boost_core/doc/swap.qbk

[/
 / Copyright (c) 2008 Joseph Gauterin
 / Copyright (c) 2008, 2009 Niels Dekker
 / Copyright (c) 2014 Glen Fernandes
 /
 / 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)
 / For more information, see http://www.boost.org
 /]

[section:swap swap]

[simplesect Authors]

* Niels Dekker
* Joseph Gauterin
* Steven Watanabe
* Eric Niebler

[endsimplesect]

[section Header <boost/core/invoke_swap.hpp>]

[^template<class T> void invoke_swap(T& left, T& right) noexcept(['see below]);]

[endsect]

[section Introduction]

The template function `boost::core::invoke_swap` allows the
values of two variables to be swapped, using argument dependent
lookup to select a specialized swap function if available. If no
specialized swap function is available, `std::swap` is used.

[endsect]

[section Rationale]

The generic `std::swap` function requires that the elements
to be swapped are assignable and copy constructible. It is
usually implemented using one copy construction and two
assignments (C++11 replaces copy operations with move) - this
is often both unnecessarily restrictive and unnecessarily slow.
In addition, where the generic swap implementation provides
only the basic guarantee, specialized swap functions are often
able to provide the no-throw exception guarantee (and it is
considered best practice to do so where possible[footnote Scott
Meyers, Effective C++ Third Edition, Item 25: "Consider support
for a non-throwing swap"].

The alternative to using argument dependent lookup in this
situation is to provide a template specialization of
`std::swap` for every type that requires a specialized swap.
Although this is legal C++, no Boost libraries use this method,
whereas many Boost libraries provide specialized swap functions
in their own namespaces.

`boost::core::invoke_swap` also supports swapping built-in arrays.
Note that `std::swap` originally did not do so, but a request to
add an overload of `std::swap` for built-in arrays has been accepted
by the C++ Standards Committee[footnote
  [@http://open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#809
  LWG Defect Report 809: std::swap should be overloaded for array
  types]].

[endsect]

[section Exception Safety]

`boost::core::invoke_swap` provides the same exception guarantee as
the underlying swap function used, with one exception; for an array
of type `T[n]`, where `n > 1` and the underlying swap function
for `T` provides the strong exception guarantee,
`boost::core::invoke_swap` provides only the basic exception guarantee.

In C++11 and later, `boost::core::invoke_swap` propagates the same
`noexcept` specification as the one specified in the underlying swap
function.

[endsect]

[section Requirements]

Either:

* `T` must be copy assignable (/since C++11:/ move assignable)
* `T` must be copy constructible (/since C++11:/ move constructible)

Or:

* A function with the signature `swap(T&, T&)` is available via
  argument dependent lookup

Or:

* A template specialization of `std::swap` exists for `T`

Or:

* `T` is a built-in array of swappable elements

[endsect]

[section Portability]

Several older compilers do not support argument dependent
lookup. On these compilers `boost::core::invoke_swap` will call
`std::swap`, ignoring any specialized swap functions that
could be found as a result of argument dependent lookup.

[endsect]

[section Credits]

* *Niels Dekker* - for implementing and documenting support for
  built-in arrays
* *Joseph Gauterin* - for the initial idea, implementation,
  tests, and documentation
* *Steven Watanabe* - for the idea to make `boost::swap` less
  specialized than `std::swap`, thereby allowing the function
  to have the name 'swap' without introducing ambiguity. However,
  later the function was renamed to `boost::core::invoke_swap`
  to avoid potential infinite recursion.

[endsect]

[endsect]