c++ - Cryptic template template parameter error -

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i'm trying create function gets keys std::map or std::unordered_map. use simple overload, first i'd love know what's wrong code.

template<typename k, typename v, template<typename, typename> class tcontainer>   std::vector<k> getkeys(const tcontainer<k, v>& mmap) {     std::vector<k> result;     for(const auto& itr(std::begin(mmap)); itr != std::end(mmap); ++itr) result.push_back(itr->first);     return result; }

when calling std::unordered_map, specifying all template typenames manually, clang++ 3.4 says:

template template argument has different template parameters corresponding template template parameter.

the problem std::map , std::unordered_map not in fact templates 2 parameters. are:

namespace std {     template <class key, class t, class compare = less<key>,               class allocator = allocator<pair<const key, t>>>     class map;      template <class key, class t, class hash = hash<key>,               class pred = equal_to<key>,               class allocator = allocator<pair<const key, t>>>     class unordered_map; }

here's similar work:

template <typename k, typename... targs, template<typename...> class tcontainer> std::vector<k> getkeys(const tcontainer<k, targs...>& mmap) {     std::vector<k> result;     (auto p : mmap)         result.push_back(p.first);     return result; }

the version prefer:

template <typename container> auto getkeys2(const container& mmap) -> std::vector<typename container::key_type> {     std::vector<typename container::key_type> result;     (auto p : mmap)         result.push_back(p.first);     return result; }

a demo program using both functions: http://ideone.com/pckcu6


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