Swift查看变量内存地址
withUnsafePointer
不说话,先放代码
withUnsafeBufferPointer(to: a) { point in
let address = UnsafeRawPointer(point)
let addressInt = Int(bitPattern: address)
print("\(addressInt)")
}
验证copy on write
值类型的赋值会对对象进行拷贝,对于一些容器,阿果做了为了避免Array、Set复制导致的性能损失, 会copy on write的机制进行优化。
对于 Array、Dictionary、Set 类型,当它们赋值的时候不会发生拷贝,只有在修改的之后才会发生拷贝。
当我们用代码验证的时候,在尝试使用withUnsafePointer
的时候,会发现数组赋值之后,两个变量的地址会不一样,当然这也是合理,毕竟值类型赋值之后两个变量在内存上应该互为独立个体才对。
如果要验证COW,Array给我们提供了另一个方法,那就是withUnsafeBufferPointer
,用此方法查看赋值后的数组,会看到输出的地址不变。
实际上withUnsafeBufferPointer
指向的地址是数组存储数据的内存地址,withUnsafePointer
指向的。
代码举例
var array1 = [1, 2, 3]
array1.withUnsafeBufferPointer { point in
let address = point.baseAddress!
let addressInt = Int(bitPattern: address)
print("\(addressInt)")
// 105553141216992
}
withUnsafePointer(to: array1) { point in
let address = UnsafeRawPointer(point)
let addressInt = Int(bitPattern: address)
print("\(addressInt)")
// 6089438336
}
var array2 = array1 // 这里并没有真正复制数据,而是共享内存
array2.withUnsafeBufferPointer { point in
let address = point.baseAddress!
let addressInt = Int(bitPattern: address)
print("\(addressInt)")
// 105553141216992
}
withUnsafePointer(to: array2) { point in
let address = UnsafeRawPointer(point)
let addressInt = Int(bitPattern: address)
print("\(addressInt)")
// 6089438320
}
array2[2] = 1
array2.withUnsafeBufferPointer { point in
let address = point.baseAddress!
let addressInt = Int(bitPattern: address)
print("\(addressInt)")
// 105553141218080
}
withUnsafePointer(to: array2) { point in
let address = UnsafeRawPointer(point)
let addressInt = Int(bitPattern: address)
print("\(addressInt)")
// 6089438280
}
本文来自博客园,作者:MrYu4,转载请注明原文链接:https://www.cnblogs.com/MrYU4/p/18432261/swift-cha-kan-de-zhi