构造函数何时抛出异常是正确的?(或者在Objective C的情况下:什么情况下init ` er才应该返回nil?)

在我看来,如果对象不完整,构造函数应该失败——因此拒绝创建对象。也就是说,构造函数应该与它的调用者有一个合同,以提供一个函数和工作对象,在哪些方法可以被有意义地调用?这合理吗?


当前回答

OP的问题有一个“语言不可知论”的标签…对于所有语言/情况,这个问题不能以同样的方式安全地回答。

下面的c#示例的类层次结构抛出了类B的构造函数,跳过了对类A的IDisposeable的立即调用。在main的使用退出时进行处置,跳过类A资源的显式处置。

例如,如果类A在构造时创建了一个套接字,连接到一个网络资源,在使用块之后可能仍然是这种情况(一个相对隐藏的异常)。

class A : IDisposable
{
    public A()
    {
        Console.WriteLine("Initialize A's resources.");
    }

    public void Dispose()
    {
        Console.WriteLine("Dispose A's resources.");
    }
}

class B : A, IDisposable
{
    public B()
    {
        Console.WriteLine("Initialize B's resources.");
        throw new Exception("B construction failure: B can cleanup anything before throwing so this is not a worry.");
    }

    public new void Dispose()
    {
        Console.WriteLine("Dispose B's resources.");
        base.Dispose();
    }
}
class C : B, IDisposable
{
    public C()
    {
        Console.WriteLine("Initialize C's resources. Not called because B throws during construction. C's resources not a worry.");
    }

    public new void Dispose()
    {
        Console.WriteLine("Dispose C's resources.");
        base.Dispose();
    }
}


class Program
{
    static void Main(string[] args)
    {
        try
        {
            using (C c = new C())
            {
            }
        }
        catch
        {           
        }

        // Resource's allocated by c's "A" not explicitly disposed.
    }
}

其他回答

Eric Lippert说有四种例外。

Fatal exceptions are not your fault, you cannot prevent them, and you cannot sensibly clean up from them. Boneheaded exceptions are your own darn fault, you could have prevented them and therefore they are bugs in your code. Vexing exceptions are the result of unfortunate design decisions. Vexing exceptions are thrown in a completely non-exceptional circumstance, and therefore must be caught and handled all the time. And finally, exogenous exceptions appear to be somewhat like vexing exceptions except that they are not the result of unfortunate design choices. Rather, they are the result of untidy external realities impinging upon your beautiful, crisp program logic.

构造函数本身不应该抛出致命异常,但它执行的代码可能会导致致命异常。像“内存不足”这样的事情不是您可以控制的,但是如果它发生在构造函数中,嘿,它就发生了。

愚蠢的异常永远不应该出现在任何代码中,所以它们应该被清除。

构造函数不应该抛出恼人的异常(例如Int32.Parse()),因为它们没有非异常情况。

最后,应该避免外生异常,但如果在构造函数中执行的某些操作依赖于外部环境(如网络或文件系统),则抛出异常是合适的。

参考链接:https://blogs.msdn.microsoft.com/ericlippert/2008/09/10/vexing-exceptions/

我不能说明Objective-C中的最佳实践,但在c++中,构造函数抛出异常是可以的。特别是没有其他方法可以确保在不调用isOK()方法的情况下报告构造过程中遇到的异常情况。

函数try块特性是专门为支持构造函数成员初始化失败而设计的(尽管它也可以用于常规函数)。这是修改或丰富将要抛出的异常信息的唯一方法。但由于其最初的设计目的(在构造函数中使用),它不允许空catch()子句包含异常。

如果无法在构造函数中初始化对象,则抛出异常,一个例子是非法参数。

根据一般经验,应该尽可能快地抛出异常,因为当问题的根源接近发出错误信号的方法时,这会使调试变得更容易。

如果你正在编写ui控件(ASPX, WinForms, WPF,…),你应该避免在构造函数中抛出异常,因为设计器(Visual Studio)在创建控件时无法处理它们。了解你的控件生命周期(控件事件),尽可能使用惰性初始化。

There is generally nothing to be gained by divorcing object initialization from construction. RAII is correct, a successful call to the constructor should either result in a fully initialized live object or it should fail, and ALL failures at any point in any code path should always throw an exception. You gain nothing by use of a separate init() method except additional complexity at some level. The ctor contract should be either it returns a functional valid object or it cleans up after itself and throws.

考虑一下,如果实现了单独的init方法,仍然必须调用它。它仍然有可能抛出异常,它们仍然必须被处理,而且它们实际上总是必须在构造函数之后立即被调用,除了现在你有4种可能的对象状态而不是2种(IE,已构造,初始化,未初始化,失败vs只是有效和不存在)。

In any case I've run across in 25 years of OO development cases where it seems like a separate init method would 'solve some problem' are design flaws. If you don't need an object NOW then you shouldn't be constructing it now, and if you do need it now then you need it initialized. KISS should always be the principle followed, along with the simple concept that the behavior, state, and API of any interface should reflect WHAT the object does, not HOW it does it, client code should not even be aware that the object has any kind of internal state that requires initialization, thus the init after pattern violates this principle.