Hi all,
I am having a problem trying to overload a function template, based on
a typedef, such as:
template<class T>
struct A
{};
template<class T>
struct B
{
typedef A<T> type;
};
template<class T>
void foo(const typename B<T>::type&)
{}
void bar()
{
foo(B<int>::type());
}
g++ 3.3.1 complains at the above code with the following message:
no matching function for call to `foo(A<int>)'
Similar code works with no problem in VC6.
Am I missing something?
Thanks in advance,
Arkadiy 5 7082
Arkadiy Vertleyb wrote: Hi all,
I am having a problem trying to overload a function template, based on a typedef, such as:
template<class T> struct A {};
template<class T> struct B { typedef A<T> type; };
template<class T> void foo(const typename B<T>::type&) {}
void bar() { foo(B<int>::type()); }
g++ 3.3.1 complains at the above code with the following message: no matching function for call to `foo(A<int>)'
Similar code works with no problem in VC6.
Am I missing something?
Even if this is not allowed by the standard (which I think it is given
some things I've seen - just a hunch) the error message is totally wrong.
Besides:
foo<int>(B<int>::type());
compiles fine.
I suggest you post a bug on gcc. Before you do, you might want to try a
gcc 3.4 devel snapshot. Some template things (like this) have been
fixed in the development branch.
On 25 Sep 2003 19:27:16 -0700, ve******@hotmail.com (Arkadiy Vertleyb)
wrote: Hi all,
I am having a problem trying to overload a function template, based on a typedef, such as:
template<class T> struct A {};
template<class T> struct B { typedef A<T> type; };
template<class T> void foo(const typename B<T>::type&)
The above contains a non-deducable context (in this case, a nested
type). This means that calls to the function cannot rely on template
argument deduction, but must use explicit template argument
specification.
The reason this is non-deducable is that the mapping from nested type
to containing type is not in general one-to-one. Often many different
Ts would give the same B<T>::type (think specialization of B).
{}
void bar() { foo(B<int>::type());
foo<int>(B<int>::type());
}
g++ 3.3.1 complains at the above code with the following message: no matching function for call to `foo(A<int>)'
Right.
Similar code works with no problem in VC6.
Yes, this is a bizarre "feature" of VC6 that it deduces this kind of
non-deducable context.
Am I missing something?
Just that VC6 is very old and non-standard.
Tom
tom_usenet wrote: On 25 Sep 2003 19:27:16 -0700, ve******@hotmail.com (Arkadiy Vertleyb) wrote:
Hi all,
I am having a problem trying to overload a function template, based on a typedef, such as:
template<class T> struct A {};
template<class T> struct B { typedef A<T> type; };
template<class T> void foo(const typename B<T>::type&)
The above contains a non-deducable context (in this case, a nested type).
What's non-deductible about it ?
const typename B<T>::type& <<>> B<int>::type
infers T is int. right ?
How does the standard define non-deductible ?
I must admit, I've never come across this kind of problem, so I really
don't know. Time to brush up ...
On 26 Sep 2003 14:09:31 GMT, Gianni Mariani <gi*******@mariani.ws>
wrote: tom_usenet wrote: On 25 Sep 2003 19:27:16 -0700, ve******@hotmail.com (Arkadiy Vertleyb) wrote:
Hi all,
I am having a problem trying to overload a function template, based on a typedef, such as:
template<class T> struct A {};
template<class T> struct B { typedef A<T> type; };
template<class T> void foo(const typename B<T>::type&)
The above contains a non-deducable context (in this case, a nested type).
What's non-deductible about it ?
It attempts to deduce a template parameter from the type of a typedef
of the template. 14.8.2.4/4 is the relevent bit of the standard.
const typename B<T>::type& <<>> B<int>::type
infers T is int. right ?
No, and the deduction can't even reliably be made. e.g.
template<>
struct B<float>
{
typedef A<int> type; //whoops, now which B?
};
Of course, this could just cause an ambiguity error, as it does in
VC6. However, the mapping from inner type to template parameter is not
always many-to-one, so the committee decided to forbid it. How does the standard define non-deductible ?
Read up on nondeduced contexts.
Tom
Hi guys,
Thanks for your responce.
Come to think about it, I do realize that overloading on a nested
typedef in this context (at least in my case) doesn't make a lot of
sense. The nested typedef may after all resolve to different types,
like:
template<class T>
struct A1
{};
template<class T>
struct A2
{};
template<class T>
struct B
{
// something like:
typedef select<some_condition_based_on_T, A1<T>, A2<T> >::type type;
};
For both A1 and A2 I need to define a separate overloaded function.
So I actually have to overload on A1<T> and A2<T>, rather than on
B<T>::type.
Regards,
Arkadiy
tom_usenet <to********@hotmail.com> wrote in message news:<9j********************************@4ax.com>. .. On 26 Sep 2003 14:09:31 GMT, Gianni Mariani <gi*******@mariani.ws> wrote:
tom_usenet wrote: On 25 Sep 2003 19:27:16 -0700, ve******@hotmail.com (Arkadiy Vertleyb) wrote:
Hi all,
I am having a problem trying to overload a function template, based on a typedef, such as:
template<class T> struct A {};
template<class T> struct B { typedef A<T> type; };
template<class T> void foo(const typename B<T>::type&)
The above contains a non-deducable context (in this case, a nested type).
What's non-deductible about it ?
It attempts to deduce a template parameter from the type of a typedef of the template. 14.8.2.4/4 is the relevent bit of the standard.
const typename B<T>::type& <<>> B<int>::type
infers T is int. right ?
No, and the deduction can't even reliably be made. e.g.
template<> struct B<float> { typedef A<int> type; //whoops, now which B? };
Of course, this could just cause an ambiguity error, as it does in VC6. However, the mapping from inner type to template parameter is not always many-to-one, so the committee decided to forbid it.
How does the standard define non-deductible ?
Read up on nondeduced contexts.
Tom This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics
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