# CMReadOnlyDataBlockBuffer.BlockRegion A contiguous region of memory within a block buffer. ``` struct BlockRegion ``` ## Topics ### Instance Properties [`var count: Int`](/documentation/CoreMedia/CMReadOnlyDataBlockBuffer/BlockRegion/count) Number of bytes in the block region [`var endIndex: Int`](/documentation/CoreMedia/CMReadOnlyDataBlockBuffer/BlockRegion/endIndex) The position one greater than the last valid subscript argument. [`var regions: CollectionOfOne`](/documentation/CoreMedia/CMReadOnlyDataBlockBuffer/BlockRegion/regions) The single buffer that makes up this region. [`let startIndex: Int`](/documentation/CoreMedia/CMReadOnlyDataBlockBuffer/BlockRegion/startIndex) The position of the first element in a nonempty collection. ### Instance Methods [`func withContiguousStorageIfAvailable((UnsafeBufferPointer) throws -> R) rethrows -> R?`](/documentation/CoreMedia/CMReadOnlyDataBlockBuffer/BlockRegion/withContiguousStorageIfAvailable(_:)) Access contents of the buffer if available as contiguous memory block. [`func withUnsafeBytes((UnsafeRawBufferPointer) throws -> ResultType) rethrows -> ResultType`](/documentation/CoreMedia/CMReadOnlyDataBlockBuffer/BlockRegion/withUnsafeBytes(_:)) Calls the given closure with a pointer to the underlying bytes of the region’s contiguous storage. ### Subscripts [`subscript(CMReadOnlyDataBlockBuffer.BlockRegion.Index) -> UInt8`](/documentation/CoreMedia/CMReadOnlyDataBlockBuffer/BlockRegion/subscript(_:)) Accesses the data byte at the specified position. ## Relationships ### Conforms To [`DataProtocol`](/documentation/Foundation/DataProtocol) [`Collection`](/documentation/Swift/Collection) [`RandomAccessCollection`](/documentation/Swift/RandomAccessCollection) [`BidirectionalCollection`](/documentation/Swift/BidirectionalCollection) [`Sequence`](/documentation/Swift/Sequence) [`ContiguousBytes`](/documentation/Foundation/ContiguousBytes) --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)