为什么基类catch块捕获派生类对象？

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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 ，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.