---
title: "validateValue(_:forKey:)"
framework: objectivec
role: symbol
role_heading: Instance Method
path: "objectivec/nsobject-swift.class/validatevalue(_:forkey:)"
---

# validateValue(_:forKey:)

Indicates whether the value specified by a given pointer is valid, or can be made valid, for the property identified by a given key.

## Declaration

```swift
func validateValue(_ ioValue: AutoreleasingUnsafeMutablePointer<AnyObject?>, forKey inKey: String) throws
```

## Parameters

- `ioValue`: A pointer to a new value for the property identified by inKey. This method may modify or replace the value in order to make it valid.
- `inKey`: The name of one of the receiver’s properties. The key must specify an attribute or a to-one relationship.

## Discussion

Discussion In Swift, this method throws an error if the value isn’t valid.  In Objective-C, it returns a Boolean value. The default implementation of this function searches the class of the receiver for a property-specific validation function with a particular signature, allowing that function to determine the outcome of the validation. For it to be found, the property-specific validation function must be exposed to Objective-C, must be named according to the pattern validate<InKey>, must take a single, optional AnyObject pointer argument, and must throw. For example, for a property named someString, the validation function is: @objc func validateSomeString(_ ioValue: AutoreleasingUnsafeMutablePointer<AnyObject?>) throws {     // Test, and possibly replace the value here; or throw an error } If you define such a function, the default implementation of validateValue(_:forKey:) calls it when asked to validate the corresponding property, allowing your function to either alter the input value or throw an error. If no such function exists for a particular property, validateValue(_:forKey:)returns YES without taking any other action. In other words, by default, the general validation call succeeds if you don’t explicitly provide a validation function for the given property. note: You typically use the validation described here only in Objective-C. In Swift, property validation is more idiomatically handled by relying on compiler support for optionals and strong type checking, while using the built-in willSet and didSet property observers to test any run-time API contracts, as described in the Property Observers section of The Swift Programming Language (Swift 4.1). See Adding Validation in Key-Value Coding Programming Guide for more information.

## See Also

### Validation

- [validateValue(_:forKeyPath:)](objectivec/nsobject-swift.class/validatevalue(_:forkeypath:).md)