c++ - Initialize std::array with a range (pair of iterators) -

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how can initialize std::array range (as defined pair of iterators)?

something this:

vector<t> v; ... // know v has n elements (e.g. called v.resize(n)) // want initialized elements array<t, n> a(???);  // put here?

i thought array have constructor taking pair of iterators, array<t, n> a(v.begin(), v.end()), appears have no constructors @ all!

i know can copy vector array, i'd rather initialize array vector contents directly, without default-constructing first. how can i?

with random access iterators, , assuming size @ compile-time, can use pack of indices so:

template <std::size_t... indices> struct indices {     using next = indices<indices..., sizeof...(indices)>; }; template <std::size_t n> struct build_indices {     using type = typename build_indices<n-1>::type::next; }; template <> struct build_indices<0> {     using type = indices<>; }; template <std::size_t n> using buildindices = typename build_indices<n>::type;  template <typename iterator> using valuetype = typename std::iterator_traits<iterator>::value_type;  // internal overload indices tag template <std::size_t... i, typename randomaccessiterator,           typename array = std::array<valuetype<randomaccessiterator>, sizeof...(i)>> array make_array(randomaccessiterator first, indices<i...>) {     return array { { first[i]... } }; }  // externally visible interface template <std::size_t n, typename randomaccessiterator> std::array<valuetype<randomaccessiterator>, n> make_array(randomaccessiterator first, randomaccessiterator last) {     // last not relevant if we're assuming size n     // i'll assert correct anyway     assert(last - first == n);      return make_array(first, buildindices<n> {}); }  // usage auto = make_array<n>(v.begin(), v.end());

this assumes compiler capable of eliding intermediate copies. think assumption not big stretch.

actually, can done input iterators well, since computation of each element in braced-init-list sequenced before computation of next element (§8.5.4/4).

// internal overload indices tag template <std::size_t... i, typename inputiterator,           typename array = std::array<valuetype<inputiterator>, sizeof...(i)>> array make_array(inputiterator first, indices<i...>) {     return array { { (void(i), *first++)... } }; }

since *first++ doesn't have i in it, need dummy i provoke pack expansion. comma operator rescue, void() silence warnings lack of effects, , preventing overloaded commas.


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