swift中任何类型和泛型类型之间的区别

Question

What is the difference between an any type and generic type in swift?

Any Type Example:

let swiftInt: Int = 1
let swiftString: String = "miao"

var array: [Any] = []
array.append(swiftInt)
array.append(swiftString)

Generic Type Example:

func duplicate<T>(item: T, numberOfTimes n: Int) -> [T] {
  var buffer : [T] = []
  for _ in 0 ..< n {
    buffer.append(item)
  }
  return buffer
}

Is this a matter of preference because both appear to solve the same problem by being able to substitute the desired type.

Answer 1

I'm not going to explain generics in details and i'll just point out the essential differences.

In the first example, you'll be able to append any type in that array, without being able to restrict beforehand your array to a specific type and to leverage compile time checks to guarantee that the array will not contain extraneous types. Not much to see in that example.

The second example contains instead a generic function that provides all of the above functionalities, consistency checks on the content of the array will come for free and if you want you'll also be able to specify additional characteristics of that generic type T , like requesting that it implements a specific protocol (eg limit duplicate() to object that implement Comparable or Equatable ).

But that is just a simple example of a generic function, you can also have parametrized classes (what you'll use the most) and there are a lot of additional functionalities.

Never use Any as a poor-man generics, real generics are way more flexible, add useful checks and make more explicit your intentions, with minimal additional effort required to implement them.

Answer 2

Any means "I don't want any type checking and I won't be able to call type-specific methods without casting"

For example, try to call:

var array: [Any] = [1, 2]
var sum = array[0] + array[1] // you cannot do this! you have to cast to Int first

A generic type is a placeholder for a type. When used, a concrete type is used instead of it (eg an Int or a String ).

In short, never use Any . There are very very few specific situations when Any is what you want to use.