Swift: 初始化抛出时如何释放属性？

Question

此示例代码存在内存泄漏。

pointer1 和 pointer2 在 Person 成功初始化之前分配。如果 init 函数抛出错误。 deinit 函数永远不会被执行。所以 pointer1 和 pointer2 将永远不会被释放。

import XCTest

class Person {

    // case1
    let pointer1: UnsafeMutablePointer<Int> = UnsafeMutablePointer<Int>.allocate(capacity: 1)

    // case2
    let pointer2: UnsafeMutablePointer<Int>

    let name: String

    init(name: String) throws {

        // case2
        self.pointer2 = UnsafeMutablePointer<Int>.allocate(capacity: 1)


        if name == "UnsupportName" {
            throw NSError()
        }
        self.name = name
    }

    deinit {
        pointer1.deallocate()
        pointer2.deallocate()
    }
}

class InterestTests: XCTestCase {

    func testExample() {
        while true {
            _ = try? Person(name: "UnsupportName")
        }
    }

}

有时候逻辑很复杂。在我的真实案例中。有很多allocate和throws与if和guard。有些很难控制。

有什么方法可以避免这种内存泄漏吗？

这是一个类似的问题：https://forums.swift.org/t/deinit-and-failable-initializers/1199

Answer 1

在您的具体示例中，解决方案很简单。在解决所有可能的故障之前不要分配任何内存：

class Person {

    let aPointer: UnsafeMutablePointer<Int> // Do not allocate here.
    let name: String

    init(name: String) throws {
        // Validate everything here
        guard name != "UnsupportName" else {
            throw NSError()
        }

        // After this point, no more throwing:

        self.name = name
        // Move the allocation here
        self.aPointer = UnsafeMutablePointer.allocate(capacity: 1)
    }

    deinit {
        aPointer.deallocate()
    }
}

但更通用的解决方案是像使用其他地方一样使用 do/catch 来管理错误：

class Person {

    let aPointer = UnsafeMutablePointer<Int>.allocate(capacity: 1)
    let name: String

    init(name: String) throws {
        do {
            if name == "UnsupportName" {
                throw NSError()
            }

            self.name = name
        } catch let e {
            self.aPointer.deallocate()
            throw e
        }

    }

    deinit {
        aPointer.deallocate()
    }
}

我很想将 .allocate 移到 init 中，只是为了让它更清楚地显示正在发生的事情。关键是你应该首先分配所有内存，然后再抛出任何东西（这样你就知道你可以全部释放），或者在最后一次抛出之后分配所有内存（这样你就知道你没有任何东西可以释放）。

查看您添加的解决方案，没问题，但建议围绕它的危险逻辑。最好将其展开以将分配放入他们自己的对象中（这几乎肯定也会摆脱 UnsafeMutablePointers；在一个 class 中需要很多这样的对象是非常可疑的）。

也就是说，IMO 有更简洁的方法来沿着这条路径构建错误处理。

extension UnsafeMutablePointer {
    static func allocate(capacity: Int, withCleanup cleanup: inout [() -> Void]) -> UnsafeMutablePointer<Pointee> {
        let result = allocate(capacity: capacity)
        result.addTo(cleanup: &cleanup)
        return result
    }

    func addTo(cleanup: inout [() -> Void]) {
        cleanup.append { self.deallocate() }
    }
}

这让 UnsafeMutablePointers 可以将清理信息附加到一个数组中，而不是创建大量 defer 块，这会增加在清理期间丢失一个块的风险。

这样，您的 init 看起来像：

init(name: String) throws {
    var errorCleanup: [() -> Void] = []
    defer { for cleanup in errorCleanup { cleanup() } }

    // deallocate helper for case1
    pointer1.addTo(cleanup: &errorCleanup)

    // case2
    self.pointer2 = UnsafeMutablePointer<Int>.allocate(capacity: 1, withCleanup: &errorCleanup)

    // case ...


    if name == "UnsupportName" {
        throw NSError()
    }
    self.name = name

    // In the end. set deallocate helpers to nil
    errorCleanup.removeAll()
}

当然，这会带来调用 allocate(capacity:) 而不是 allocate(capacity:withCleanup:) 的危险。所以你可以通过将它包装成另一种类型来解决这个问题；自动释放自身的引用类型。

class SharedPointer<Pointee> {
    let ptr: UnsafeMutablePointer<Pointee>
    static func allocate(capacity: Int) -> SharedPointer {
        return .init(pointer: UnsafeMutablePointer.allocate(capacity: capacity))
    }
    init(pointer: UnsafeMutablePointer<Pointee>) {
        self.ptr = pointer
    }
    deinit {
        ptr.deallocate()
    }
}

有了这个，这就变成了（不需要 deinit）：

class Person {

    // case1
    let pointer1 = SharedPointer<Int>.allocate(capacity: 1)

    // case2
    let pointer2: SharedPointer<Int>

    let name: String

    init(name: String) throws {

        // case2
        self.pointer2 = SharedPointer<Int>.allocate(capacity: 1)

        if name == "UnsupportName" {
            throw NSError()
        }
        self.name = name
    }
}

您可能想要编写各种帮助程序来处理 .ptr。

当然，这可能会导致您构建特定版本的 SharedPointer 来处理各种事情（例如 "father" 而不是 "int"）。如果你继续沿着这条路走下去，你会发现 UnsafeMutablePointers 蒸发了，问题就消失了。但您不必走那么远，SharedPointer 会为您完成这项工作。

Answer 2

我找到了解决问题的方法。

import XCTest

class Person {

    // case1
    let pointer1: UnsafeMutablePointer<Int> = UnsafeMutablePointer<Int>.allocate(capacity: 1)

    // case2
    let pointer2: UnsafeMutablePointer<Int>

    let name: String

    init(name: String) throws {

        // deallocate helper for case1
        var deallocateHelper1: UnsafeMutablePointer<Int>? = self.pointer1
        defer {
            deallocateHelper1?.deallocate()
        }

        // case2
        self.pointer2 = UnsafeMutablePointer<Int>.allocate(capacity: 1)
        var deallocateHelper2: UnsafeMutablePointer<Int>? = self.pointer2
        defer {
            deallocateHelper2?.deallocate()
        }

        // case ... 


        if name == "UnsupportName" {
            throw NSError()
        }
        self.name = name

        // In the end. set deallocate helpers to nil
        deallocateHelper1 = nil
        deallocateHelper2 = nil
    }

    deinit {
        pointer1.deallocate()
        pointer2.deallocate()
    }
}

class InterestTests: XCTestCase {

    func testExample() {
        while true {
            _ = try? Person(name: "UnsupportName")
        }
    }

}

Answer 3

另一个解决方案。

class Person {
    let name: String
    let pointer1: UnsafeMutablePointer<Int>
    let pointer2: UnsafeMutablePointer<Int>

    init(name: String) throws {
        var pointers: [UnsafeMutablePointer<Int>] = []
        do {
            let pointer1 = UnsafeMutablePointer<Int>.allocate(capacity: 1)
            pointers.append(pointer1)
            let pointer2 = UnsafeMutablePointer<Int>.allocate(capacity: 1)
            pointers.append(pointer2)
            if name == "Unsupported Name" {
                throw NSError()
            }
            self.pointer1 = pointer1
            self.pointer2 = pointer2
            self.name = name
        } catch {
            pointers.forEach { [=10=].deallocate() }
            throw error
        }
    }

    deinit {
        pointer1.deallocate()
        pointer2.deallocate()
    }
}

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