send(to,from,count)
register short *to,*from;
register count;
{
register n = (count + 7) / 8;
switch (count % 8) {
case 0: do { *to = *from++;
case 7: *to = *from++;
case 6: *to = *from++;
case 5: *to = *from++;
case 4: *to = *from++;
case 3: *to = *from++;
case 2: *to = *from++;
case 1: *to = *from++;
} while (--n > 0);
}
}
现在,在Swif 2.1中,switch-case control flows并没有像我们在Swift文档中读到的那样隐含起来：
No Implicit Fallthrough
In contrast with switch statements in C and Objective-C,switch
statements in Swift do not fall through the bottom of each case and
into the next one by default. Instead,the entire switch statement
finishes its execution as soon as the first matching switch case is
completed,without requiring an explicit break statement. This makes
the switch statement safer and easier to use than in C,and avoids
executing more than one switch case by mistake.
现在,鉴于在Swift中明确存在一个突破性副作用,有一个通过条款：
Fallthrough
Switch statements in Swift do not fall through the bottom of each case
and into the next one. Instead,the entire switch statement completes
its execution as soon as the first matching case is completed. By
contrast,C requires you to insert an explicit break statement at the
end of every switch case to prevent fallthrough. Avoiding default
fallthrough means that Swift switch statements are much more concise
and predictable than their counterparts in C,and thus they avoid
executing multiple switch cases by mistake.
这非常像：
let integerToDescribe = 5
var description = "The number \(integerToDescribe) is"
switch integerToDescribe {
case 2,3,5,7,11,13,17,19:
description += " a prime number,and also"
fallthrough
default:
description += " an integer."
}
print(description)
// prints "The number 5 is a prime number,and also an integer."
考虑到维基百科提醒我们,这些设备来自这个问题
A straightforward code to copy items from an array to a memory-mapped output register might look like this:
do { /* count > 0 assumed */
*to = *from++; /* "to" pointer is NOT incremented,see explanation below */
} while(--count > 0);
哪个是Swift中Duff设备的确切实现？
这只是一种语言和编码问题,它不适用于真正的Swift应用程序.
特别是在Swift中,您不必担心复制数组(Duff设备的原始应用程序),因为Swift假装在分配数组时使用“copy on write”复制数组.这意味着只要您只是从它们中读取它就会对两个变量使用相同的数组,但只要您修改其中一个变量,它就会在后台创建一个副本.
例如,从https://developer.apple.com/documentation/swift/array开始
修改数组的副本
Each array has an independent value that includes the values of all of its elements. For simple types such as integers and other structures,this means that when you change a value in one array,the value of that element does not change in any copies of the array. For example: var numbers = [1,2,4,5] var numbersCopy = numbers numbers[0] = 100 print(numbers) // Prints "[100,5]" print(numbersCopy) // Prints "[1,5]"
