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## Chapter 4. Boost.Any

### Kevlin Henney

Copyright © 2001 Kevlin Henney

Copyright © 2013-2019 Antony Polukhin

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

There are times when a generic (in the sense of general as opposed to template-based programming) type is needed: variables that are truly variable, accommodating values of many other more specific types rather than C++'s normal strict and static types. We can distinguish three basic kinds of generic type:

- Converting types that can hold one of a number of possible value types, e.g. `int` and `string`, and freely convert between them, for instance interpreting `5` as `"5"` or vice-versa. Such types are common in scripting and other interpreted languages. `boost::lexical_cast` supports such conversion functionality.

- Discriminated types that contain values of different types but do not attempt conversion between them, i.e. `5` is held strictly as an `int` and is not implicitly convertible either to `"5"` or to `5.0`. Their indifference to interpretation but awareness of type effectively makes them safe, generic containers of single values, with no scope for surprises from ambiguous conversions.

- Indiscriminate types that can refer to anything but are oblivious to the actual underlying type, entrusting all forms of access and interpretation to the programmer. This niche is dominated by `void *`, which offers plenty of scope for surprising, undefined behavior.

The `boost::any` class (based on the class of the same name described in "Valued Conversions" by Kevlin Henney, C++ Report 12(7), July/August 2000) is a variant value type based on the second category. It supports copying of any value type and safe checked extraction of that value strictly against its type. A similar design, offering more appropriate operators, can be used for a generalized function adaptor, `any_function`, a generalized iterator adaptor, `any_iterator`, and other object types that need uniform runtime treatment but support only compile-time template parameter conformance.