Example using the exception handling functions (C++ only)

The following example shows the flow of control and special functions used in exception handling:

#include <iostream>
#include <exception>
using namespace std;

class X { };
class Y { };
class A { };

// pfv type is pointer to function returning void
typedef void (*pfv)();

void my_terminate() {
  cout << "Call to my terminate" << endl;
  abort();
}

void my_unexpected() {
  cout << "Call to my_unexpected()" << endl;
  throw;
}

void f() throw(X,Y, bad_exception) {
  throw A();
}

void g() throw(X,Y) {
  throw A();
}

int main()
{
  pfv old_term = set_terminate(my_terminate);
  pfv old_unex = set_unexpected(my_unexpected);
  try {
    cout << "In first try block" << endl;
    f();
  }
  catch(X) {
    cout << "Caught X" << endl;
  }
  catch(Y) {
    cout << "Caught Y" << endl;
  }
  catch (bad_exception& e1) {
    cout << "Caught bad_exception" << endl;
  }
  catch (...) {
    cout << "Caught some exception" << endl;
  }

  cout << endl;

  try {
    cout << "In second try block" << endl;
    g();
  }
  catch(X) {
    cout << "Caught X" << endl;
  }
  catch(Y) {
    cout << "Caught Y" << endl;
  }
  catch (bad_exception& e2) {
    cout << "Caught bad_exception" << endl;
  }
  catch (...) {
    cout << "Caught some exception" << endl;
  }
}

The following is the output of the above example:

In first try block
Call to my_unexpected()
Caught bad_exception

In second try block
Call to my_unexpected()
Call to my terminate

At run time, this program behaves as follows:

1. The call to set_terminate() assigns to old_term the address of the function last passed to set_terminate() when set_terminate() was previously called.
2. The call to set_unexpected() assigns to old_unex the address of the function last passed to set_unexpected() when set_unexpected() was previously called.
3. Within the first try block, function f() is called. Because f() throws an unexpected exception, a call to unexpected() is made. unexpected() in turn calls my_unexpected(), which prints a message to standard output. The function my_unexpected() tries to rethrow the exception of type A. Because class A has not been specified in the exception specification of function f(), my_unexpected() throws an exception of type bad_exception.
4. Because bad_exception has been specified in the exception specification of function f(), the handler catch (bad_exception& e1) is able to handle the exception.
5. Within the second try block, function g() is called. Because g() throws an unexpected exception, a call to unexpected() is made. The unexpected() throws an exception of type bad_exception. Because bad_exception has not been specified in the exception specification of g(), unexpected() calls terminate(), which calls the function my_terminate().
6. my_terminate() displays a message then calls abort(), which terminates the program.

Note that the catch blocks following the second try block are not entered, because the exception was handled by my_unexpected() as an unexpected throw, not as a valid exception.