Using std::copy and std::back_inserter to copy between vectors of different types

copy_using_back_inserter_between_different_typed_vectors.cpp

/*
 * By adding a converting constructor to `person` (that converts from `thingy` to `person`)
 * we can allow the std::back_inserter to work when copying from vector<thingy> to vector<person>
 *
 * This allows a one-liner that calls `push_back` over and over.
 *
 * Note: The repeated calls to `push_back` might have a performance cost.
 *       The alternative is to do it in two lines:
 *
 *         people.reserve(N);
 *         copy(ids.begin(), ids.end(), people.begin());
 *
 *       By calling `reserve` first, we pre-allocate in one go. It avoids repeated reallocations
 *       (and copies of old data to new memory). And the simpler `copy` can rely on all the
 *       locations in the vector already having been allocated.
 *
 * Note #2: Maybe use std::transform instead of std::copy, and implement the conversion
 *          using lambda or a function-object.
 *          [See https://stackoverflow.com/a/11686976/54491]
 */

#include <algorithm>
#include <vector>
#include <iterator>

struct thingy
{
    int _id;
};

struct person
{
    // This non-explicit converting constructor allows std::back_inserter
    // to insert from a vector of 'thingy' to a vector of 'person'
    person( thingy const & src )
    {
        _id = src._id;
    }

    int _id;
};

int main()
{
    std::vector<thingy> ids;
    ids.reserve(2);
    ids.push_back(thingy{11});
    ids.push_back(thingy{22});

    std::vector<person> people;

    copy( ids.begin(), ids.end(), back_inserter( people ) );

    return 0;    
}