| Thinking in C++ Vol 2 - Practical Programming |
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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:
//: C01:Autoexcp.cpp
// No matching conversions.
#include <iostream>
using namespace std;
class Except1 {};
class Except2 {
public:
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 matched
by converting an Except1 object into an Except2 using the converting
constructor, 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:
//: C01: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 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.
| Thinking in C++ Vol 2 - Practical Programming |
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