c++ - 如果抛出的值匹配多个 catch 子句会发生什么？

Question

从字面上看，如果抛出的一个值与多个 catch 子句匹配会发生什么？

编译器决定还是出错？

下面是一个抛出一个值并匹配三个 catch 子句的示例。

我编译它并得到错误。但是不知道那个错误是什么意思...

    #include <iostream>
    using namespace std;

    class AAA
    {
    public:
        void ShowYou() { cout<<"AAA exception!"<<endl; }
    };

    class BBB : public AAA
    {
    public:
        void ShowYou() { cout<<"BBB exception!"<<endl; }
    };

    class CCC : public BBB
    {
    public:
        void ShowYou() { cout<<"CCC exception!"<<endl; }
    };

    void ExceptionGenerator(int expn)
    {
        if(expn==1)
            throw AAA();
        else if(expn==2)
            throw BBB();
        else
            throw CCC();
    }

    int main(void)
    {
        try
        {
            ExceptionGenerator(1);
            ExceptionGenerator(1);
            ExceptionGenerator(1);
        }

        catch(AAA& expn)
        {
            cout<<"catch(AAA& expn)"<<endl;
            expn.ShowYou();
        }   

        catch(AAA& expn)
        {
            cout<<"catch(BBB& expn)"<<endl;
            expn.ShowYou();
        }

        catch(AAA& expn)
        {
              cout<<"catch(CCC& expn)"<<endl;
              expn.ShowYou();
        }
      system("pause"); return 0;
}

错误 1 ​​error C2312: 'AAA &' : is caught by 'AAA &' on line 40

我得到上面的错误。

这是什么意思？

Answer 1

来自 n3337 标准草案，15.3/4

4 The handlers for a try block are tried in order of appearance. That makes it possible to write handlers that can never be executed, for example by placing a handler for a derived class after a handler for a corresponding base class.

这意味着第二个和第三个catch block 基本上是无法访问的代码。不过，您的程序格式正确，符合规范的编译器不应拒绝它(例如，我的程序只会发出警告)。

当你有一个异常层次结构时，让它们表现出多态性以区分它们:

#include <iostream>
using namespace std;

class AAA {
public:
    virtual void ShowYou() { cout<<"AAA exception!"<<endl; }
};

class BBB : public AAA {
public:
    void ShowYou() { cout<<"BBB exception!"<<endl; }
};

class CCC : public BBB {
public:
    void ShowYou() { cout<<"CCC exception!"<<endl; }
};

void ExceptionGenerator()
{
    int expn = 0;
    cin >> expn;
    if(expn==1)
        throw AAA();
    else if(expn==2)
        throw BBB();
    else
        throw CCC();
}

int main(void)
{
    try
    {
        ExceptionGenerator();
    }
    catch(AAA& expn)
    {
        expn.ShowYou();
    }

    system("pause"); return 0;
}