Sometimes, particular relationships need to be maintained among a target's build properties. For example, you might want to set specific #define when a library is built as shared, or when a target's release variant is built. This can be achieved using conditional requirements.

lib network : network.cpp
    : <link>shared:<define>NETWORK_LIB_SHARED
     <variant>release:<define>EXTRA_FAST
    ;

In the example above, whenever network is built with <link>shared, <define>NETWORK_LIB_SHARED will be in its properties, too. Also, whenever its release variant is built, <define>EXTRA_FAST will appear in its properties.

Sometimes the ways a target is built are so different that describing them using conditional requirements would be hard. For example, imagine that a library actually uses different source files depending on the toolset used to build it. We can express this situation using target alternatives:

lib demangler : dummy_demangler.cpp ;                      # alternative 1
lib demangler : demangler_gcc.cpp : <toolset>gcc ;   # alternative 2
lib demangler : demangler_msvc.cpp : <toolset>msvc ; # alternative 3

When building demangler, Boost.Build will compare requirements for each alternative with build properties to find the best match. For example, when building with <toolset>gcc alternative 2, will be selected, and when building with <toolset>msvc alternative 3 will be selected. In all other cases, the most generic alternative 1 will be built.