---
title: MDQueryCreateValueFunction
framework: coreservices
role: symbol
role_heading: Type Alias
path: coreservices/mdquerycreatevaluefunction
---

# MDQueryCreateValueFunction

Callback function usedto create the value objects stored and returned by a query.

## Declaration

```swift
typealias MDQueryCreateValueFunction = (MDQuery?, CFString?, CFTypeRef?, UnsafeMutableRawPointer?) -> UnsafeRawPointer?
```

## Parameters

- `query`: The query instance.
- `attrName`: The attribute name of the value.
- `attrValue`: The default value of the value.
- `context`: The user-defined context parameter provided in the MDQuerySetCreateValueFunction function.

## Return Value

Return Value The function must return a pointer-sized value that can be managed with the callback which were set at the same time the create function was given to the query. The value must be returned with a reference (such as if the retain callback had been called on it), as implied by the Create name. If this function doesn't wish to create a new object, it can return the given CFTypeRef, but must also return it with a new retain, and the callbacks must be able to handle a CFTypeRef as an input value.

## Discussion

Discussion The function may hold onto the given attribute name and/or value in some other data structure, but must retain them for them to remain valid

## See Also

### Callbacks

- [MDQuerySortComparatorFunction](coreservices/mdquerysortcomparatorfunction.md)
- [MDQueryCreateResultFunction](coreservices/mdquerycreateresultfunction.md)