Here are the classes, structs, unions and interfaces with brief descriptions:

[detail level 1234]

▼Nosmium	Namespace for everything in the Osmium library
►Narea	Code related to the building of areas (multipolygons) from relations
►CAssembler
CMPFilter
CAssemblerConfig
CMultipolygonCollector
CProblemReporter
CProblemReporterException
CProblemReporterOGR
CProblemReporterStream
►Nbuilder	Classes for building OSM objects and other items in buffers
CAreaBuilder
CBuilder
CNodeRefListBuilder
CObjectBuilder
COSMObjectBuilder
CRelationMemberListBuilder
CTagListBuilder
CWayBuilder
►Ndiff_handler	Osmium diff handlers provide access to differences between OSM object versions
CDiffHandler
►Nexperimental	Experimental code that is not "officially" supported
CFlexReader
►Ngeom	Everything related to geometry handling
CCoordinates
►CCRS
CProjCRSDeleter
CGeometryFactory
CIdentityProjection
CMercatorProjection
CProjection
CTile
►Nhandler	Osmium handlers provide callbacks for OSM objects
►CChainHandler
Ccall_flush
Ccall_flush< SIZE, SIZE, THandlers >
CDiskStore
CDump
CDynamicHandler
CHandler
CNodeLocationsForWays
CObjectRelations
►Nindex	Indexing of OSM data, Locations, etc
►Nmap	Key-value containers with unique integer values for a key
Ccreate_map
Ccreate_map< TId, TValue, DenseFileArray >
Ccreate_map< TId, TValue, SparseFileArray >
CDummy
CMap
CSparseMemMap
►Nmultimap	Key-value containers with multiple values for an integer key
CHybrid
CHybridIterator
CMultimap
CSparseMemMultimap
CBoolVector
CMapFactory
►Nio	Everything related to input and output of OSM data
CBzip2BufferDecompressor
CBzip2Compressor
CBzip2Decompressor
CCompressionFactory
CCompressor
CDecompressor
CFile
CGzipBufferDecompressor
CGzipCompressor
CGzipDecompressor
CHeader
CInputIterator
CNoCompressor
CNoDecompressor
►COutputIterator
Cbuffer_wrapper
CReader
CWriter
►Nmemory	Memory management of items in buffers and iterators over this data
CBuffer
CCollection
CCollectionIterator
CItem
CItemIterator
►Nrelations	Code related to the assembly of OSM relations
►CCollector
CHandlerPass1
CHandlerPass2
►Ntags	Code related to working with OSM tags
►CFilter
CRule
Cmatch_key
Cmatch_key_prefix
Cmatch_value
Cmatch_value< std::regex >
Cmatch_value< void >
►Nthread	Threading-related low-level code
►Cfunction_wrapper
Cimpl_base
Cimpl_type
►CPool
Cthread_joiner
CQueue
CSortedQueue
►Nutil	Helpful utility classes and functions not strictly OSM related
CAnonymousMemoryMapping
CAnonymousTypedMemoryMapping
CDataFile
CDeltaDecode
CDeltaEncode
CMemoryMapping
COptions
CTypedMemoryMapping
CVerboseOutput
Care_real_integers
CArea
CBox
Cbuffer_is_full
Cbzip2_error
CChangeset	An OSM Changeset, a group of changes made by a single user over a short period of time
CDiffIterator
CDiffObject
CDiffObjectDerived
Cgeometry_error
Cgeos_geometry_error
Cgzip_error
CInnerRing
Cinvalid_location
Cio_error
CLocation
Clocation_equal
Clocation_less
Cmax_op
Cmin_op
CNode
CNodeRef
CNodeRefList
Cnot_found
Cobject_equal_type_id
Cobject_equal_type_id_version
Cobject_order_type_id_reverse_version
Cobject_order_type_id_version
CObjectPointerCollection
COSMEntity	OSMEntity is the abstract base class for the OSMObject and Changeset classes
COSMObject
COuterRing
Cprojection_error
CRelation
CRelationMember
CRelationMemberList
CSegment
CTag
CTagList
CTimestamp
CUndirectedSegment
Cunknown_type
CWay
CWayNodeList