[boost] [gil] where is the mysterious boost::gil::read_image function?

I would like to read a jpeg image from a memory location, not from a file. According to this post http://boost.2283326.n4.nabble.com/gil-io-new-review-Reading-images-from-in-... there is a boost::gil::read_image(input_stream, image, image_tag) function but I cannot find it in the header files. Thanks, Frédéric