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## Chapter 33. Boost.Signals2

### Douglas Gregor

### Frank Mori Hess

Copyright © 2001-2004 Douglas Gregor

Copyright © 2007-2009 Frank Mori Hess

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)

## Introduction

The Boost.Signals2 library is an implementation of a managed signals and slots system. Signals represent callbacks with multiple targets, and are also called publishers or events in similar systems. Signals are connected to some set of slots, which are callback receivers (also called event targets or subscribers), which are called when the signal is "emitted."

Signals and slots are managed, in that signals and slots (or, more properly, objects that occur as part of the slots) can track connections and are capable of automatically disconnecting signal/slot connections when either is destroyed. This enables the user to make signal/slot connections without expending a great effort to manage the lifetimes of those connections with regard to the lifetimes of all objects involved.

When signals are connected to multiple slots, there is a question regarding the relationship between the return values of the slots and the return value of the signals. Boost.Signals2 allows the user to specify the manner in which multiple return values are combined.

### Signals2

This documentation describes a thread-safe variant of the original Boost.Signals library. There have been some changes to the interface to support thread-safety, mostly with respect to automatic connection management. This implementation was written by Frank Mori Hess. Acknowledgements are also due to Timmo Stange, Peter Dimov, and Tony Van Eerd for ideas and feedback, and to Douglas Gregor for the original version of Boost.Signals this effort was based on.