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  1. #include <iostream>
  2. #include <utility>
  3. #include <vector>
  4. #include <algorithm>
  5. #include <iterator>
  6. #include <functional>
  7. using namespace std;
  8.  
  9. int get_first(const std::pair<int, int> p) { return std::get<0>(p); }
  10.  
  11. int main() {
  12.   std::vector<int> items;
  13.   std::vector<std::pair<int, int>> pairs;
  14.  
  15.   pairs.push_back(std::make_pair(1, 3));
  16.   pairs.push_back(std::make_pair(5, 7));
  17.  
  18.   // With custom function.
  19.   std::transform(pairs.begin(), pairs.end(), std::back_inserter(items),
  20.                  get_first);
  21.  
  22.   // With std::get<0>, but this is super nasty. :(
  23.   std::transform(pairs.begin(), pairs.end(), std::back_inserter(items),
  24.                  (const int& (*)(const std::pair<int, int>&))std::get<0>);
  25.  
  26.   // With lambda; it's not much better.
  27.   std::transform(pairs.begin(), pairs.end(), std::back_inserter(items),
  28.                  [](const std::pair<int, int>& p) { return std::get<0>(p); });
  29.  
  30.   std::copy(items.begin(), items.end(),
  31.             std::ostream_iterator<int>(std::cout, " "));
  32.   cout << endl;
  33.  
  34.   return 0;
  35. }
Success #stdin #stdout 0s 3412KB
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1 5 1 5 1 5
https://ideone.com/6dfzxz
language:
C++14 (gcc 8.3)
created:
9 years ago
visibility:
public

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