Combinatorial arg explosion for function accepting initializer list literals

[Alf P. Steinbach]

In C++03 perfect forwarding of a sequence of arguments of arbitrary
types, caused a combinatorial explosion in const versus non-const.

In C++11 and later even just writing a function that /accepts/ arbitrary
argument types, when it's desirable to accept std::initializer_list
literals such as `{1, 2, 3}`, causes a combinatorial explosion in
initializer_list (which must known to the compiler as such) versus others.

E.g. in order to be able to call

multi( {1, 2, 3}, {5, 6, 7} )

with up to 3 arguments, a bunch of silly overloads are apparently needed:

template< class... Collections >
class Multidimensional_view_
{
public:
Multidimensional_view_( Collections const&... )
{}
};

template< class... Collections >
inline $f make_multidimensional_view( ref_<const Collections>... c )
-> Multidimensional_view_< Collections... >
{ return Multidimensional_view_< Collections... >{ c... }; }

template< class... Collections >
inline auto multi( ref_<Collections>... c )
{ return make_multidimensional_view( c... ); }

template< class Value >
// i
inline auto multi(
ref_<const initializer_list<Value>> list
)
{ return make_multidimensional_view( list ); }

template< class Collection, class Value >
// ci
inline auto multi(
ref_<const Collection> c,
ref_<const initializer_list<Value>> list
)
{ return make_multidimensional_view( c, list ); }

template< class Value, class Collection >
// ic
inline auto multi(
ref_<const initializer_list<Value>> list,
ref_<const Collection> c
)
{ return make_multidimensional_view( list, c ); }

template< class Value_1, class Value_2 >
// ii
inline $f multi(
ref_<const initializer_list<Value_1>> list_1,
ref_<const initializer_list<Value_2>> list_2
)
{ return make_multidimensional_view( list_1, list_2 ); }

template< class Collection_1, class Collection_2, class Value >
// cci
inline auto multi(
ref_<const Collection_1> c_1,
ref_<const Collection_2> c_2,
ref_<const initializer_list<Value>> list
)
{ return make_multidimensional_view( c_1, c_2, list ); }

template< class Collection_1, class Value, class Collection_2 >
// cic
inline auto multi(
ref_<const Collection_1> c_1,
ref_<const initializer_list<Value>> list,
ref_<const Collection_2> c_2
)
{ return make_multidimensional_view( c_1, list, c_2 ); }

template< class Collection, class Value_1, class Value_2 >
// cii
inline auto multi(
ref_<const Collection> c,
ref_<const initializer_list<Value_1>> list_1,
ref_<const initializer_list<Value_2>> list_2
)
{ return make_multidimensional_view( c, list_1, list_2 ); }

template< class Value, class Collection_1, class Collection_2 >
// icc
inline auto multi(
ref_<const initializer_list<Value>> list,
ref_<const Collection_1> c_1,
ref_<const Collection_2> c_2
)
{ return make_multidimensional_view( list, c_1, c_2 ); }

template< class Value_1, class Collection, class Value_2 >
// ici
inline auto multi(
ref_<const initializer_list<Value_1>> list_1,
ref_<const Collection> c,
ref_<const initializer_list<Value_2>> list_2
)
{ return make_multidimensional_view( list_1, c, list_2 ); }

template< class Value_1, class Value_2, class Collection >
// iic
inline auto multi(
ref_<const initializer_list<Value_1>> list_1,
ref_<const initializer_list<Value_2>> list_2,
ref_<const Collection> c
)
{ return make_multidimensional_view( list_1, list_2, c ); }

template< class Value_1, class Value_2, class Value_3 >
// iii
inline auto multi(
ref_<const initializer_list<Value_1>> list_1,
ref_<const initializer_list<Value_2>> list_2,
ref_<const initializer_list<Value_3>> list_3
)
{ return make_multidimensional_view( list_1, list_2, list_3 ); }

How can this be avoided?


Cheers!,

- Alf

[Alf P. Steinbach]

On 30-Mar-17 1:40 PM, Alf P. Steinbach wrote:
> [snip]
>

Sorry about the formatting, the non-aligned comments: it's Thunderbird
messing things up with its flowed format implementation.

Cheers!,

- Alf

[Juha Nieminen]

Alf P. Steinbach <alf.p.stein...@gmail.com> wrote:
> inline $f make_multidimensional_view( ref_<const Collections>... c )

Perhaps use C++, for starters?

[Alf P. Steinbach]

Oh sorry. It is C++ but with use of a library. I think the code example
can be sufficiently understood to illustrate the problem, without me
using much time on replacing the library stuff with plain direct C++.

Here just replace `$f` with `auto`, and the `ref_` is self-explanatory I
should think. :)

Cheers!,

- Alf