text: /////////////////////////////////////////////////////////////////////////////
  // Name:        backwardcompatibility.h
  // Purpose:     topic overview
  // Author:      wxWidgets team
  // RCS-ID:      $Id: backwardcompatibility.h 59598 2009-03-18 09:29:05Z VZ $
  // Licence:     wxWindows license
  /////////////////////////////////////////////////////////////////////////////
  
  /**
  
  @page overview_backwardcompat Backwards 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:
  
  @verbatim
  major.minor.release.sub-release
  @endverbatim
  
  A stable release of wxWidgets will have an even number for @e minor, e.g.
  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 2.6.x releases, such as 2.6.1 and 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 @e minor, for
  example 2.7.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: 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 2.8.0 then it should
  also compile fine with all later 2.8.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 2.6.1 your program will
  compile fine with wxWidgets 2.8.0 providing you don't use any 2.8.1 specific
  features.
  
  For some platforms binary compatibility is also supported, see
  @ref overview_backwardcompat_libbincompat below.
  
  Between minor versions, for example between 2.4.x, 2.6.x and 2.8.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_6
      // deprecated feature
      ...
  #endif
  @endcode
  
  By default the @c WXWIN_COMPATIBILITY_X_X macro is set to 1 for the previous
  stable branch, for example in 2.8.x, @c WXWIN_COMPATIBILITY_2_6 = 1. For the
  next earlier stable branch the default is 0, so @c WXWIN_COMPATIBILITY_2_4 = 0
  for 2.8.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-compat26 and @c --enable-compat24 options to
  configure.
  
  They can be useful in two ways:
  
  @li Changing @c WXWIN_COMPATIBILITY_2_6 to 0 can be useful to find uses of
      deprecated features in your program that should eventually be removed.
  @li Changing @c WXWIN_COMPATIBILITY_2_4 to 1 can be useful to compile a program
      developed using 2.4.x that no longer compiles with 2.8.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 2.6.0, 2.6.1 and 2.6.2. Since 2.6.2 is backward compatible with the
  earlier versions, it should be enough to install just wxWidgets 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 2.6.0, 2.6.1 and
  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. 2.6.1, continue to work with shared
  libraries of a later binary compatible version, for example 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. 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 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 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 2.6.x using
  wxWidgets 2.8.x.
  
  */
  
  