1.BOOL与bool的区别

• bool 是0和1 分别对应 -> false/true
• BOOL 是0和非0（-256~255除0外） 分别对应->NO/YES

bool boolA = 1;
bool boolB = 0;
bool boolC = 256;
bool boolD = -1;
bool boolE = 13;


BOOL BOOLA = 1;
BOOL BOOLB = 0;
BOOL BOOLC = 256;
BOOL BOOLD = -1;
BOOL BOOLE = 13;


NSLog(@"boolA = %d",boolA);
NSLog(@"boolB = %d",boolB);
NSLog(@"boolC = %d",boolC);
NSLog(@"boolD = %d",boolD);
NSLog(@"boolE = %d",boolE);

NSLog(@"===========");

NSLog(@"BOOLA = %d",BOOLA);
NSLog(@"BOOLB = %d",BOOLB);
NSLog(@"BOOLC = %d",BOOLC);
NSLog(@"BOOLD = %d",BOOLD);
NSLog(@"BOOLE = %d",BOOLE);

结果输出为：

boolA = 1  // true
boolB = 0  // false
boolC = 1  // true
boolD = 1  // true
boolE = 1  // true
===========
BOOLA = 1  // YES
BOOLB = 0  // NO
BOOLC = 0  // NO
BOOLD = -1  // YES
BOOLE = 13  // YES

由以上结果可看出：bool的结果只有两个：0和1。除了0是0，其他任意数字都是1，没有大小限制。
BOOL的范围为8位二进制数字，如果超出了8位，就截取后8位。当8个位置上都为0时，返回0，所以0会返回0，256会返回0，但是257会返回1，因为只截取后8位。

2.使用事项

• bool
  Introduced to standard C in the C99 spec. (The C99 standard was published in 1999, but it took some years after that to become widespread in use.) Prior to that, "plain" C had no built-in Boolean type, so libraries that built on top of C often defined their own. (And often continued using their own types for source/binary compatibility even after they embraced C99 compilers.)
  Use this if you're writing ISO C and aren't working in the context of higher level libraries with their own Boolean types.

• Boolean
  Defined by Carbon (the early-OSX-days compatibility bridge from the even older Mac Toolbox), which you might still see in some projects (due to transitive #include of headers that are really only around for compatibility with really old source code).
  Don't use this.

• BOOL
  Defined by ObjC because NeXTSTEP needed its own Boolean type back in 1988. (The oldest objc.h I can find on my office bookshelf dates to 1992 and includes a definition of BOOL.)
  ObjC BOOL has often been defined as typedef signed char, meaning that it can hold more values than just YES (1) and NO (0). That can be a problem if you aren't careful. (Why even do that? Because if a type is one bit wide, it's hard to pack into well-aligned memory for good performance.)
  However, in iOS 64-bit (including tvOS) and watchOS, the compiler defines OBJC_BOOL_IS_BOOL, which makes ObjC BOOL just an alias for C99 bool. That means the language/compiler ensures that nonzero values are always stored as 1, so you don't have the issues that come from typedef signed char BOOL. (Still gotta worry about them on macOS or 32-bit iOS, though.)

• TLDR
  If you're working in ObjC with ObjC frameworks (like Cocoa, UIKit, etc), you should use BOOL for consistency with the APIs you're interacting with. (Besides, YES and NO are much louder than true and false, and it's good to be emphatic when you're talking about absolute truth, right?)

DO NOT COMPARE TO YES

在条件判断语句中，不要直接使用x == YES，或x != YES这种写法

参考：
BOOL’s sharp corners
[Objc 中 “== YES” 的愚蠢行为有多可怕]
(https://www.jianshu.com/p/75b88d2a0380)
is-there-any-difference-between-bool-boolean-and-bool-in-objective-c
iOS中BOOL跟bool的区别