Hi Chris,
Its just as easy to do this outside the message_digest class using input(void*,size_t)
I'm not a fan of this constructor for this reason and 1) because I like minimal class interfaces - the same functionality can be implemented as free function. 2) because it doesn't handle io errors in a useful way, i.e. it is not discernible if the digest is wrong because of an io error (not enough bytes could be read) or because the file is different from what was expected. Also it's generally not a good idea to do expensive operations in a ctor. Reading a file qualifies as expensive.
This may not be the best impl. I use basic_ios because its the first class in the ios_base hierarchy that provides rdbuf().
I would have expected an istream& argument to make clear that an input stream is required. Kevin