从构造函数捕获异常意味着我的实例之后超出范围 [英] Catching exceptions from a constructor means that my instance is out of scope afterward

问题描述

我有一个类，它的构造函数可能会抛出异常.下面是一些可以捕获异常的代码:

I have a class whose constructor may throw an exception. Here’s some code that will catch the exception:

try {
    MyClass instance(3, 4, 5);
}
catch (MyClassException& ex) {
    cerr << "There was an error creating the MyClass." << endl;
    return 1;
}

但是当然在 try/catch 之后没有代码可以看到 instance 因为它现在超出了范围.解决此问题的一种方法是分别声明和定义 instance:

But of course no code after the try/catch can see instance because it’s now out of scope. One way to resolve this would be to declare and define instance separately:

MyClass instance;
try {
    MyClass instance(3, 4, 5);
}
...

除了我的类没有合适的零参数构造函数.事实上，这里的这种情况是唯一一个这样的构造函数甚至有意义的情况:MyClass 对象旨在是不可变的，从某种意义上说，它的数据成员在构造后都不会改变.如果我要添加一个零参数构造函数，我需要引入一些实例变量，例如 is_initialized_ 然后检查每个方法以确保该变量之前是 true进行.对于这样一个简单的模式来说，这似乎太过冗长了.

except that my class doesn’t have the appropriate zero-argument constructor. In fact, this case right here is the only one in which such a constructor would even make sense: the MyClass object is intended to be immutable, in the sense that none of its data members change after construction. If I were to add a zero-argument constructor I’d need to introduce some instance variable like is_initialized_ and then have every method check to make sure that that variable is true before proceeding. That seems like far too much verbosity for such a simple pattern.

处理这种事情的惯用方式是什么?我是否需要接受它并允许在初始化之前声明我的类的实例?

What is the idiomatic way to deal with this kind of thing? Do I need to suck it up and allow instances of my class to be declared before they’re initialized?

推荐答案

您应该在 try 块中内做您需要做的一切:

You should be doing everything you need to do inside the try block:

try {
    MyClass instance(3, 4, 5);

    // Use instance here
}
catch (MyClassException& ex) {
    cerr << "There was an error creating the MyClass." << endl;
    return 1;
}

毕竟只有在try块内，instance才被成功创建，可以使用.

After all, it is only within the try block that instance has been successfully created and so can be used.

我确实想知道您的 catch 块是否真的在处理异常.如果你不能做任何事情来解决这种情况，你应该让它传播.

I do wonder whether your catch block is really handling the exception. If you can't do anything to resolve the situation, you should be letting it propagate.

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