苹果文档中的Swift代码编译报错：无法将不可变值作为inout参数传递——‘pointee’是只读属性

Let's break down the problem first: The error pops up because you're using withUnsafePointer(to: &ss) which gives you an immutable pointer (UnsafePointer<sockaddr_storage>). When you rebound this to AddrType, you still end up with an immutable UnsafePointer<AddrType>—its pointee property is read-only, so you can't pass &$0.pointee as an inout argument to your body closure (which expects a mutable reference).

The Fix: Use a Mutable Pointer #

To fix this, you need to work with a mutable pointer to your sockaddr_storage variable. Here's the corrected version of your extension:

import Foundation
import Darwin

extension sockaddr_storage {
    //...
    static func fromSockAddr<AddrType, ReturnType>(_ body: (_ sax: inout AddrType) throws -> ReturnType) rethrows -> (ReturnType, sockaddr_storage) {
        precondition(MemoryLayout<AddrType>.size <= MemoryLayout<sockaddr_storage>.size)
        var ss = sockaddr_storage()
        let result = try withUnsafeMutablePointer(to: &ss) { // Switch to mutable pointer
            try $0.withMemoryRebound(to: AddrType.self, capacity: 1) { mutableAddrPtr in
                try body(&mutableAddrPtr.pointee) // Now valid: mutable pointer's pointee can be passed as inout
            }
        }
        return (result, ss)
    }
}

Key Changes Explained: #

• Replaced withUnsafePointer(to: &ss) with withUnsafeMutablePointer(to: &ss): This gives us an UnsafeMutablePointer<sockaddr_storage>, which lets us modify the underlying memory of our ss variable.
• After rebounding, we get an UnsafeMutablePointer<AddrType>. The pointee of a mutable pointer can be referenced with & to create a valid inout argument, which matches exactly what your body closure expects.

This approach stays safe because we fully own the sockaddr_storage variable (var ss) throughout the scope, so modifying its memory via the mutable pointer complies with Swift's memory safety rules.

内容的提问来源于stack exchange，提问作者shallowThought