<!--
{
  "availability" : [
    "iOS: 13.0.0 -",
    "iPadOS: 13.0.0 -",
    "macCatalyst: -",
    "macOS: 10.15.0 -",
    "tvOS: 13.0.0 -",
    "visionOS: 1.0.0 -",
    "watchOS: 6.0.0 -"
  ],
  "documentType" : "symbol",
  "framework" : "CoreMedia",
  "identifier" : "/documentation/CoreMedia/CMFormatDescription/Extensions-swift.struct",
  "metadataVersion" : "0.1.0",
  "role" : "Structure",
  "symbol" : {
    "kind" : "Structure",
    "modules" : [
      "Core Media",
      "CoreMedia"
    ],
    "preciseIdentifier" : "s:So22CMFormatDescriptionRefa9CoreMediaE10ExtensionsV"
  },
  "title" : "CMFormatDescription.Extensions"
}
-->

# CMFormatDescription.Extensions

A type that describes format description extensions.

```
struct Extensions
```

## Topics

### Creating Extensions

[`init()`](/documentation/CoreMedia/CMFormatDescription/Extensions-swift.struct/init())

Creates an empty `Extensions` structure.

[`init(base: [CFString : CFPropertyList]?)`](/documentation/CoreMedia/CMFormatDescription/Extensions-swift.struct/init(base:))

Creates an `Extensions` structure with existing values.

### Finding Extension Elements

[`subscript(CFString) -> CFPropertyList?`](/documentation/CoreMedia/CMFormatDescription/Extensions-swift.struct/subscript(_:)-1c6vg)

[`subscript(CMFormatDescription.Extensions.Key) -> CMFormatDescription.Extensions.Value?`](/documentation/CoreMedia/CMFormatDescription/Extensions-swift.struct/subscript(_:)-80zh8)

### Constants

[`struct Key`](/documentation/CoreMedia/CMFormatDescription/Extensions-swift.struct/Key)

[`struct Value`](/documentation/CoreMedia/CMFormatDescription/Extensions-swift.struct/Value)

## Relationships

### Conforms To

[`Sendable`](/documentation/Swift/Sendable)

[`Copyable`](/documentation/Swift/Copyable)

[`Equatable`](/documentation/Swift/Equatable)

[`Escapable`](/documentation/Swift/Escapable)

[`Sequence`](/documentation/Swift/Sequence)

[`Hashable`](/documentation/Swift/Hashable)

[`Collection`](/documentation/Swift/Collection)

[`SendableMetatype`](/documentation/Swift/SendableMetatype)

---

Copyright &copy; 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)