## extern_vptr

### Synopsis

Defined in <boost/openmethod/policies/basic_policy.hpp>.

```c++
namespace boost::openmethod::policies {

struct extern_vptr : policy {};

}
```

### Description

`extern_vptr` is a policy that stores and returns pointers to v-tables for
registered classes.

### Requirements

#### register_vptrs

```c++
template<typename ForwardIterator>
auto register_vptrs(ForwardIterator first, ForwardIterator last) -> void;
```

`ForwardIterator` is a forward iterator over a range of objects that contain
information about the type ids and the vptr of a registered class. They have the
following member functions:

```c++
auto type_id_begin() const -> type_id_forward_iterator;
auto type_id_end() const -> type_id_forward_iterator;
auto vptr() const -> const vptr_type&;
```

`type_id_begin` and `type_id_end` return iterators delimiting a range of
`type_id`s for a class.

`vptr` returns a _reference_ to a _static_ variable containing a pointer to the
v-table for a the class. Its value is set by `initialize`. While the value of
the variable changes with each call to `initialize`, the variable itself remains
the same.

## indirect_vptr

### Synopsis

```c++
struct indirect_vptr : policy {};
```

### Description

`indirect_vptr` is a policy that makes `virtual_ptr`{empty}s and `inplace_vptr` use
pointers to pointers to v-tables, instead of straight pointers. As a
consequence, they remain valid after a call to `initialize`.

### Requirements

None. The policy is its own implementation.