The descriptions seem to be the same to me. "required" vs "needed" what does that mean?

// Returns the number of bytes required to store the String in a given encoding.
lengthOfBytes(using:)

// Returns the maximum number of bytes needed to store the receiver in a given encoding.
maximumLengthOfBytes(using:)

CodePudding user response：

The lengthOfBytes(using:) returns the exact number, while maximumLengthOfBytes(using:) returns an estimate, which "may be considerably greater than the actual length" (in Apple own words)

CodePudding user response：

The main difference between these methods is given by the Discussion sections of their documentation.

lengthOfBytes(using:):

The result is exact and is returned in O(n) time.

maximumLengthOfBytes(using:):

The result is an estimate and is returned in O(1) time; the estimate may be considerably greater than the actual length needed.

An example where they may differ: the UTF-8 string encoding requires between 1 and 4 bytes to represent a code point, but the exact representation depends on which code point is being represented. lengthOfBytes(using:) will go through the string, calculating the exact number of bytes for every single character, while maximumLengthOfBytes(using:) is allowed to round up to 4 for every code point without looking at the actual value in the string. In this case, the maximum returned is 3× as much as actually needed:

import Foundation

let str = "Hello, world!"
print(str.lengthOfBytes(using: .utf8)) // => 13
print(str.maximumLengthOfBytes(using: .utf8)) // => 39

maximumLengthOfBytes(using:) can give you an immediate answer with little to no computation, at the cost of overestimating, sometimes greatly. The tradeoff of which to use depends on your specific use-case.