boost_1_45_0/libs/config/doc/rationale.qbk

[/
    Boost.Config

    Copyright (c) 2001 Beman Dawes
    Copyright (c) 2001 Vesa Karvonen
    Copyright (c) 2001 John Maddock

    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)
]

[section Rationale]

The problem with many traditional "textbook" implementations of configuration
headers (where all the configuration options are in a single "monolithic"
header) is that they violate certain fundamental software engineering
principles which would have the effect of making boost more fragile, more
difficult to maintain and more difficult to use safely. You can find a
description of the principles from the __PRINCIPLES_AND_PATTERNS_ARTICLE__.

[section The problem]

Consider a situation in which you are concurrently developing on multiple
platforms. Then consider adding a new platform or changing the platform
definitions of an existing platform. What happens? Everything, and this does
literally mean everything, recompiles. Isn't it quite absurd that adding a
new platform, which has absolutely nothing to do with previously existing
platforms, means that all code on all existing platforms needs to be
recompiled?

Effectively, there is an imposed physical dependency between platforms that
have nothing to do with each other. Essentially, the traditional solution
employed by configuration headers does not conform to the Open-Closed
Principle:

[: [*"A module should be open for extension but closed for modification."]]

Extending a traditional configuration header implies modifying existing code.

Furthermore, consider the complexity and fragility of the platform detection
code. What if a simple change breaks the detection on some minor platform?
What if someone accidentally or on purpose (as a workaround for some other
problem) defines some platform dependent macros that are used by the
detection code? A traditional configuration header is one of the most
volatile headers of the entire library, and more stable elements of
Boost would depend on it. This violates the Stable Dependencies Principle:

[: [*"Depend in the direction of stability."]]

After even a minor change to a traditional configuration header on one minor
platform, almost everything on every platform should be tested if we follow
sound software engineering practice.

Another important issue is that it is not always possible to submit changes
to `<boost/config.hpp>`. Some boost users are currently working on platforms
using tools and libraries that are under strict Non-Disclosure Agreements.
In this situation it is impossible to submit changes to a traditional
monolithic configuration header, instead some method by which the user
can insert their own configuration code must be provided.

[endsect]

[section The solution]

The approach taken by boost's configuration headers is to separate
configuration into three orthogonal parts: the compiler, the standard
library and the platform. Each compiler/standard library/platform gets
its own mini-configuration header, so that changes to one compiler's
configuration (for example) does not affect other compilers. In addition
there are measures that can be taken both to omit the compiler/standard
library/platform detection code (so that adding support to a new platform
does not break dependencies), or to freeze the configuration completely;
providing almost complete protection against dependency changes.

[endsect]

[endsect]