Jonathan Turkanis wrote:
Dear All,
I'm trying to write a metafunction rebind which, when fed a specialization T<P1,...,Pn> of an n-ary template T, returns T<mpl::_, ...., mpl::_>. This is straightforward if default template arguments are not considered when matching template template parameters. (See http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_closed.html#150.) E.g.,
[snip code]
I have two questions:
1. is my analysis correct?
I think so.
2. is there a better solution?
I don't know if it's better, but you can get rid of the SFINAE by doing something like: struct _ {}; template<class T, int N = 3> struct rebind : rebind<T, N - 1> {}; template<class T> struct rebind<T, 0> { typedef T type; }; template<template<class> class T, class P1> struct rebind<T<P1>, 1> { typedef T<_> type; }; template<template<class, class> class T, class P1, class P2> struct rebind<T<P1, P2>, 2> { typedef T<_, _> type; }; template< template<class, class, class> class T , class P1, class P2, class P3
struct rebind<T<P1, P2, P3>, 3> { typedef T<_, _, _> type; }; Or you could just use template_arity to pass the arity to the specialized template, instead of recursively trying every arity. I suspect template_arity has to do something equivalent to this though, so maybe this saves a few instantiations. HTH, -- Daniel Wallin