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# CFBagRetainCallBack

Prototype of a callback function used to retain a value being added to a bag.

```
typealias CFBagRetainCallBack = (CFAllocator?, UnsafeRawPointer?) -> UnsafeRawPointer?
```

## Parameters

`allocator`

The bag’s allocator.

`value`

The value being added to the bag.

## Return Value

The value to store in the bag, which is usually the `value` parameter passed to this callback, but may be a different   value if a different value should be stored in the collection.

## Discussion

This callback is passed to [`CFBagCreate(_:_:_:_:)`](/documentation/CoreFoundation/CFBagCreate(_:_:_:_:)) in a [`CFBagCallBacks`](/documentation/CoreFoundation/CFBagCallBacks) structure.

---

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