docs/doxygen/overviews/backwardcompatibility.h

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