常量的按位或 [英] Bitwise OR of constants

问题描述

在阅读一些文件<一个href=\"http://developer.apple.com/mac/library/documentation/cocoa/reference/foundation/Classes/NSCalendar_Class/Reference/NSCalendar.html#//apple_ref/occ/instm/NSCalendar/components%3afromDate%3a\"相对=nofollow>这里，我碰到这个就来了：

While reading some documentation here, I came across this:

unsigned unitFlags = NSYearCalendarUnit | NSMonthCalendarUnit |  NSDayCalendarUnit;

我不知道如何工作的。我就在C位运算符读了，但我不明白你如何适应三个（或更多！）使用一个INT内部常量，后来能以某种方式提取它们从INT回来？进一步挖掘的文件下来，我也发现了这一点，这可能与

I have no idea how this works. I read up on the bitwise operators in C, but I do not understand how you can fit three (or more!) constants inside one int and later being able to somehow extract them back from the int? Digging further down the documentation, I also found this, which is probably related:

typedef enum {
   kCFCalendarUnitEra = (1 << 1),
   kCFCalendarUnitYear = (1 << 2),
   kCFCalendarUnitMonth = (1 << 3),
   kCFCalendarUnitDay = (1 << 4),
   kCFCalendarUnitHour = (1 << 5),
   kCFCalendarUnitMinute = (1 << 6),
   kCFCalendarUnitSecond = (1 << 7),
   kCFCalendarUnitWeek = (1 << 8),
   kCFCalendarUnitWeekday = (1 << 9),
   kCFCalendarUnitWeekdayOrdinal = (1 << 10),
} CFCalendarUnit;

如何在（1 <<; 3;）语句/变量的工作？我很抱歉，如果这是小事，但可能有人请解释或者或者发布一个链接到一个很好的解释？

How do the (1 << 3) statements / variables work? I'm sorry if this is trivial, but could someone please enlighten me by either explaining or maybe posting a link to a good explanation?

推荐答案

基本上，常量重新presented只是一位，所以如果你有一个32位整数，你可以在上面安装32常数。您的常量必须是两个大国，所以他们只有一个设置位重新present。

Basically, the constants are represented just by one bit, so if you have a 32 bit integer, you can fit 32 constants in it. Your constants have to be powers of two, so they take only one "set" bit to represent.

例如：

#define CONSTANT_1 0x01 // 0001 in binary
#define CONSTANT_2 0x02 // 0010 in binary
#define CONSTANT_3 0x04 // 0100 in binary

那么你可以做

int options = CONSTANT_1 | CONSTANT_3; // will have 0101 in binary.

正如你可以看到，每一位再presents特定不变。所以，你可以二进制和在code和测试的presence每个常量，如：

As you can see, each bit represents that particular constant. So you can binary AND in your code and test for the presence of each constant, like:

if (options & CONSTANT_3)
{
    // CONSTANT_3 is set
}

我建议你到约二元运算阅读（他们的工作就像逻辑运算符，但在位水平），如果你神交这个东西，它会使你一个程序员的好一点。

I recommend you to read about binary operations (they work like LOGICAL operators, but at the bit level), if you grok this stuff, it will make you a bit better of a programmer.

干杯。

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