Swift struct 相当于 C++ 可变关键字？

Question

Swift 中是否有 C++ mutable 关键字的等效项？我有一个实现包含非变异函数的协议(protocol)的结构，但是内部实现确实会改变内部状态。有没有办法让类的成员变量通过这个函数可变，而不改变协议(protocol)定义，使函数变异？

例如

public protocol P {
    // f is semantically unmutating
    func f
}

public struct S : P {
    // f is semantically unmutating, but does change some internal state
    public func f {
        v += 1
    }
    private var v: Int // How to mark this as mutable for internal purposes?
    ...
}

Answer 1

与 C++ 中的相同

public struct S : P {
    // f is semantically unmutating, but does change some internal state
    public func f {
       increment()
    }
    private mutating func increment() {
       v += 1
    }
    private var v: Int // How to mark this as mutable for internal purposes?
    ...
}