Help with Code Validation of Variadic templates

Help with Code Validation of Variadic templates

[Rene Buergel]

I'm currently playing around using mainline with the variadic template 
feature and would like to hear some comments on code gcc is rejecting.

#1:
=====
1 template<typename T, T a, T... Args> T max()
2 {
3     return a > max<T, Args>() ? a : max<T, Args>();
4 };
5
6 template<typename T, T a, T b>T max()
7 {
8     return a > b ? a : b;
9 };

int main()
{
    unsigned int a = max<unsigned int, 3, 2>();
    unsigned int a = max<unsigned int, 3, 2, 5>();
}

=====
Using the non-variadic template, everything works fine. With the 
variadic templates, gcc issues an error in line 3:
error: parameter packs not expanded with `...':
note:         'Args'
I think, that this code is valid. If not, please point it out to me.


#2:
=====
template<template<typename... Params> class Comp, typename... Params> 
void f( Params... p)
{
  static_assert( Comp<Params>::value, "" );
}

template <typename T>
class Foo
{
    static const int value=1;
};

int main()
{
        f<Foo, int, int>( );
}

=====
Here I'm not that sure about the validity of the code, my intention was 
to use tr1/type_traits in macros on ordinary variables instead of types. 
So, the variadic template f would resolve the type for me. That intended 
use was something like f<std::tr1::is_same>( _var1, _var2 );, which 
would compile-time-check, if _var1 and _var2 are exaclty the same type. 
But i'm not even coming that far, as gcc issues: "error: no matching 
function for call to 'f()'". Any Ideas, what's wrong here?

Re: Help with Code Validation of Variadic templates

[Larry Evans]

On 12/07/07 15:12, Rene Buergel wrote:
> I'm currently playing around using mainline with the variadic template 
> feature and would like to hear some comments on code gcc is rejecting.
> 
> #1:
> =====
> 1 template<typename T, T a, T... Args> T max()
> 2 {
> 3     return a > max<T, Args>() ? a : max<T, Args>();
> 4 };
> 5
> 6 template<typename T, T a, T b>T max()
> 7 {
> 8     return a > b ? a : b;
> 9 };
> 
> int main()
> {
>    unsigned int a = max<unsigned int, 3, 2>();
>    unsigned int a = max<unsigned int, 3, 2, 5>();
> }
> 
> =====
> Using the non-variadic template, everything works fine. With the 
> variadic templates, gcc issues an error in line 3:
> error: parameter packs not expanded with `...':
> note:         'Args'
[snip]
I think this is an instance of bug 40092:

   http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40092

A 1 line patch to gcc/cp/tree.c has been proposed there.
You might try the patch an see if it solves your problem.

-regards,
Larry

Re: Help with Code Validation of Variadic templates

[Larry Evans]

[-- Attachment #1: Type: text/plain, Size: 1149 bytes --]

On 08/16/09 09:32, Larry Evans wrote:
> On 12/07/07 15:12, Rene Buergel wrote:
>> I'm currently playing around using mainline with the variadic template 
>> feature and would like to hear some comments on code gcc is rejecting.
>>
>> #1:
>> =====
>> 1 template<typename T, T a, T... Args> T max()
>> 2 {
>> 3     return a > max<T, Args>() ? a : max<T, Args>();
>> 4 };
>> 5
>> 6 template<typename T, T a, T b>T max()
>> 7 {
>> 8     return a > b ? a : b;
>> 9 };
>>
>> int main()
>> {
>>    unsigned int a = max<unsigned int, 3, 2>();
>>    unsigned int a = max<unsigned int, 3, 2, 5>();
>> }
>>
>> =====
>> Using the non-variadic template, everything works fine. With the 
>> variadic templates, gcc issues an error in line 3:
>> error: parameter packs not expanded with `...':
>> note:         'Args'
> [snip]
> I think this is an instance of bug 40092:
[snip]
OOPS.  It's unrealted to bug 40092.
Part of the problem is Args needs to be expanded as follows:

    Args...

I think *maybe* the other problem is template function specialization.
Anyway, the workaround is to use template class specializations
as shown in the attached.

HTH.

-Larry

[-- Attachment #2: maxv.cpp --]
[-- Type: text/x-c++src, Size: 533 bytes --]

template<typename T, T... Args>struct maxc
;

template<typename T, T Head>struct maxc<T,Head>
{
    static T const value=Head;
};

template<typename T, T Head, T... Tail>struct maxc<T,Head,Tail...>
{
 private:
    static T const tail=maxc<T,Tail...>::value;
 public:
    static T const value=Head>tail?Head:tail;
};
  
  
template<typename T, T... Args> T maxf()
{
    return maxc<T,Args...>::value;
}

int main()
{
   unsigned int a = maxf<unsigned int, 3, 2>();
   unsigned int b = maxf<unsigned int, 3, 2, 5>();
   return a==b;
}