[boost] why not Boost.signals are supported with only header files, like std::vector

I appreciate Boost.signals, but i'am very curious about its implementation. I don't understand why its implementation need build into library, like Boost.thread that depends on OS. I think the best way that its implementation is only in header file, like std::vector. Thanks! -- HongChao Zhao Laboratory of Spatial Information Technology Division of System Architecture Institute of Computing Technology Chinese Academy of Sciences