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# copyFormatDescription()

Creates a format description based on the receiver’s items.

```
func copyFormatDescription() -> CMMetadataFormatDescription?
```

## Return Value

An instance of <doc://com.apple.documentation/documentation/CoreMedia/CMMetadataFormatDescription> sufficient to describe the contents of all the items referenced by the object.

## Discussion

The returned format description is suitable for use as the format hint parameter when creating an instance of [`AVAssetWriterInput`](/documentation/AVFoundation/AVAssetWriterInput).

Each item referenced by the receiver must carry a non-`nil` value for its `dataType` property.  An exception will be thrown if any item does not have a data type.

---

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