小编ema*_*cos的帖子

我试图explain在查询中解释mysql(以两种不同的方式编写),这是表:

    create table text_mess(
    datamess timestamp(3) DEFAULT 0,
    sender bigint ,
    recipient bigint ,
    roger boolean,
    msg char(255),
    foreign key(recipient) 
            references users (tel) 
                        on delete cascade
                        on update cascade,
primary key(datamess,sender)
)
engine = InnoDB

这是第一种查询类型:

    EXPLAIN
    select /*!STRAIGHT_JOIN*/datamess, sender,recipient,roger,msg
    from text_mess join (select max(datamess)as dmess
                    from text_mess 
                    where roger = true
                    group by sender,recipient) as max
                    on text_mess.datamess=max.dmess ; 

这是第二个:

    EXPLAIN
    select /*!STRAIGHT_JOIN*/datamess, sender,recipient,roger,msg
    from  (select max(datamess)as dmess
                    from text_mess 
                    where roger = true
                    group by sender,recipient) …

有谁知道如何使用 Swift 语言将自定义输入源附加到运行循环？我正在关注这个文档：特别是运行循环：“配置运行循环源”，但我还没有弄清楚如何快速使用它。

我正在尝试了解Swift中指针的使用，尤其是：Unsafe[Mutable]Pointer和UnsafeRaw[Mutable]Pointer。关于这个问题，我有几个问题。

1. 是UnsafePointer <T>等于const T * Pointer在？和UnsafeMutablePointer <T>等于T * Pointer在C 2

2. Unsafe[Mutable]Pointer和之间有什么区别UnsafeRaw[Mutable]Pointer？

3. 为什么编译

func receive(pointer: UnsafePointer<Int> ) {
    print("param value is: \(pointer.pointee)")
}

var a: Int = 1
receive(pointer: &a) // prints 1

但这给我一个错误？

var a: Int = 1
var pointer: UnsafePointer<Int> = &a // error : Cannot pass immutable value of type 'Int' as inout argument

我想弄清楚 NSMapTable 是如何工作的所以我在操场上尝试以下代码：

class Person {
    var name: String


    init(name: String ) {
        self.name = name
        print("\(name) is being initialized")
    }

    deinit {
        print("\(name) is being deinitialized")
    }
}

var hobyePerson : NSMapTable? = NSMapTable<Person, NSMutableString>
(keyOptions: .weakMemory, valueOptions: .weakMemory)

var rob : Person? = Person(name: "Rob Appleseed") // print : Rob Appleseed is being initialized

hobyePerson?.setObject("golf", forKey: rob)
hobyePerson?.count // return : 1


rob = nil // print : Rob Appleseed is being deinitialized
hobyePerson?.count // return : 1 …

我正在学习本教程,特别是在使用Swift语言转换此函数时遇到问题:

- (id)init
{
    CFRunLoopSourceContext    context = {0, self, NULL, NULL, NULL, NULL, NULL,
                                    &RunLoopSourceScheduleRoutine,
                                    RunLoopSourceCancelRoutine,
                                    RunLoopSourcePerformRoutine};

    runLoopSource = CFRunLoopSourceCreate(NULL, 0, &context);
    commands = [[NSMutableArray alloc] init];

    return self;
}

对此,在init函数中是context变量给我带来的问题.

在上面的代码中,context是一个类型的变量:CFRunLoopSourceContext在apple文档中初始化这个对象是这样的

所以,我在初始化时,使用了以下代码,专注于schedule参数:

var context = CFRunLoopSourceContext(version: 0, info: bridge(obj: self) ,
                                         retain: nil,
                                         release: nil,
                                         copyDescription: nil,
                                         equal: nil,
                                         hash: nil,
                                         schedule: RunLoopSourceScheduleRoutine,
                                         cancel: nil,
                                         perform: nil)

功能RunLoopSourceScheduleRoutine如下:

    func RunLoopSourceScheduleRoutine(info:UnsafeMutableRawPointer? ,rl:CFRunLoop? , mode:CFRunLoopMode?)  {

    let obj :  RunLoopSource …