Modularizing a Library with CMake

Boost's CMake-based build system supports the notion of "modular" libraries, which are libraries that are contained entirely within a single directory structure. Since modular libraries are self-contained, it is easier to bring in libraries of different versions and select specific subsets of libraries. Additionally, modular libraries explicitly declare their dependencies on other libraries (or "modules"), making it possible to build and install coherent subsets of Boost. For example, the binary installer for Windows allows one to turn on or off installation of each modular library.

Layout of a modular library

A modular library has a similar layout to non-modular libraries. The main difference is in the handling of include files, which are stored within the library's directory in libs/libname/include rather than in the main "boost" include directory. A modular library will typically have the following subdirectories:

libs/libname - Main library directory
  include/   - Library headers. Since most Boost headers go into boost/, the actual library headers will be in the subdirectory include/boost (or its subdirectoiries)
  src/       - Source files for compiled library binaries (if any)
  test/      - Regression tests
  example/   - Example programs, libraries, and applications
  doc/       - Documentation

Throughout this document, we will use the Filesystem library as an example of a modular library. Please refer to the contents of libs/filesystem to see a fully-working modular library's description.

Restructuring the include directory

For most Boost libraries, the only changes needed to the directory structure is to introduce the include directory. To do so, create an empty directories include and then include/boost in libs/libname. Then, add these two new directories to Subversion. If you're using the command-line Subversion, you can do this with the following command run from libs/libname:

svn add include

Next, we need to identify each of the include files that are part of this library (but *not* part of libraries that it depends on) and move each of these libraries from the main Boost include directory into our library-specific include directory. We handle library-specific subdirectories of the Boost include directories (e.g., boost/filesystem) slightly differently from individual headers (e.g., boost/shared_ptr.hpp):

• Library-specific include directories are handled by ​Subversion externals. To move the directory boost/filesystem, for example, one should first delete boost/filesystem entirely from the main Boost include directory. With the command-line Subversion, this can be done by changing into the top-level boost include directory (e.g., $BOOST/boost) and running

  svn rm filesystem
  

  Next, change into the include directory within the library-specific directory, e.g., libs/filesystem/include. Add a new Subversion svn:externals property to this directory that references the corresponding include directory from the main Boost directory tree. For example, we want our filesystem directory to point at branches/release/boost/filesystem. This way, our modularized version of the library automatically picks up fixes from the main release branch. The svn:externals property contains one or more lines corresponding to external definitions. Each line contains the local directory name (e.g., filesystem) followed by a space and then the Subversion directory that this directory will come from, e.g., ​https://svn.boost.org/svn/branches/release/boost/filesystem. For example:

  filesystem https://svn.boost.org/svn/boost/branches/release/boost/filesystem
  

  The svn:externals property with this value must be attached to the library-specific boost subdirectory, because each line is a subdirectory within boost. Using the command-line Subversion client, this can be done with:

  svn propset svn:externals "filesystem https://svn.boost.org/svn/boost/branches/release/boost/filesystem" boost
  

  Note that, to see the actual changes this involves, you will need to commit all of your changes to the Subversion repository and then execute an update operation.

• Individual headers are handled by moving the headers from the main Boost include directory into the library-specific include directory. This is effectively just a rename operation, e.g., to rename boost/shared_ptr.hpp to libs/smart_ptr/include/boost/shared_ptr.hpp. To perform this rename operation via the command-line Subversion client, change to the top-level Boost directory and execute

  svn move boost/shared_ptr.hpp libs/smart_ptr/include/boost/
  

Once all of the headers have been moved and the changes have been committed, there should be no remaining headers in the main Boost include directory.