Swift中带返回值与无返回值函数的核心差异是什么？

Hey there! Let's break down the difference between functions with return values and those without in Swift, then dive into the two examples you shared.

In Swift, these two function types serve distinct purposes:

• 无返回值函数（隐式返回Void）: These functions are built to handle side effects — like printing to the console, modifying external variables, updating UI, or triggering an action. Even if you don’t write a return type, Swift automatically makes them return Void (an empty tuple ()). Their main job is to "do something," not to pass data back to the caller.
• 带返回值函数: These functions exist to compute and deliver data. You declare their return type with -> Type, and they must return a value of that type (either explicitly with return or implicitly for single-expression functions). The caller can then use this returned value for further work: assign it to a variable, pass it to another function, or include it in calculations. They often prioritize producing a result over causing side effects.

Let’s break down each function to see their key differences:

1. func sqr(number: Int) { print (number * number) } #

This is a typical void-return function:

• It has no declared return type, so it implicitly returns Void.
• Its only job is to perform a side effect: directly printing the squared value to the console.
• When you call it (e.g., sqr(number: 5)), you can’t capture the squared value for later use. Even if you try let result = sqr(number: 5), result will just be () — useless for any numerical operations.

2. func sqr1(number: Int) -> Int { return number * number } #

This is a value-returning function:

• It explicitly states it returns an Int, so it must send back a squared integer via return.
• Its core responsibility is to compute and deliver the squared value — no side effects are baked into this function (unless you add them, which this example doesn’t).
• When you call it, you’re in full control of the result:

  let squared = sqr1(number: 5) // squared holds the value 25
  print(squared) // You choose to print it if needed
  let total = squared + 15 // Or use it in another calculation
  

The big-picture difference is responsibility separation:

• sqr ties computation and output into a single function.
• sqr1 focuses only on computation, leaving the decision of what to do with the result (print, store, etc.) entirely up to the caller.

内容的提问来源于stack exchange，提问作者Ian Solomein