C++ iterator challenge

iterator_maker.cpp

#include <vector>

using namespace std;

class C
{
  typedef struct {
    double position;
    double value1;
    double value2;
  } ElementA;
  
  typedef struct {
    double position;
    double value3;
    double value4;
  } ElementB;
  
  typedef enum {
    Field1,
    Field2,
    Field3,
    Field4
  } ValueSource;
  
  vector<ElementA> vecElementA;
  vector<ElementB> vecElementB;
  
  //By implementing "transform_iterator" or otherwise, how do I create an accessor
  //that transforms elements of either vecElementA or vecElementB, depending on
  //value of "theSource", and gives me access to them one-by-one, on-the-fly?
  vector<pair<double,double>>::const_iterator MakePairs(ValueSource theSource)
  {
    switch(theSource)
    {
      case ValueSource::Field1:
        return transform_iterator(vecElementA, [](const auto elem&) { make_pair(elem.position, elem.value1); });
      
      case ValueSource::Field2:
        return transform_iterator(vecElementA, [](const auto elem&) { make_pair(elem.position, elem.value2); });

      case ValueSource::Field3:
        return transform_iterator(vecElementB, [](const auto elem&) { make_pair(elem.position, elem.value3); });

      case ValueSource::Field4:
        return transform_iterator(vecElementB, [](const auto elem&) { make_pair(elem.position, elem.value4); });
    }
  }
}

#include <iostream>

int main(int, char**)
{
  C c;
  c.vecElementA = <some data source>;
  c.vecElementB = <some other data source>;
  
  for(auto pair : c.MakePairs(ValueSource::Field1))
  {
    cout << pair.first << "," << pair.second << endl;
  }
  
  return 0;
}