C++ Exception Handling: The wrong catch statement catches an exception! Why?

[KevinHall]

Q: The wrong catch statement catches an exception in the following code! Why?

#include <iostream>

using namespace std;

class A
{
};

class B : public A
{
};

int main()
{
    B b;

    try
    {
        throw b;
    }
    catch (A& ea)
    {
        cout << "Caught an instance of A" << endl;
    }
    catch (B& eb)
    {
        cout << "Caught an instance of B" << endl;
    }
    return 0;
}

A: No, the code works the way it is supposed to. When a thrown class encounters a parent class in an a catch statement, the class will be recast and copied -- you will not be able to recast a caught pointer back to the original type!

Catching happens on a "first come, first serve" basis. The first catch statement that can match will. In order to get the above code to work as expected, the catch statement for the B class needs to occur before the catch statement for the A class:

#include <iostream>

using namespace std;

class A
{
};

class B : public A
{
};

int main()
{
    B b;

    try
    {
        throw b;
    }
    catch (B& eb)
    {
        cout << "Caught an instance of B" << endl;
    }
    catch (A& ea)
    {
        cout << "Caught an instance of A" << endl;
    }
    return 0;
}