Many of the client facing headers in mongodb use system functions without including the headers that define those functions: mongo/bson/bsonobjbuilder.h for instance, uses 'pow', but does not include <cmath>, uses 'strlen', but does not include <cstring>, etc. Similarly, the BSON headers are not complete among themselves: bsonobjbuilder.h uses util/builder.h, but does not include it.

Every client facing header should compile cleanly if included as the only text in a .cc file.