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Copyright © 2001 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
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.