vector<X> to vector<Y> implicit casting

#include <vector>
#include <string>
#include <iostream>

struct B {
  B() {}
  std::string str() const { return "B" + std::to_string(y); }
  int y;
};

struct A {
  A(int a) { x = a; }
  operator B () const { B b; b.y = x; return b; }
  std::string str() const { return "A" + std::to_string(x); }
  int x;
};


struct intvector : std::vector<int> {
  using base = std::vector<int>; using base::base; //< using-declarations don't like template syntax
  operator std::vector<A>() const {
    std::vector<A> rtn(this->begin(), this->end());
    return rtn;
  }
  operator std::vector<B>() const {
    std::vector<B> rtn(size()); // or rtn.reserve(size());
    for (size_t i = 0; i < size(); ++i) rtn[i].y = at(i);
    return rtn;
  }
};

struct avector : std::vector<A> {
  using base = std::vector<A>; using base::base; //< using-declarations don't like template syntax
  avector(const std::initializer_list<int>& ints) { for (int i : ints) push_back(i); }
  avector(const intvector& ints) { for (int i : ints) push_back(i); }
  operator std::vector<B>() const {
    std::vector<B> rtn(begin(), end());
    // std::vector<B> rtn(size()); // or rtn.reserve(size());
    // for (size_t i = 0; i < size(); ++i) rtn[i] = at(i);
    return rtn;
  }
};


int main() {
  intvector vec = {1,2,3,4,5};
  std::vector<A> avec = vec;
  for (const A& a : avec) std::cout << a.str() << std::endl;
  std::vector<B> bvec = vec;
  for (const B& b : bvec) std::cout << b.str() << std::endl;
  avector avec2 = vec;
  std::vector<B> bvec2 = avec2;
  for (const B& b : bvec) std::cout << b.str() << std::endl;
}