docs/doxygen/overviews/backwardcompatibility.h

   1 /////////////////////////////////////////////////////////////////////////////
   2 // Name:        backwardcompatibility.h
   3 // Purpose:     topic overview
   4 // Author:      wxWidgets team
   5 // RCS-ID:      $Id$
   6 // Licence:     wxWindows licence
   7 /////////////////////////////////////////////////////////////////////////////
   8
   9 /**
  10
  11 @page overview_backwardcompat Backwards Compatibility
  12
  13 @tableofcontents
  14
  15 Many of the GUIs and platforms supported by wxWidgets are continuously
  16 evolving, and some of the new platforms wxWidgets now supports were quite
  17 unimaginable even a few years ago. In this environment wxWidgets must also
  18 evolve in order to support these new features and platforms.
  19
  20 However the goal of wxWidgets is not only to provide a consistent programming
  21 interface across many platforms, but also to provide an interface that is
  22 reasonably stable over time, to help protect its users from some of the
  23 uncertainty of the future.
  24
  25
  26
  27 @section overview_backwardcompat_versionnumbering The Version Numbering Scheme
  28
  29 wxWidgets version numbers can have up to four components, with trailing zeros
  30 sometimes omitted:
  31
  32 @verbatim
  33 major.minor.release.sub-release
  34 @endverbatim
  35
  36 A stable release of wxWidgets will have an even number for @e minor, e.g.
  37 2.6.0. Stable, in this context, means that the API is not changing. In truth,
  38 some changes are permitted, but only those that are backward compatible. For
  39 example, you can expect later 2.6.x releases, such as 2.6.1 and 2.6.2 to be
  40 backward compatible with their predecessor.
  41
  42 When it becomes necessary to make changes which are not wholly backward
  43 compatible, the stable branch is forked, creating a new development branch of
  44 wxWidgets. This development branch will have an odd number for @e minor, for
  45 example 2.7.x. Releases from this branch are known as development snapshots.
  46
  47 The stable branch and the development branch will then be developed in parallel
  48 for some time. When it is no longer useful to continue developing the stable
  49 branch, the development branch is renamed and becomes a new stable branch, for
  50 example: 2.8.0. And the process begins again. This is how the tension between
  51 keeping the interface stable, and allowing the library to evolve is managed.
  52
  53 You can expect the versions with the same major and even minor version number
  54 to be compatible, but between minor versions there will be incompatibilities.
  55 Compatibility is not broken gratuitously however, so many applications will
  56 require no changes or only small changes to work with the new version.
  57
  58
  59 @section overview_backwardcompat_sourcecompat Source Level Compatibility
  60
  61 Later releases from a stable branch are backward compatible with earlier
  62 releases from the same branch at the source level. This means that, for
  63 example, if you develop your application using wxWidgets 2.8.0 then it should
  64 also compile fine with all later 2.8.x versions.
  65
  66 The converse is also true providing you avoid any new features not present in
  67 the earlier version. For example if you develop using 2.6.1 your program will
  68 compile fine with wxWidgets 2.8.0 providing you don't use any 2.8.1 specific
  69 features.
  70
  71 For some platforms binary compatibility is also supported, see
  72 @ref overview_backwardcompat_libbincompat below.
  73
  74 Between minor versions, for example between 2.4.x, 2.6.x and 2.8.x, there will
  75 be some incompatibilities. Wherever possible the old way of doing something is
  76 kept alongside the new for a time wrapped inside:
  77
  78 @code
  79 #if WXWIN_COMPATIBILITY_2_6
  80     // deprecated feature
  81     ...
  82 #endif
  83 @endcode
  84
  85 By default the @c WXWIN_COMPATIBILITY_X_X macro is set to 1 for the previous
  86 stable branch, for example in 2.8.x, @c WXWIN_COMPATIBILITY_2_6 = 1. For the
  87 next earlier stable branch the default is 0, so @c WXWIN_COMPATIBILITY_2_4 = 0
  88 for 2.8.x. Earlier than that, obsolete features are removed.
