a linking error, not compilation error. I tested your code in
VS2005 and got the linking error. After defining the foo(), every
thing gone well.

"Joaqu  n M L  pez Mu  oz
"
> Hello,
>
> What is the reason for the standard to not allow common conversions
> like derived-to-base for non-type template-parameters of type pointer
> to
> object?
>
> struct base{};
> struct derived:base{};
>
> template<base*> void foo();
>
> derived d;
>
> int main()
> {
>   // error: no instance of function template "foo" matches the
>   // argument list
>   foo<&d>();
> }
>
> Thank you in advance,
>
> Joaqu  n M L  pez Mu  oz
> Telef  nica, Investigaci  n y Desarrollo
>
>
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On Mar 6, 8:35 am, "h.yuz...@gmail.com" <h.yuz...@gmail.com> wrote:
> Did you forgot to define the function body of the foo()? If so, you
> will get a linking error, not compilation error. I tested your code in
> VS2005 and got the linking error. After defining the foo(), every
> thing gone well.

Strictly speaking you are right. As I learned recently, using an
undefined
entity belongs to the UB realm.
But even if he would fix this issue by simply providing a proper
definition,
e.g.

template<base*> void foo() {}

this program should not compile (as Joaqu   n correctly pointed out),
because of what 14.3.2/5 demands:

"for a non-type template-parameter of type pointer to object,
qualification
conversions (4.4) and the array-to-pointer conversion (4.2) are
applied.
[ Note: In particular, neither the null pointer conversion (4.10) nor
the
derived-to-base conversion (4.10) are applied.[..]"

So, if VS2005 compiles this successfully (modulo function definition),
it behaves non-conforming in this regard (Even VS2005-SP1
does accept this program).

Greetings from Bremen,

Daniel Kr   gler


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