Options

The following CMake variables can be set before calling FetchBoostContent_Populate to influence its behaviour:

Option	Default	Description
Logging
FETCH_BOOST_CONTENT_VERBOSE	0	Verbose output level
FETCH_BOOST_CONTENT_QUIET	OFF	quiet output
Directories
FETCH_BOOST_CONTENT_INCLUDE	""	additional subdirectoroes to scan
FETCH_BOOST_CONTENT_EXCLUDE	""	exclude a default subdirectory ("include", "src") from scan
Behavior
FETCH_BOOST_CONTENT_PRUNE_DEPENDENCIES	OFF	prune transitive dependencies on which library doesn't depend
FETCH_BOOST_CONTENT_IGNORE_CACHE	OFF	ignore cached dependency list (rescan files every time)
FETCH_BOOST_CONTENT_IGNORE	""	exclude top-level dependency even when found in scan

The FETCH_BOOST_CONTENT_PRUNE_DEPENDENCIES option will prune any transitive header-only dependencies on which the main library does not rely. For instance, considering these header-only libraries:

A:
boost/a/a1.hpp
    includes boost/b/b1.hpp
    includes boost/b/b2.hpp

B:
boost/b/b1.hpp
    includes boost/c/c1.hpp
    includes boost/c/c2.hpp
boost/b/b2.hpp

C:
boost/c/c1.hpp
boost/c/c2.hpp
boost/c/c3.hpp
    includes boost/d/d1.hpp

D:
boost/d/d1.hpp

When FETCH_BOOST_CONTENT_PRUNE_DEPENDENCIES is OFF, this would fetch the libraries A, B, C, and D, because of the transitive relationship A -> B -> C -> D. When FETCH_BOOST_CONTENT_PRUNE_DEPENDENCIES is ON, this would only fetch the libraries A, B, and C, because fetching the dependency D for the transitive relationship C -> D is only required when A is indirectly using boost/c/c3.hpp.

Pruning makes the process of fetching faster. This is particularly useful when a given library happens to be at a high dependency level only for requiring a smaller feature from a library with many other transitive dependencies. However, the scripts in C might depend on targets created by D. In this case, we cannot include C with add_subdirectory. This is usually not a problem, as long as the libraries are header-only, as we can just use Boost::headers. For compiled libraries, it's often easier to use add_subdirectory.