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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
Thesvn:externals
property with this value must be attached to the library-specificboost
subdirectory, because each line is a subdirectory withinboost
. 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
tolibs/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 executesvn 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.
Informing CMake that the library is modular
The CMake build system needs to know that the layout of the Boost library follows the rules of a modular library, which also instructs it to add the appropriate include paths when compiling itself and any of its dependencies. To label the library as modular, edit the CMakeLists.txt
file contained in the library's subdirectory (e.g., libs/filesystem/CMakeLists.txt
, and add the argument MODULAR
to the use of [wikiCMakeLibraryProject boost_library_project
]. After this change, Filesystem library's CMakeLists.txt
looks like this:
boost_library_project( Filesystem SRCDIRS src TESTDIRS test MODULAR DESCRIPTION "Provides portable facilities to query and manipulate paths, files, and directories." AUTHORS "Beman Dawes <bdawes -at- acm.org>" )
If the library you're modularizing does not have DESCRIPTION
, AUTHORS
, or MAINTAINERS
arguments, please add them! Short library descriptions are available at http://www.boost.org/doc/ along with author information; additional maintainer information can be found in http://svn.boost.org/svn/boost/trunk/libs/maintainers.txt.