Chapter 5. Boost.Any

Kevlin Henney

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)

Table of Contents

Introduction

Examples

Reference

ValueType requirements
Header <boost/any.hpp>

Acknowledgements

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.