通过引用捕获异常 [英] Catching exceptions by reference

问题描述

在此C ++教程中，在标准例外一节中，有一个示例代码使用从STL中的标准异常类派生的类：

In this C++ tutorial, in the section titled "Standard exceptions", there is this example code which uses a class derived from the standard exception class in the STL:

// standard exceptions
#include <iostream>
#include <exception>
using namespace std;

class myexception: public exception
{
  virtual const char* what() const throw()
  {
    return "My exception happened";
  }
} myex; //Declares an instance of myexception outside of the main function

int main () {
  try
  {
    throw myex;
  }
  catch (exception& e) //My question is regarding this line of code
  {
    cout << e.what() << endl;
  }
  return 0;
}

代码打印出来 c $ c>。但是，如果我删除&符号，它会输出 std :: exception ，这是当你调用 what（）

That code prints out My exception happened. However, if I remove the ampersand, it prints out std::exception, which is what happens when you call what() with the standard exception class, not the derived class.

网站给出了这个解释：

我们放置了一个处理程序，通过引用
捕获异常对象（注意&之后的类型），因此这也捕获了从异常派生的
类，像myex对象类
myexception。

We have placed a handler that catches exception objects by reference (notice the ampersand & after the type), therefore this catches also classes derived from exception, like our myex object of class myexception.

抛出 myex 调用 catch 函数，并将 myex 作为参数传递。因为在这种情况下，我想象无论你是否通过值或引用抛出异常（这是和号做的正确吗？），因为你仍然抛出一个 myexception 而不是异常。由于动态绑定和多态性等， e.what（）应该仍然打印出我的异常发生而不是 std :: exception 。

Is throwing myex kind of like "calling a the catch function, and passing myex as a parameter"? Because it that case, I would imagine that it doesn't matter whether you throw the exception by value or by reference (that's what the ampersand does right?), because you are still throwing a myexception rather than an exception. And due to dynamic binding and polymorphism or something like that, e.what() should still print out My exception happened rather than std::exception.

推荐答案

对象切片。

如果你将一个派生类的对象赋给一个基类的一个实例（就像使用拷贝c'tor传递值一样） ）所有派生类的信息将丢失（切片）。

If you assign an object of a derived class to an instance of a base class (as in the case of using a copy c'tor when passing by value) all the information of the derived class will be lost (sliced).

例如，

class Base {
   int baseInfo;
};

class Derived : public Base
{
   int someInfo;
};

然后，如果你这样写：

Derived myDerivedInstance;

Base baseInstance = myDerivedInstance;

然后， myDerivedInstance 中的someInfo在 baseInstance 中切片。

Then the "someInfo" in myDerivedInstance is sliced away in baseInstance.

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