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## swap

### Authors

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

### Header <boost/core/invoke_swap.hpp>

`template<class T> void invoke_swap(T& left, T& right) noexcept(see below);`

### 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.

### 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[1].

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[2].

### 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.

### 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

### 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.

### 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.

[1] Scott Meyers, Effective C++ Third Edition, Item 25: "Consider support for a non-throwing swap"

[2] LWG Defect Report 809: std::swap should be overloaded for array types