懒得找出处了~！
In fact, when __attribute__ was first introduced to GCC, it was faced with some resistance by some who suggested that #pragma be used exclusively for the same purposes.
There were, however, two very good reasons why __attribute__ was added:
It was impossible to generate #pragma commands from a macro (before the C99 _Pragma operator).
There is no telling what the same #pragma might mean in another compiler.
Quoth the GCC Documentation for Function Attributes:
These two reasons applied to almost any application that might have been proposed for #pragma. It was basically a mistake to use #pragma for anything.
Indeed, if you look at modern Objective-C–in the headers of Apple frameworks and well-engineered open-source projects–__attribute__ is used for myriad purposes. (By contrast, #pragma’s main claim to fame these days is decoration: #pragma mark)
- 子类调用该方法的时候需要用 super
__attribute__((objc_requires_super))
//eg:略
- 定义的的结构体也可以使用语法糖
__attribute__((objc_boxable))
typedef struct __attribute__((objc_boxable)) {
CGFloat x, y, width, height;
} YYGRect;
YYGRect rect2 = {1, 2, 3, 4};
NSValue *value2 = @(rect2); //编译通过
- 在程序的主函数之前运行
#include<stdio.h>
__attribute__((constructor)) void before_main() {
printf("before main\n");
}
__attribute__((destructor)) void after_main() {
printf("after main\n");
}
int main(int argc, char **argv) {
printf("in main\n");
return 0;
}
- 修饰一个变量在该变量作用域结束后，自动调用一个指定的方法
所谓作用域结束，包括大括号结束、return、goto、break、exception等各种情况。可以修饰的变量不止NSString，包括自定义Class或基本类型都是可以的。以压栈的方式调用，在函数调用dealloc之前执行：
__attribute__((cleanup))
// 指定一个cleanup方法，注意入参是所修饰变量的地址，类型要一样
// 对于指向objc对象的指针(id *)，如果不强制声明__strong默认是
// __autoreleasing，造成类型不匹配
__strong NSString *string __attribute__((cleanup(Show))) = @"Yin Yu 思密达";
...//这里写N多代码，都是最后执行下面的函数
static void Show(__strong NSString **string)
{
NSLog(@"代码以为我为标志结束!%@",*string);
}
//eg:
// 函数里面：
// 加了个`unused`的attribute用来消除`unused variable`的warning
__strong void(^block)(void) __attribute__((cleanup(blockCleanUp), unused)) = ^{
NSLog(@"代码以为我标志结束！");
};
//所调用的函数
// void(^block)(void)的指针是void(^*block)(void)
static void blockCleanUp(__strong void(^*block)(void))
{
(*block)();
}
//定义宏
#define onExit __strong void(^block)(void) __attribute__((cleanup(blockCleanUp), unused)) = ^
//使用的地方
onExit {
NSLog(@"代码以为为结束");
};
//下面~~~~~~~~N多代码
# 所调用的函数
// void(^block)(void)的指针是void(^*block)(void)
static void blockCleanUp(__strong void(^*block)(void))
{
(*block)();
}
typedef void (^mtl_cleanupBlock_t)();
#define metamacro_concat_(A, B) A ## B
#define metamacro_concat(A, B) \
metamacro_concat_(A, B)
#define onExit \
try {} @finally {} \
__strong mtl_cleanupBlock_t metamacro_concat(mtl_exitBlock_, __LINE__) __attribute__((cleanup(mtl_executeCleanupBlock), unused)) = ^
+ (void)enumeratePropertiesUsingBlock:(void (^)(objc_property_t property, BOOL *stop))block {
Class cls = self;
BOOL stop = NO;
while (!stop && ![cls isEqual:MTLModel.class]) {
unsigned count = 0;
objc_property_t *properties = class_copyPropertyList(cls, &count);
cls = cls.superclass;
if (properties == NULL) continue;
//注意这里的用法
@onExit {
free(properties);
};
for (unsigned i = 0; i < count; i++) {
block(properties[i], &stop);
if (stop) break;
}
}
}
//@onExit 宏展开之后
@try {} @finally {}
__strong mtl_cleanupBlock_t mtl_exitBlock___LINE__ __attribute__((cleanup(mtl_executeCleanupBlock), unused)) = ^{
free(properties);
};
//可以保证 程序在即将运行出 propertties的作用时释放 properties
- 定义若干函数，函数名字相同但是方法不同，编译器能够自动识别调用的函数
__attribute__((overloadable)) void logAnything(id arr)
{
NSLog(@"%@", arr);
}
__attribute__((overloadable)) void logAnything(int num)
{
NSLog(@"%@", @(num));
}
__attribute__((overloadable)) void logAnything(CGRect rect)
{
NSLog(@"%@", NSStringFromCGRect(rect));
}
#调用
//传递数值
logAnything(@[@"hello",@"world"]);
//int 数值
logAnything(66);
//Rect
logAnything(CGRectMake(0, 0, 0, 0));
- 混淆
_attribute__((objc_runtime_name("FalseEunuch")))
//相当于给Eunuch起个学名
@interface Eunuch : NSObject
+(void)show;
@end
//示例：
NSLog(@"%@",[Eunuch class]);
//得到的打印消息
//2016-08-03 14:16:46.882 0803-黑魔法[24329:1921649] FalseEunuch
- attribute((availability))
- (CGSize)sizeWithFont:(UIFont *)font NS_DEPRECATED_IOS(2_0, 7_0, "Use -sizeWithAttributes:") __TVOS_PROHIBITED;
//来看一下 后边的宏
#define NS_DEPRECATED_IOS(_iosIntro, _iosDep, ...) CF_DEPRECATED_IOS(_iosIntro, _iosDep, __VA_ARGS__)
define CF_DEPRECATED_IOS(_iosIntro, _iosDep, ...) __attribute__((availability(ios,introduced=_iosIntro,deprecated=_iosDep,message="" __VA_ARGS__)))
//宏展开以后如下
__attribute__((availability(ios,introduced=2_0,deprecated=7_0,message=""__VA_ARGS__)));
//iOS即是iOS平台
//introduced 从哪个版本开始使用
//deprecated 从哪个版本开始弃用
//警告的消息
//其实还可以再加一个参数例如
void f(void) __attribute__((availability(macosx,introduced=10.4,deprecated=10.6,obsoleted=10.7)));
//obsoleted完全禁止使用的版本
