从构造函数中捕获异常意味着以后我的实例超出范围 [英] Catching exceptions from a constructor means that my instance is out of scope afterward

问题描述

我有一个其构造函数可能会引发异常的类。以下是一些捕获异常的代码：

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;
}

当然，在try / catch之后没有代码可以看到实例，因为它现在不在范围内。解决此问题的一种方法是分别声明并定义 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);
}
...

除了我的班级没有适当的零参数构造函数。实际上，在这种情况下，只有这样一个构造函数才有意义： MyClass 对象是不可变的，因为它没有一个数据成员在构造后会更改。如果要添加零参数构造函数，则需要引入一些实例变量，例如 is_initialized _ ，然后进行所有方法检查以确保该变量为 true ，然后继续。对于这种简单的模式，这似乎太冗长了。

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？我需要吸收它并允许在初始化类的实例之前对其进行声明吗？

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

做内部 尝试块所需的一切：

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，它只在 try 块，表明已成功创建实例，因此可以使用。

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。如果您无能为力，请把它传播出去。

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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