Qt uses a major.minor.patch numbering scheme for Qt releases. For example, Qt 5.0.1 represents the 1st patch release of Qt 5.0. Each release has restrictions on what kind of changes are acceptable, in order to provide a predictable and stable API. This enables users who are only interested in certain types of improvements to be sure that they won't be adversely affected by upgrading to a newer version.
We do our best to maintain compatibility between versions. However, in order to improve Qt it is sometimes necessary to break compatibility.
Major releases may break backwards binary and source compatibility, although source compatibility may be maintained.
Minor releases are backwards binary and source compatible.
Patch releases are both backwards and forwards binary and source compatible.
Binary Compatibility Guidelines
A library is binary compatible, if a program linked dynamically to a former version of the library continues running with newer versions of the library without the need to recompile.
Source Compatibility Guidelines
If a program needs to be recompiled to run with a new version of library but doesn't require any further modifications, the library is source compatible.
Compatibility in Modules
The modules in Qt Essentials have the same forward and backward compatibility promises as Qt 4: new Qt 5 minor releases will not break binary compatibility, and patch level releases are forward and backward compatible. Qt Add-On modules specify their compatibility promise separately.
For more information on which branch you should submit a change to depending on its compatibility, see the branch guidelines.
When preparing for a new release, we perform an API change review to check for any changes incompatible with this policy.
For examples of things you cannot do in C++ while maintaining binary compatibility, see Binary Compatibility Examples.