Array extension to remove object by value

extension Array {
    func removeObject<T where T : Equatable>(object: T) {
        var index = find(self, object)
        self.removeAtIndex(index)
    }
}

However, I get an error on var index = find(self, object)

'T' is not convertible to 'T'

I also tried with this method signature: func removeObject(object: AnyObject), however, I get the same error:

'AnyObject' is not convertible to 'T'

What is the proper way to do this?

As of Swift 2, this can be achieved with a protocol extension method. removeObject() is defined as a method on all types conforming to RangeReplaceableCollectionType (in particular on Array) if the elements of the collection are Equatable:

extension RangeReplaceableCollectionType where Generator.Element : Equatable {

    // Remove first collection element that is equal to the given `object`:
    mutating func removeObject(object : Generator.Element) {
        if let index = self.indexOf(object) {
            self.removeAtIndex(index)
        }
    }
}

Example:

var ar = [1, 2, 3, 2]
ar.removeObject(2)
print(ar) // [1, 3, 2]

Update for Swift 2 / Xcode 7 beta 2: As Airspeed Velocity noticed in the comments, it is now actually possible to write a method on a generic type that is more restrictive on the template, so the method could now actually be defined as an extension of Array:

extension Array where Element : Equatable {
    
    // ... same method as above ...
}

The protocol extension still has the advantage of being applicable to a larger set of types.

Update for Swift 3:

extension Array where Element: Equatable {
    
    // Remove first collection element that is equal to the given `object`:
    mutating func remove(object: Element) {
        if let index = index(of: object) {
            remove(at: index)
        }
    }
}

Update for Swift 5:

extension Array where Element: Equatable {
    
    /// Remove first collection element that is equal to the given `object` or `element`:
    mutating func remove(element: Element) {
        if let index = firstIndex(of: element) {
            remove(at: index)
        }
    }
}

You cannot write a method on a generic type that is more restrictive on the template.

NOTE: as of Swift 2.0, you can now write methods that are more restrictive on the template. If you have upgraded your code to 2.0, see other answers further down for new options to implement this using extensions.

The reason you get the error 'T' is not convertible to 'T' is that you are actually defining a new T in your method that is not related at all to the original T. If you wanted to use T in your method, you can do so without specifying it on your method.

The reason that you get the second error 'AnyObject' is not convertible to 'T' is that all possible values for T are not all classes. For an instance to be converted to AnyObject, it must be a class (it cannot be a struct, enum, etc.).

Your best bet is to make it a function that accepts the array as an argument:

func removeObject<T : Equatable>(object: T, inout fromArray array: [T]) {
}

Or instead of modifying the original array, you can make your method more thread safe and reusable by returning a copy:

func arrayRemovingObject<T : Equatable>(object: T, fromArray array: [T]) -> [T] {
}

As an alternative that I don't recommend, you can have your method fail silently if the type stored in the array cannot be converted to the the methods template (that is equatable). (For clarity, I am using U instead of T for the method's template):

extension Array {
    mutating func removeObject<U: Equatable>(object: U) {
        var index: Int?
        for (idx, objectToCompare) in enumerate(self) {
            if let to = objectToCompare as? U {
                if object == to {
                    index = idx
                }
            }
        }

        if(index != nil) {
            self.removeAtIndex(index!)
        }
    }
}

var list = [1,2,3]
list.removeObject(2) // Successfully removes 2 because types matched
list.removeObject("3") // fails silently to remove anything because the types don't match
list // [1, 3]

Edit To overcome the silent failure you can return the success as a bool:

extension Array {
  mutating func removeObject<U: Equatable>(object: U) -> Bool {
    for (idx, objectToCompare) in self.enumerate() {  //in old swift use enumerate(self) 
      if let to = objectToCompare as? U {
        if object == to {
          self.removeAtIndex(idx)
          return true
        }
      }
    }
    return false
  }
}
var list = [1,2,3,2]
list.removeObject(2)
list
list.removeObject(2)
list