How to convert a binary to decimal in Swift?

Update for Swift 2: All integer types have an

public init?(_ text: String, radix: Int = default)

method now, which converts a string to an integer according to
a given base:

let binary = "11001"
if let number = Int(binary, radix: 2) { 
    print(number) // Output: 25
}

(Previous answer:) You can simply use the BSD library function strtoul(), which converts a string to
a number according to a given base:

let binary = "11001"
let number = strtoul(binary, nil, 2)
println(number) // Output: 25