Bruce Eckel's Thinking in C++, 2nd Ed Contents | Prev | Next

Exception matching

When an exception is thrown, the exception-handling system looks through the “nearest” handlers in the order they are written. When it finds a match, the exception is considered handled, and no further searching occurs.

Matching an exception doesn’t require a perfect match between the exception and its handler. An object or reference to a derived-class object will match a handler for the base class. (However, if the handler is for an object rather than a reference, the exception object is “sliced” as it is passed to the handler; this does no damage but loses all the derived-type information.) If a pointer is thrown, standard pointer conversions are used to match the exception. However, no automatic type conversions are used to convert one exception type to another in the process of matching. For example, 

//: C23:Autoexcp.cpp
// No matching conversions
#include <iostream>
using namespace std;

class Except1 {};
class Except2 {
public:
  Except2(Except1&) {}
};

void f() { throw Except1(); }

int main() {
  try { f();
  } catch (Except2) {
    cout << "inside catch(Except2)" << endl;
  } catch (Except1) {
    cout << "inside catch(Except1)" << endl;
  }
} ///:~

Even though you might think the first handler could be used by converting an Except1 object into an Except2 using the constructor conversion, the system will not perform such a conversion during exception handling, and you’ll end up at the Except1 handler.

The following example shows how a base-class handler can catch a derived-class exception: 

//: C23:Basexcpt.cpp
// Exception hierarchies
#include <iostream>
using namespace std;

class X {
public:
  class Trouble {};
  class Small : public Trouble {};
  class Big : public Trouble {};
  void f() { throw Big(); }
};

int main() {
  X x;
  try {
    x.f();
  } catch(X::Trouble) {
    cout << "caught Trouble" << endl;
  // Hidden by previous handler:
  } catch(X::Small) {
    cout << "caught Small Trouble" << endl;
  } catch(X::Big) {
    cout << "caught Big Trouble" << endl;
  }
} ///:~

Here, the exception-handling mechanism will always match a Trouble object, or anything derived from Trouble, to the first handler. That means the second and third handlers are never called because the first one captures them all. It makes more sense to catch the derived types first and put the base type at the end to catch anything less specific (or a derived class introduced later in the development cycle).

In addition, if Small and Big represent larger objects than the base class Trouble (which is often true because you regularly add data members to derived classes), then those objects are sliced to fit into the first handler. Of course, in this example it isn’t important because there are no additional members in the derived classes and there are no argument identifiers in the handlers anyway. You’ll usually want to use reference arguments rather than objects in your handlers to avoid slicing off information.

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