implicit conversion of vector from one type to another c++

Question

Is it possible to convert a vector of one type to another implicitly?

i.e some way to make this code work (obviously this is a simplified problem of what I am trying to do)

std::vector<int> intVec;
intVec.push_back(1);

std::vector<double> doubleVec = intVec;
std::vector<double> doubleVec2;
doubleVec2 = intVec;

Answer 1

No, there is no conversion (implicit or otherwise) between different vector types.

You could initialise it from an iterator range:

std::vector<double> doubleVec(intVec.begin(), intVec.end());

perhaps wrapping this in a function:

template <typename To, typename From>
To container_cast(From && from) {
    using std::begin; using std::end; // Koenig lookup enabled
    return To(begin(from), end(from));
}

auto doubleVec = container_cast<std::vector<double>>(intVec);

Answer 2

template<class T, class A=std::allocator<T>>
struct magic_vector:std::vector<T,A> {
  using base=std::vector<T,A>;
  using base::base;
  magic_vector(magic_vector const&)=default;
  magic_vector(magic_vector &&)=default;
  magic_vector& operator=(magic_vector const&)=default;
  magic_vector& operator=(magic_vector &&)=default;
  magic_vector()=default;

  template<class U, class B,
    class=typename std::enable_if<std::is_convertible<U,T>::value>::type
  >
  magic_vector( magic_vector<U,B> const& o ):
    base( o.begin(), o.end() )
  {}
  template<class U, class B,
    class=typename std::enable_if<
      std::is_convertible<U,T>::value
      && noexcept( T(std::declval<U&&>()) )
    >::type
  >
  magic_vector( magic_vector<U,B>&& o ):
    base(
      std::make_move_iterator(o.begin()),
      std::make_move_iterator(o.end())
    )
  {}
};

magic_vectors are vectors that auto convert from other magic_vectors.

If you have a pointer to a magic_vector that you convert into a pointer to vector, then delete it as a vector, the result is undefined behavior. (However in practice, there will be no harm in every C++ implementation I have checked). This is, however, a strange way to act with vectors.

Replace use of vector with magic_vector. So long as you don't have specializations on the exact type of a container in your code, it should be a drop-in replacement, except now it will auto-convert between them.

Work could be done to have magic_vectors auto-convert with vectors instead of just with magic_vectors.