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[wxWidgets.git] / docs / doxygen / overviews / backwardcompatibility.h

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

/*!

 @page backwardcompatibility_overview 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.
 @ref versionnumbering_overview
 @ref sourcecompatibility_overview
 @ref libbincompatibility_overview
 @ref appbincompatibility_overview


 @section 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 sourcecompatibility 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:


Changing @c WXWIN_COMPATIBILITY_2_4 to 0 can be useful to
find uses of deprecated features in your program.
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 libbincompatibility 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 appbincompatibility 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.

        */