Exception matching

When an exception is thrown, the exception-handling system looks through the ?nearest? handlers in the order they appear in the source code. When it finds a match, the exception is considered handled and no further searching occurs.

Matching an exception doesn?t require a perfect correlation 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?? truncated to the base type ? as it is passed to the handler; this does no damage but loses all the derived-type information.) For this reason, as well as to avoid making yet another copy of the exception object, it is always better to catch an exception by reference instead of by value . If a pointer is thrown, the usual standard pointer conversions are used to match the exception. However, no automatic type conversions are used to convert from one exception type to another in the process of matching, for example:

// No matching conversions
#include <iostream>
using namespace std;
class Except1 {};
class Except2 {
Except2(const 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:

// Exception hierarchies
#include <iostream>
using namespace std;
class X {
class Trouble {};
class Small : public Trouble {};
class Big : public Trouble {};
void f() { throw Big(); }
int main() {
X x;
try {
} 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 that is a Trouble (through public inheritance ), 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.
Notice that these examples catch exceptions by reference, although for these classes 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 value arguments in your handlers to avoid slicing off information.

Re-throwing an exception

You usually want to re-throw an exception when you have some resource that needs to be released, such as a network connection or heap memory that needs to be deallocated. (See the section ?Resource Management? later in this chapter for more detail). If an exception occurs, you don?t necessarily care what error caused the exception?you just want to close the connection you opened previously. After that, you?ll want to let some other context closer to the user (that is, higher up in the call chain) handle the exception. In this case the ellipsis specification is just what you want. You want to catch any exception, clean up your resource, and then re-throw the exception so that it can be handled elsewhere. You re-throw an exception by using throw with no argument inside a handler:

catch(...) {
cout << "an exception was thrown" << endl;
// Deallocate your resource here, and then re-throw?

Any further catch clauses for the same try block are still ignored?the throw causes the exception to go to the exception handlers in the next-higher context. In addition, everything about the exception object is preserved, so the handler at the higher context that catches the specific exception type can extract any information the object may contain.