boost_smart_ptr/doc/smart_ptr/pointer_cast.adoc

////
Copyright 2017 Peter Dimov

Distributed under the Boost Software License, Version 1.0.

See accompanying file LICENSE_1_0.txt or copy at
http://www.boost.org/LICENSE_1_0.txt
////

[#pointer_cast]
# Generic Pointer Casts
:toc:
:toc-title:
:idprefix: pointer_cast_

## Description

The pointer cast function templates (`static_pointer_cast`,
`dynamic_pointer_cast`, `const_pointer_cast`, and `reinterpret_pointer_cast`)
provide a way to write generic pointer castings for raw pointers,
`std::shared_ptr` and `std::unique_ptr`.

There is test and example code in
link:../../test/pointer_cast_test.cpp[pointer_cast_test.cpp]

## Rationale

Boost smart pointers usually overload those functions to provide a mechanism
to emulate pointers casts. For example, `shared_ptr<T>` implements a static
pointer cast this way:

```
template<class T, class U>
  shared_ptr<T> static_pointer_cast(const shared_ptr<U>& p);
```

Pointer cast functions templates are overloads of `static_pointer_cast`,
`dynamic_pointer_cast`, `const_pointer_cast`, and `reinterpret_pointer_cast`
for raw pointers, `std::shared_ptr` and `std::unique_ptr`. This way when
developing pointer type independent classes, for example, memory managers or
shared memory compatible classes, the same code can be used for raw and smart
pointers.

## Synopsis

The generic pointer casts are defined in `<boost/pointer_cast.hpp>`.

```
namespace boost {
  template<class T, class U> T* static_pointer_cast(U* p) noexcept;
  template<class T, class U> T* dynamic_pointer_cast(U* p) noexcept;
  template<class T, class U> T* const_pointer_cast(U* p) noexcept;
  template<class T, class U> T* reinterpret_pointer_cast(U* p) noexcept;

  template<class T, class U> std::shared_ptr<T>
    static_pointer_cast(const std::shared_ptr<U>& p) noexcept;
  template<class T, class U> std::shared_ptr<T>
    dynamic_pointer_cast(const std::shared_ptr<U>& p) noexcept;
  template<class T, class U> std::shared_ptr<T>
    const_pointer_cast(const std::shared_ptr<U>& p) noexcept;
  template<class T, class U> std::shared_ptr<T>
    reinterpret_pointer_cast(const std::shared_ptr<U>& p) noexcept;

  template<class T, class U> std::unique_ptr<T>
    static_pointer_cast(std::unique_ptr<U>&& p) noexcept;
  template<class T, class U> std::unique_ptr<T>
    dynamic_pointer_cast(std::unique_ptr<U>&& p) noexcept;
  template<class T, class U> std::unique_ptr<T>
    const_pointer_cast(std::unique_ptr<U>&& p) noexcept;
  template<class T, class U> std::unique_ptr<T>
    reinterpret_pointer_cast(std::unique_ptr<U>&& p) noexcept;
}
```

## Free Functions

### static_pointer_cast
```
template<class T, class U> T* static_pointer_cast(U* p) noexcept;
```
::
Returns::: `static_cast<T*>(p)`

```
template<class T, class U> std::shared_ptr<T>
  static_pointer_cast(const std::shared_ptr<U>& p) noexcept;
```
::
Returns::: `std::static_pointer_cast<T>(p)`

```
template<class T, class U> std::unique_ptr<T>
  static_pointer_cast(std::unique_ptr<U>&& p) noexcept;
```
::
Requires::: The expression `static_cast<T*>((U*)0)` must be well-formed.
Returns::: `std::unique_ptr<T>(static_cast<typename
std::unique_ptr<T>::element_type*>(p.release()))`.

CAUTION: The seemingly equivalent expression
`std::unique_ptr<T>(static_cast<T*>(p.get()))` will eventually result in
undefined behavior, attempting to delete the same object twice.

### dynamic_pointer_cast

```
template<class T, class U> T* dynamic_pointer_cast(U* p) noexcept;
```
::
Returns::: `dynamic_cast<T*>(p)`

```
template<class T, class U> std::shared_ptr<T>
  dynamic_pointer_cast(const std::shared_ptr<U>& p) noexcept;
```
::
Returns::: `std::dynamic_pointer_cast<T>(p)`

```
template<class T, class U> std::unique_ptr<T>
  dynamic_pointer_cast(std::unique_ptr<U>&& p) noexcept;
```
::
Requires:::
* The expression `static_cast<T*>((U*)0)` must be well-formed.
* `T` must have a virtual destructor.
Returns:::
* When `dynamic_cast<typename std::unique_ptr<T>::element_type*>(p.get())`
returns a non-zero value, `std::unique_ptr<T>(dynamic_cast<typename
std::unique_ptr<T>::element_type*>(p.release()));`.
* Otherwise, `std::unique_ptr<T>()`.

### const_pointer_cast

```
template<class T, class U> T* const_pointer_cast(U* p) noexcept;
```
::
Returns::: `const_cast<T*>(p)`

```
template<class T, class U> std::shared_ptr<T>
  const_pointer_cast(const std::shared_ptr<U>& p) noexcept;
```
::
Returns::: `std::const_pointer_cast<T>(p)`

```
template<class T, class U> std::unique_ptr<T>
  const_pointer_cast(std::unique_ptr<U>&& p) noexcept;
```
::
Requires::: The expression `const_cast<T*>((U*)0)` must be well-formed.
Returns::: `std::unique_ptr<T>(const_cast<typename
std::unique_ptr<T>::element_type*>(p.release()))`.

### reinterpret_pointer_cast

```
template<class T, class U> T* reinterpret_pointer_cast(U* p) noexcept;
```
::
Returns::: `reinterpret_cast<T*>(p)`

```
template<class T, class U> std::shared_ptr<T>
  reinterpret_pointer_cast(const std::shared_ptr<U>& p) noexcept;
```
::
Returns::: `std::reinterpret_pointer_cast<T>(p)`

```
template<class T, class U> std::unique_ptr<T>
  reinterpret_pointer_cast(std::unique_ptr<U>&& p) noexcept;
```
::
Requires::: The expression `reinterpret_cast<T*>((U*)0)` must be well-formed.
Returns::: `std::unique_ptr<T>(reinterpret_cast<typename
std::unique_ptr<T>::element_type*>(p.release()))`.

## Example

The following example demonstrates how the generic pointer casts help us
create pointer independent code.

```
#include <boost/pointer_cast.hpp>
#include <boost/shared_ptr.hpp>

class base {
public:
  virtual ~base() { }
};

class derived : public base { };

template<class Ptr>
void check_if_it_is_derived(const Ptr& ptr)
{
  assert(boost::dynamic_pointer_cast<derived>(ptr) != 0);
}

int main()
{
  base* ptr = new derived;
  boost::shared_ptr<base> sptr(new derived);

  check_if_it_is_derived(ptr);
  check_if_it_is_derived(sptr);

  delete ptr;
}
```