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From: Thomas Petit on 29 Mar 2010 06:21
On 15 mar, 13:35, PGK <graham.k...(a)gmail.com> wrote:
> In the class "Foo" below, is it possible to use a constructor
> initialization list to initialize "cs", a tuple of complex values?
>
> // #include <iostream>,<tuple>,<complex>,<type_traits>
>
> template <typename T, typename ... Ts>
> struct Foo {
> std::tuple<std::complex<T>, std::complex<Ts>...> cs;
>
> template<typename ... Us>
> Foo(T c1, T c2, Us ... ts) : ???? { }
>
> };
>
> int main(int argc, char *argv[]) {
> Foo<int,bool,double> a(1,2,true,false,42.0,43.0); // For example
>
> }

Phew, what a great brain teaser !
I found an another way :

template <typename T>
std::tuple<std::complex<T>>
make_tuple_complex(T real, T im)
{
return std::make_tuple(std::complex<T>(real, im));
}

template <typename Head, typename... Tail>
auto make_tuple_complex(Head real, Head im, Tail... tail)
-> decltype(std::tuple_cat( make_tuple_complex(real, im),
make_tuple_complex(tail...)))
{
return std::tuple_cat( make_tuple_complex(real, im),
make_tuple_complex(tail...));
}

template <typename... Ts>
struct Foo
{
std::tuple<std::complex<Ts>...> cs;

template <typename... Us>
Foo(Us... us) : cs(make_tuple_complex(us...)) {}
};

int main()
{
Foo<int, double> foo(4, 5, 1.0, 4.0);
}


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From: PGK on 31 Mar 2010 07:34
> Phew, what a great brain teaser !
> I found an another way :
>
> template <typename T>
> std::tuple<std::complex<T>>
> make_tuple_complex(T real, T im)
> {
> return std::make_tuple(std::complex<T>(real, im));
>
> }
>
> template <typename Head, typename... Tail>
> auto make_tuple_complex(Head real, Head im, Tail... tail)
> -> decltype(std::tuple_cat( make_tuple_complex(real, im),
> make_tuple_complex(tail...)))
> {
> return std::tuple_cat( make_tuple_complex(real, im),
> make_tuple_complex(tail...));
>
> }
>
> template <typename... Ts>
> struct Foo
> {
> std::tuple<std::complex<Ts>...> cs;
>
> template <typename... Us>
> Foo(Us... us) : cs(make_tuple_complex(us...)) {}
>
> };
>
> int main()
> {
> Foo<int, double> foo(4, 5, 1.0, 4.0);
>
> }

Glad you're enjoying the puzzle; me too. Unfortunately your approach
doesn't
work with more than 4 arguments. (I think you need a one-to-one
correspondence between template and normal arguments(?))

Shame, it would be nice if it could be shortened.



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