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## Chapter 25. Boost.Ref

### Jaakko Järvi

### Peter Dimov

### Douglas Gregor

### Dave Abrahams

Copyright © 1999, 2000 Jaakko Järvi

Copyright © 2001, 2002 Peter Dimov

Copyright © 2002 David Abrahams

Subject to 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.

## Introduction

The Ref library is a small library that is useful for passing references to function templates (algorithms) that would usually take copies of their arguments. It defines the class template `boost::reference_wrapper<T>`, two functions `boost::ref` and `boost::cref` that return instances of `boost::reference_wrapper<T>`, a function `boost::unwrap_ref` that unwraps a `boost::reference_wrapper<T>` or returns a reference to any other type of object, and the two traits classes `boost::is_reference_wrapper<T>` and `boost::unwrap_reference<T>`.

The purpose of `boost::reference_wrapper<T>` is to contain a reference to an object of type T. It is primarily used to "feed" references to function templates (algorithms) that take their parameter by value.

To support this usage, `boost::reference_wrapper<T>` provides an implicit conversion to `T&`. This usually allows the function templates to work on references unmodified.

`boost::reference_wrapper<T>` is both CopyConstructible and Assignable (ordinary references are not Assignable).

The expression `boost::ref(x)` returns a `boost::reference_wrapper<X>(x)` where X is the type of x. Similarly, `boost::cref(x)` returns a `boost::reference_wrapper<X const>(x)`.

The expression `boost::unwrap_ref(x)` returns a `boost::unwrap_reference<X>::type&` where X is the type of x.

The expression `boost::is_reference_wrapper<T>::value` is true if T is a `reference_wrapper`, and false otherwise.

The type-expression `boost::unwrap_reference<T>::type` is T::type if T is a `reference_wrapper`, T otherwise.

Last revised: June 03, 2009 at 15:36:08 +0100 |