Map和Dictionary

NSMutableDictionary会对key进行强引用，对value进行copy，key必须有copyWithZone方法。
NSMapTable可以灵活配置key和value的内存管理方法，
更多类型见NSPointerFunctionsOptions枚举值

[NSMutableDictionary dictionary]

等同于

[NSMapTable mapTableWithKeyOptions:NSPointerFunctionsCopyIn valueOptions:NSPointerFunctionsStrongMemory]

比较一下区别

  NSMutableDictionary *aMapTable = [NSMutableDictionary dictionary];
    {
        NSObject *object = [[NSObject alloc] init];
        NSURL *key = [NSURL URLWithString:@"urlkey"];
        [aMapTable setObject:object forKey:key];
        uintptr_t count1 = _objc_rootRetainCount(key);
        uintptr_t count2 = _objc_rootRetainCount(object);
        NSLog(@"key retainCount:%lu, value retainCount:%lu",count1, count2);
        NSLog(@"NSMapTable:%@", aMapTable);
    }
    NSLog(@"NSMapTable:%@", aMapTable);

结果

key retainCount:1, value retainCount:2
NSMapTable:{
    urlkey = "<NSObject: 0x600000af6f60>";
}
NSMapTable:{
    urlkey = "<NSObject: 0x600000af6f60>";
}

key是copy的，value是强引用的，大括号作用域结束后，临时变量被释放，value引用计数减1。

  NSMapTable *aMapTable = [NSMapTable mapTableWithKeyOptions:NSPointerFunctionsStrongMemory valueOptions:NSPointerFunctionsWeakMemory];
    {
        NSObject *object = [[NSObject alloc] init];
        NSURL *key = [NSURL URLWithString:@"urlkey"];
        [aMapTable setObject:object forKey:key];
        uintptr_t count1 = _objcrootRetainCount(key);
        uintptr_t count2 = _objc_rootRetainCount(object);
        NSLog(@"key retainCount:%lu, value retainCount:%lu",count1, count2);
        NSLog(@"NSMapTable:%@", aMapTable);
    }

    NSLog(@"NSMapTable:%@", aMapTable);

结果：

key retainCount:1, value retainCount:1
NSMapTable:NSMapTable {
[6] urlkey -> <NSObject: 0x600001d5c940>
}
NSMapTable:NSMapTable {
}

key是copy的，value是弱引用，大括号作用域结束后，临时变量被释放，值不再存在。