API and ABI versioning

API Versioning

  • We use semantic versioning.
  • bsoncxx and mongocxx both define corresponding CMake variables for MAJOR, MINOR, and PATCH.

ABI Versioning

  • Both bsoncxx and mongocxx both have a single scalar ABI version.
  • Only bump ABI version on incompatible ABI change (not for ABI additions).
  • We stay on ABI version _noabi (without bumping for incompatible changes) until ABI is stable.

Parallel Header Installation

  • For mongocxx, install all headers to $PREFIX/mongocxx/v$ABI/.
  • For bsoncxx, install all headers to $PREFIX/bsoncxx/v$ABI/.
  • We install a pkg-config file to shield consumers from this complexity.

Note that below examples are given for libmongocxx, but also apply to libbsoncxx

DSO = Dynamic Shared Object, to use Ulrich Drepper’s terminology

  • Physical filename for a DSO is libmongocxx.so.$MAJOR.$MINOR.$PATCH

Note that the physical filename is disconnected from ABI version/soname. This looks a bit strange, but allows multiple versions of the library with the same ABI version to be installed on the same system.

  • soname for a DSO is libmongocxx.$ABI

We provide a soname symlink that links to the physical DSO. We also provide a dev symlink that links to the soname symlink of the highest ABI version of the library installed.

Inline namespaces

  • We provide inline namespace macros for both mongocxx and bsoncxx.
  • This allows multiple, ABI incompatible versions of the library to be linked into the same application.
  • The name of the namespace is v$ABI. We create them from ABI version to maintain forwards compatibibility.


Occasionally we will phase features out of use in the driver. In the release that marks a feature as deprecated we offer several transition options:

  1. The original method, marked with MONGOCXX_DEPRECATED. This will raise deprecation warnings when compiled.
  2. A variant of the feature suffixed with _deprecated. This will require only small code changes and will not raise deprecation warnings.
  3. A new feature that provides alternate functionality. The new feature may not be a drop-in replacement for the deprecated feature and switching to it may require code changes. In the rare case where we remove a feature without replacing it, this third option will not be available.

In the following release, the original feature marked with MONGOCXX_DEPRECATED and its suffixed _deprecated equivalent will be removed.

// release 1 with a supported feature
void do_thing();

// release 2 where the feature is deprecated in favor of a new feature
MONGOCXX_DEPRECATED void do_thing();
void do_thing_deprecated();
void do_new_thing();

// release 3 where the original feature is removed
void do_new_thing();

In the case of a feature rename we do not offer a suffixed _deprecated variant, as one can simply switch to using the new name with the same amount of effort.

// release 1 with a supported feature
void do_thing();

// release 2 where the feature name is renamed
MONGOCXX_DEPRECATED void do_thing();
void do_stuff();

// release 3 where the original feature name is removed
void do_stuff();