Swift4.0 strtoul、strtod、strtof、strtol等字符串函数的改变
在新的iOS 11的SDK中，这几个函数参数类型发生了改变
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| public func strtod(_: UnsafePointer<Int8>!, _: UnsafeMutablePointer<UnsafeMutablePointer<Int8>?>!) -> Double
public func strtof(_: UnsafePointer<Int8>!, _: UnsafeMutablePointer<UnsafeMutablePointer<Int8>?>!) -> Float
public func strtol(_ __str: UnsafePointer<Int8>!, _ __endptr: UnsafeMutablePointer<UnsafeMutablePointer<Int8>?>!, _ __base: Int32) -> Int
public func strtoll(_ __str: UnsafePointer<Int8>!, _ __endptr: UnsafeMutablePointer<UnsafeMutablePointer<Int8>?>!, _ __base: Int32) -> Int64
/* !__DARWIN_NO_LONG_LONG */
public func strtoul(_ __str: UnsafePointer<Int8>!, _ __endptr: UnsafeMutablePointer<UnsafeMutablePointer<Int8>?>!, _ __base: Int32) -> UInt
public func strtoull(_ __str: UnsafePointer<Int8>!, _ __endptr: UnsafeMutablePointer<UnsafeMutablePointer<Int8>?>!, _ __base: Int32) -> UInt64
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从原来可以直接传的char*变成了一个UnsafePointer。那么我们用到函数的地方也要改变一下，具体代码如下：
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| let s = hex[hex.characters.index(hex.startIndex, offsetBy: start)..<hex.characters.index(hex.startIndex, offsetBy: end)]
return CGFloat(s.withCString{strtoul($0, nil, 16)})
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这里的s是一个String类型，他有一个withCString方法。利用这个方法可以将字符串中的字符转换成UnsafePointer类型。
