感受一下字符串相关的源文件个数



String 概览

• 是一个结构体
• 只有一个变量，类型是 _StringGuts

 如上所示，String 真正的内容在__StringStorage或者__SharedStringStorage里面。

private static func create(    realCodeUnitCapacity: Int, countAndFlags: CountAndFlags  ) -> __StringStorage {    let storage = Builtin.allocWithTailElems_2(      __StringStorage.self,      realCodeUnitCapacity._builtinWordValue, UInt8.self,      1._builtinWordValue, Optional<_StringBreadcrumbs>.self)#if arch(i386) || arch(arm)    storage._realCapacity = realCodeUnitCapacity    storage._count = countAndFlags.count    storage._flags = countAndFlags.flags#else    storage._realCapacityAndFlags =      UInt64(truncatingIfNeeded: realCodeUnitCapacity)    storage._countAndFlags = countAndFlags#endif    storage._breadcrumbsAddress.initialize(to: nil)    storage.terminator.pointee = 0 // nul-terminated    // NOTE: We can't _invariantCheck() now, because code units have not been    // initialized. But, _StringGuts's initializer will.    return storage  }

这里是真正分配内存的地方。

标记位

String 里有若干标记位，表示不同类型，一共有4位，被称为 discriminator。

On 64-bit platforms, the discriminator is the most significant 4 bits of the bridge object.



字符串粗略可以分为Small strings 和 Large strings

几乎所有的字符串操作，都根据是否是Small string来做了区分，比如判断是否是ACSCII

//  // Whether the string is all ASCII  //  @inlinable  internal var isASCII: Bool {    @inline(__always) get {      if isSmall { return smallIsASCII }      return _countAndFlags.isASCII    }  }

在_StringObject里获取并判断标记位

获取标记位

internal var discriminatedObjectRawBits: UInt64 {    return Builtin.reinterpretCast(_object)  }

即bridge object的最高位。

判断是否可变

@inlinable  internal var isImmortal: Bool {    @inline(__always) get {      return (discriminatedObjectRawBits & 0x8000_0000_0000_0000) != 0    }  }

判断是否是 small string

internal var isSmall: Bool {    @inline(__always) get {      return (discriminatedObjectRawBits & 0x2000_0000_0000_0000) != 0    }  }

判断是否提供了连续的UTF8 code point

// Whether this string can provide access to contiguous UTF-8 code units:  //   - Small strings can by spilling to the stack  //   - Large native strings can through an offset  //   - Shared strings can:  //     - Cocoa strings which respond to e.g. CFStringGetCStringPtr()  //     - Non-Cocoa shared strings  @inlinable  internal var providesFastUTF8: Bool {    @inline(__always) get {      return (discriminatedObjectRawBits & 0x1000_0000_0000_0000) == 0    }  }