...one of the most highly
regarded and expertly designed C++ library projects in the
world. — Herb Sutter and Andrei
Copyright © 2001 Kevlin Henney
Copyright © 2013-2020 Antony Polukhin
Distributed under the Boost Software License, Version 1.0.
(See accompanying file
LICENSE_1_0.txt or copy at
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.
string, and freely convert between them, for
vice-versa. Such types are common in scripting and other
supports such conversion functionality.
Discriminated types that contain values of different types but
do not attempt conversion between them, i.e.
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
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
void *, which offers plenty of scope
for surprising, undefined behavior.
(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