为什么基类catch块捕获派生类对象？ [英] Why Base Class catch block catch derived class object?

问题描述

为什么 Base catch handler catch a Derived 对象，如：

Why does the Base catch handler catch a Derived object, as in:

#include <iostream>
using namespace std;
class Base {};
class Derived: public Base {};

int main()
{
   Derived d;
   try {
        throw d;
   }    
   catch(Base b)
   {
        cout << "Caught Base Exception";
   }    
   catch(...)
   {
       cout << "Default\n";
   }
   return 0;
}

我得到的输出是捕获基地异常。我期待着默认。

The output I get is "Caught Base Exception". I was expecting "Default".

推荐答案

Catch子句按照列出的顺序进行评估。当catch子句参数满足与throw表达式相对应的一些前提条件时，搜索可行的catch子句将停止：

Catch clauses are evaluated in the order in which they are listed. A search for a viable catch clause stops when a catch clause parameter satisfies some prerequisite corresponding to the throw expression:

当类型 E 由复合语句中的任何语句抛出，它与形式参数的类型匹配 T 处理程序-seq 中的每个 catch子句的c $ c>，按照catch子句列出的顺序。如果满足以下条件之一，则为例外：

When an exception of type E is thrown by any statement in compound-statement, it is matched against the types of the formal parameters T of each catch-clause in handler-seq, in the order in which the catch clauses are listed. The exception is a match if any of the following is true:

• E T 是相同的类型（忽略 T 上的顶级cv限定符）


• T 是一个左值（可能是cv-qualified）的引用 E


• T 是一个明确的公共基类 E


• [...]

• E and T are the same type (ignoring top-level cv-qualifiers on T)
• T is an lvalue-reference to (possibly cv-qualified) E
• T is an unambiguous public base class of E
• […]

code>是 Derived 的明确基础，因此选择了第一个catch块。由于一个全部处理程序（ catch（...））只能出现在catch处理程序列表的最后，它是最不可行的一个catch处理程序的候选人。

Base is an unambiguous base of Derived, so the first catch block is chosen. Since a catch-all handler (catch(...)) can only appear last in the list of catch handlers, it is the least viable candidate for a catch handler.

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