TITLE: catching exceptions by value

PROBLEM: clgonsal@undergrad.math.uwaterloo.ca (Carl Laurence Gonsalves)

I'm having a problem with derived exceptions [...]

#include <iostream.h>

class BaseExcept {
public:
    virtual void print(){ cout << "Base\n"; }
};

class SubExcept : BaseExcept {
public:
    void print(){ cout << "Sub\n"; }
};

int main(){
    try{
	throw SubExcept();
	cout << "not caught";
    } catch( BaseExcept v ) {
	v.print();
    }
}


RESPONSE: clamage@Eng.Sun.COM (Steve Clamage), 3 Sept 95

SubExcept is privately derived from BaseExcept, so there is no
implicit conversion, and thus no available handler. Make BaseExcept
publicly-derived and it should work.

In addition, since you are catching the exception by value, the
caught exception will always be "sliced" down to the base class,
no matter what was thrown. You probably want to catch the
exception by reference.

[ I think its bad programming practice to catch exceptions by value.
  You never should assume that derived implementations will always
  throw the same exception type. -adc ]