Question

I have a processing pipeline implemented, but I would like to improve it like this :

#include <iostream>

// buffers
struct src{}; struct b1{}; struct snk{};
// filters
struct f1
{
    f1( const src &, b1 & ) { std::cout << "f1( src, b1 )" << std::endl; }
};
struct f2
{
    f2( const b1 &, snk & ) { std::cout << "f2( b1, snk )" << std::endl; }
};
// the pipeline
template< typename... Filters >
struct pipeline
{
    template< typename LastB >
    pipeline( const LastB & )
    {}
};
template < typename T1, typename... T >
struct pipeline< T1, T... > : pipeline< T... >
{
    template< typename... Buffs, typename Bin, typename Bout >
    pipeline( Buffs &... buffs, Bin & bin, Bout & bout ) :
        pipeline< T... >( buffs..., bin ),
        filter( bin, bout )
    {
    }

    T1 filter;
};

int main()
{
    src ba; b1  bb; snk bc;

    pipeline< f1 > p1( ba, bb );
    pipeline< f1, f2 > p2( ba, bb, bc ); // the problem is in this line!
}

Unfortunately, the example above produces next errors :

sda_variadic.cpp: In function 'int main()':
sda_variadic.cpp:40:39: error: no matching function for call to 'pipeline<f1, f2>::pipeline(src&, b1&, snk&)'
sda_variadic.cpp:40:39: note: candidates are:
sda_variadic.cpp:26:5: note: template<class ... Buffs, class Bin, class Bout> pipeline<T1, T ...>::pipeline(Buffs& ..., Bin&, Bout&)
sda_variadic.cpp:23:8: note: constexpr pipeline<f1, f2>::pipeline(const pipeline<f1, f2>&)
sda_variadic.cpp:23:8: note:   candidate expects 1 argument, 3 provided
sda_variadic.cpp:23:8: note: constexpr pipeline<f1, f2>::pipeline(pipeline<f1, f2>&&)
sda_variadic.cpp:23:8: note:   candidate expects 1 argument, 3 provided

What is the reason for this error?
How to fix it?

Just a small explanation. I expect that the example above will first create an object of the unspecialized pipeline<>( snk ), then the specialized object pipeline< f1 >(b1,snk), and then the specialized object pipeline< f1, f2 >(src,b1,snk).
btw take a note that the above example works for 1 filter (pipeline< f1).

Answer 1

template< typename... Buffs, typename Bin, typename Bout >
pipeline( Buffs &... buffs, Bin & bin, Bout & bout ) :
    pipeline< T... >( buffs..., bin ),
    filter( bin, bout )
{
}

Since the function parameter pack (Buffs) is not in the last position, it cannot be deduced. From 14.8.2.1 Deducing template arguments from a function call [temp.deduct.call] paragraph 1:

1. […] For a function parameter pack that does not occur at the end of the parameter-declaration-list, the type of the parameter pack is a non-deduced context. […]

Since you can’t explicitly pass template parameters to a constructor, it can’t be called at all (not that it matters for your problem though).

I’d recommend using std::tuple to manipulate the variadic arguments, such that you call the constructor like so: pipeline(std::forward_as_tuple(b0, b1, b2), in, out) and an empty tuple would be passed to the last stage.