wxWidgets/docs/doxygen/overviews/backwardcompatibility.h

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// Name:        backwardcompatibility.h
// Purpose:     topic overview
// Author:      wxWidgets team
// RCS-ID:      $Id$
// Licence:     wxWindows license
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/*!

 @page overview_backwardcompat Backward compatibility

 Many of the GUIs and platforms supported by wxWidgets are continuously
 evolving, and some of the new platforms wxWidgets now supports were quite
 unimaginable even a few years ago. In this environment wxWidgets must also
 evolve in order to support these new features and platforms.

 However the goal of wxWidgets is not only to provide a consistent
 programming interface across many platforms, but also to provide an
 interface that is reasonably stable over time, to help protect its users
 from some of the uncertainty of the future.

 @li @ref overview_backwardcompat_versionnumbering
 @li @ref overview_backwardcompat_sourcecompat
 @li @ref overview_backwardcompat_libbincompat
 @li @ref overview_backwardcompat_appbincompat


 <hr>


 @section overview_backwardcompat_versionnumbering The version numbering scheme

 wxWidgets version numbers can have up to four components, with trailing
 zeros sometimes omitted:

 @code
 major.minor.release.sub-release
 @endcode

 A stable release of wxWidgets will have an even number for @c minor, e.g. @c 2.6.0.
 Stable, in this context, means that the API is not changing. In truth, some
 changes are permitted, but only those that are backward compatible. For
 example, you can expect later @c 2.6.x.x releases, such as @c 2.6.1
 and @c 2.6.2 to be backward compatible with their predecessor.

 When it becomes necessary to make changes which are not wholly backward
 compatible, the stable branch is forked, creating a new development
 branch of wxWidgets. This development branch will have an odd number
 for @c minor, for example @c 2.7.x.x. Releases from this branch are
 known as development snapshots.

 The stable branch and the development branch will then be developed in
 parallel for some time. When it is no longer useful to continue developing
 the stable branch, the development branch is renamed and becomes a new
 stable branch, for example @c 2.8.0. And the process begins again.
 This is how the tension between keeping the interface stable, and allowing
 the library to evolve is managed.

 You can expect the versions with the same major and even minor
 version number to be compatible, but between minor versions there will be
 incompatibilities. Compatibility is not broken gratuitously however, so
 many applications will require no changes or only small changes to work
 with the new version.


 @section overview_backwardcompat_sourcecompat Source level compatibility

 Later releases from a stable branch are backward compatible with earlier
 releases from the same branch at the source level.
 This means that, for example, if you develop your application using
 wxWidgets @c 2.6.0 then it should also compile fine with all later @c 2.6.x versions. 

 The converse is also @true providing you avoid any new
 features not present in the earlier version. For example if you develop
 using @c 2.6.1 your program will compile fine with wxWidgets @c 2.6.0
 providing you don't use any @c 2.6.1 specific features.

 For some platforms binary compatibility is also supported, see 'Library
 binary compatibility' below.

 Between minor versions, for example between @c 2.2.x, @c 2.4.x and @c 2.6.x, there 
 will be some incompatibilities. Wherever possible the old way of doing something 
 is kept alongside the new for a time wrapped inside:

 @code
 #if WXWIN_COMPATIBILITY_2_4
    // deprecated feature
    ...
 #endif
 @endcode

 By default the @c WXWIN_COMPATIBILITY@e _X_X macro is set
 to 1 for the previous stable branch, for example
 in @c 2.6.x @c WXWIN_COMPATIBILITY_2_4 = 1. For the next earlier
 stable branch the default is 0, so @c WXWIN_COMPATIBILITY_2_2 = 0
 for @c 2.6.x. Earlier than that, obsolete features are removed.

 These macros can be changed in @c setup.h. Or on UNIX-like systems you can
 set them using the @c --disable-compat24 and @c --enable-compat22
 options to @c configure.

 They can be useful in two ways:

 @li changing @c WXWIN_COMPATIBILITY_2_4 to 0 can be useful to
     find uses of deprecated features in your program.
 @li changing @c WXWIN_COMPATIBILITY_2_2 to 1 can be useful to
     compile a program developed using @c 2.2.x that no longer compiles
     with @c 2.6.x.
 
 A program requiring one of these macros to be 1 will become
 incompatible with some future version of wxWidgets, and you should consider
 updating it.


 
 @section overview_backwardcompat_libbincompat Library binary compatibility
 
 For some platforms, releases from a stable branch are not only source level
 compatible but can also be binary compatible.

 Binary compatibility makes it possible to get the maximum benefit from
 using shared libraries, also known as dynamic link libraries (DLLs) on
 Windows or dynamic shared libraries on OS X.

 For example, suppose several applications are installed on a system requiring
 wxWidgets @c 2.6.0, @c 2.6.1 and @c 2.6.2. Since @c 2.6.2 is
 backward compatible with the earlier versions, it should be enough to
 install just wxWidgets @c 2.6.2 shared libraries, and all the applications
 should be able to use them. If binary compatibility is not supported, then all
 the required versions @c 2.6.0, @c 2.6.1 and @c 2.6.2 must be
 installed side by side.

 Achieving this, without the user being required to have the source code
 and recompile everything, places many extra constraints on the changes
 that can be made within the stable branch. So it is not supported for all
 platforms, and not for all versions of wxWidgets. To date it has mainly
 been supported by wxGTK for UNIX-like platforms.

 Another practical consideration is that for binary compatibility to work,
 all the applications and libraries must have been compiled with compilers
 that are capable of producing compatible code;
 that is, they must use the
 same ABI (Application Binary Interface). Unfortunately most different C++
 compilers do not produce code compatible with each other, and often even
 different versions of the same compiler are not compatible.


 
 @section overview_backwardcompat_appbincompat Application binary compatibility
 
 The most important aspect of binary compatibility is that applications
 compiled with one version of wxWidgets, e.g. @c 2.6.1, continue to work
 with shared libraries of a later binary compatible version, for example @c 2.6.2.
 The converse can also be useful however. That is, it can be useful for a
 developer using a later version, e.g. @c 2.6.2 to be able to create binary
 application packages that will work with all binary compatible versions of
 the shared library starting with, for example @c 2.6.0.

 To do this the developer must, of course, avoid any features not available
 in the earlier versions. However this is not necessarily enough;
 in some cases an application compiled with a later version may depend on it even
 though the same code would compile fine against an earlier version.

 To help with this, a preprocessor symbol @c wxABI_VERSION can be defined
 during the compilation of the application (this would usually be done in the
 application's makefile or project settings). It should be set to the lowest
 version that is being targeted, as a number with two decimal digits for each
 component, for example @c wxABI_VERSION=20600 for @c 2.6.0.

 Setting @c wxABI_VERSION should prevent the application from implicitly
 depending on a later version of wxWidgets, and also disables any new features
 in the API, giving a compile time check that the source is compatible with
 the versions of wxWidgets being targeted.

 Uses of @c wxABI_VERSION are stripped out of the wxWidgets sources when
 each new development branch is created. Therefore it is only useful to help
 achieve compatibility with earlier versions with the same major
 and even minor version numbers. It won't, for example, help you write
 code compatible with @c 2.4.x using wxWidgets @c 2.6.x.

*/