  89
  90 These macros can be changed in @c setup.h. Or on UNIX-like systems you can set
  91 them using the @c --disable-compat26 and @c --enable-compat24 options to
  92 configure.
  93
  94 They can be useful in two ways:
  95
  96 @li Changing @c WXWIN_COMPATIBILITY_2_6 to 0 can be useful to find uses of
  97     deprecated features in your program that should eventually be removed.
  98 @li Changing @c WXWIN_COMPATIBILITY_2_4 to 1 can be useful to compile a program
  99     developed using 2.4.x that no longer compiles with 2.8.x.
 100
 101 A program requiring one of these macros to be 1 will become incompatible with
 102 some future version of wxWidgets, and you should consider updating it.
 103
 104
 105 @section overview_backwardcompat_libbincompat Library Binary Compatibility
 106
 107 For some platforms, releases from a stable branch are not only source level
 108 compatible but can also be binary compatible.
 109
 110 Binary compatibility makes it possible to get the maximum benefit from using
 111 shared libraries, also known as dynamic link libraries (DLLs) on Windows or
 112 dynamic shared libraries on OS X.
 113
 114 For example, suppose several applications are installed on a system requiring
 115 wxWidgets 2.6.0, 2.6.1 and 2.6.2. Since 2.6.2 is backward compatible with the
 116 earlier versions, it should be enough to install just wxWidgets 2.6.2 shared
 117 libraries, and all the applications should be able to use them. If binary
 118 compatibility is not supported, then all the required versions 2.6.0, 2.6.1 and
 119 2.6.2 must be installed side by side.
 120
 121 Achieving this, without the user being required to have the source code and
 122 recompile everything, places many extra constraints on the changes that can be
 123 made within the stable branch. So it is not supported for all platforms, and
 124 not for all versions of wxWidgets. To date it has mainly been supported by
 125 wxGTK for UNIX-like platforms.
 126
 127 Another practical consideration is that for binary compatibility to work, all
 128 the applications and libraries must have been compiled with compilers that are
 129 capable of producing compatible code; that is, they must use the same ABI
 130 (Application Binary Interface). Unfortunately most different C++ compilers do
 131 not produce code compatible with each other, and often even different versions
 132 of the same compiler are not compatible.
 133
 134
 135 @section overview_backwardcompat_appbincompat Application Binary Compatibility
 136
 137 The most important aspect of binary compatibility is that applications compiled
 138 with one version of wxWidgets, e.g. 2.6.1, continue to work with shared
 139 libraries of a later binary compatible version, for example 2.6.2. The converse
 140 can also be useful however. That is, it can be useful for a developer using a
 141 later version, e.g. 2.6.2 to be able to create binary application packages that
 142 will work with all binary compatible versions of the shared library starting
 143 with, for example 2.6.0.
 144
 145 To do this the developer must, of course, avoid any features not available in
 146 the earlier versions. However this is not necessarily enough; in some cases an
 147 application compiled with a later version may depend on it even though the same
 148 code would compile fine against an earlier version.
 149
 150 To help with this, a preprocessor symbol @c wxABI_VERSION can be defined during
 151 the compilation of the application (this would usually be done in the
 152 application's makefile or project settings). It should be set to the lowest
 153 version that is being targeted, as a number with two decimal digits for each
 154 component, for example @c wxABI_VERSION=20600 for 2.6.0.
 155
 156 Setting @c wxABI_VERSION should prevent the application from implicitly
 157 depending on a later version of wxWidgets, and also disables any new features
 158 in the API, giving a compile time check that the source is compatible with the
 159 versions of wxWidgets being targeted.
 160
 161 Uses of @c wxABI_VERSION are stripped out of the wxWidgets sources when each
 162 new development branch is created. Therefore it is only useful to help achieve
 163 compatibility with earlier versions with the same major and even minor version
 164 numbers. It won't, for example, help you write code compatible with 2.6.x using
 165 wxWidgets 2.8.x.
 166
 167 */