c++ - Implementing const range with template -

i tried use template avoid code duplication in implementation of const , non-const range on std::vector.

the non-const version works, const version not. don't know why.

test.cpp. compile g++ test.cpp:

#include <vector> #include <iostream> enum cv {     constant,     non_constant };  template <typename t, typename index, cv cv_enum> class vector_range_facade {     typedef typename std::conditional <             cv_enum == cv::constant,                     const std::vector<t>, std::vector<t>     >::type vec; public:     typedef typename vec::iterator iterator;     vector_range_facade(vec& vec, const index start_id, const index size)             : vec_{vec},               it_begin_{vec_.begin() + start_id},               it_end_  {vec_.begin() + start_id + size}     {}       iterator& begin() {return it_begin_;}     iterator& end()   {return it_end_;} private:     vec& vec_;     iterator it_begin_;     iterator it_end_; };  int main() {     std::vector<int> a;     a.resize(100);     vector_range_facade<int, int, cv::constant> range(a,0,10);     (auto x : range) {         std::cout << x <<"\n";     }    } 

std::vector::iterator evaluates type of non-const iterator, typedef defined inside std::vector cannot know if it's being called on const or non-const instance.

you need conditionally select iterator type, between std::vector::iterator , std::vector::const_iterator. example:

typedef typename std::conditional <         cv_enum == cv::constant,         typename std::vector<t>::const_iterator,          typename std::vector<t>::iterator >::type iterator; 

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