c++ - Using pair in variadic templates -

okay, i'm trying implement range tree in n-dimensions using variadic templates. have basic setup working, want able optionally pass compare function object in template list sort tree other std::less.

my first thought overload template list version includes pair first element, pair containing type , compare funcition. can't see template declaration line compile. visual c++ (2015) starts c2079: "std::pair::first uses undefined class 't'".

anyways, on code. here's small snippet show i'm trying do:

template <class... args> class rangetree{ };  template <class t, class... args> class rangetree<t, args...> { public:   map <t, rangetree<args...> * > tree; }; 

this works normally. when add version of rangetree, pair first template member, have problems:

template <pair<class t, class compare>, class... args> class rangetree<pair<t, compare>, args...>{ public:   map <t, rangetree <args...> *, compare> tree; }; 

this part can seem formatted in way compiler happy. idea optionally pair template members compare functions if other less should used.

the syntax you're looking is:

template <class t, class compare, class... args> class rangetree<pair<t, compare>, args...> {     ... }; 

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alternatively, if you're doing here providing ability add comparator, can outsource work metafunction:

template <typename t> struct range_types {     using key = t;     using compare = std::less<t>; };  template <typename t, typename comp> struct range_types<pair<t, comp>> {     using key = t;     using compare = comp; };  template <class t, class... args> class rangetree<t, args...> {     map <typename range_types<t>::key,          rangetree <args...> *,          typename range_types<t>::compare          > tree; };