Handle when FUSION_MAX_MAP_SIZE is not defined. FUSION_MAX_MAP_SIZE is only defined when BOOST_FUSION_HAS_VARIADIC_MAP is not defined. The conditions in which BOOST_FUSION_HAS_VARIADIC_MAP are defined can be seen in the fusion file C:\Programming\VersionControl\modular-boost\libs\fusion\include\boost\fusion\container\map\map_fwd.hpp.