问题
Most languages use the true/false
keywords for boolean values. I found that even Smalltalk is using true/false
. I know Objective-C is just borrowing concepts from Smalltalk, not the language itself, but I'm curious why it's using YES/NO
instead of the more widely-used true/false
. Is there any historical reason?
回答1:
Objective-C was designed to be (and still is) a strict superset of C. The creators worked very hard to ensure that they did not break compatibility with C in any way. They also tried to make their modifications somewhat obvious so that it would be easy to tell which parts of the code use Objective-C and which parts use plain C. Case in point, the @
used to denote NSStrings rather than just using quotes. This allows plain C strings to coexist with the new ones.
C already had an informal system of TRUE/FALSE macros. I suspect the designers of Objective-C chose the YES/NO macros to avoid conflict and to make it obvious that the code is actually Objective-C. Notice also the usage nil
for the 'empty' object rather than just modifying the behavior of good old NULL.
回答2:
Objective-C is a very verbose language, all methods are very descriptive, and using YES/NO for boolean values instead of true/false makes it more human readable.
You would probably find the following conversation strange, if it happened in real life: A: "Did you see the movie?" B: "True"
If B had answered "yes" (or "no"), it would sound perfectly normal, and code looks more like plain english by using YES/NO instead of true/false.
回答3:
Apple have always tried to make things easier to use. If you read some system boolean methods and ask yourself what makes more sense to answer a boolean question with, either using YES|NO
or TRUE|FALSE
, you'll see thank the answer is YES|NO
in my opinion.
Otherwise you can always use TRUE|FALSE
in your code.
回答4:
It is strange, but I find code is more readable using the YES
/NO
macros rather than TRUE
/FALSE
(which also work).
However, Objective-C is a superset of C99 now, so you should be using the C99 boolean
type and true
and false
wherever possible. I was toying with the idea of defining yes
and no
to true
and false
but have resisted it so far.
回答5:
The best way to think of this is that it's parallel evolution.
Objective-C's BOOL
and YES
/NO
dates all the way back to early 1980s, and was likely intended to not only mimic other languages but miss C's future development. _Bool
, true
/false
in C were only made part of the standard in 1999.
So are YES
and NO
historical? Yes. Are they only historical? No. Just as NULL
is not the result of 3-3 in a pure sense (despite NULL
often being defined as 0
, or casually usable if it were), true
is not a value for BOOL
.
You would not (I think) write this code:
int matches = NULL;
for (int i = 0; i<count; ++i) {
if (array[i] == value) ++matches;
}
This is less obviously wrong, but it's on the same spectrum:
BOOL foundMatch = false;
for (int i = 0; i<count; ++i) {
if (array[i] == value) {
foundMatch = YES;
break;
}
}
来源:https://stackoverflow.com/questions/6420987/why-does-objective-c-use-yes-no-macro-convention-instead-of-true-false