Exceptions

In C every call had to be checked for error values - which could double the code size. C++ exceptions are an alternative to traditional techniques when they are insufficient, inelegant, and error-prone. Three keywords are involved

try: - specifies an area of code where exceptions will operate
catch: - deals with the exceptional situation produced in the previous try clause.
throw: - causes an exceptional situation

When an exception is 'thrown' it will be 'caught' by the local ``catch'' clause if one exists, otherwise it will be passed up through the call hierarchy until a suitable catch clause is found. The default response to an exception is to terminate.

There are many types of exception - they form a class hierarchy of their own. Here just a few will be introduced.

try {
// code
}
// catch a standard exception
catch(std:exception& e){
 // order of the catch clauses matters; they're tried 
 // in the given order
}
// catch all the other types
catch(...) {
// cleanup
 throw;   // throw the exception up the hierarchy
}

Here's an example that sooner or later will produce an exception.

try{
  for(;;)
    new char[10000];
}
catch(bad_alloc) {
  cerr << "No memory left";
}

An exception example is online.

You can override the routine that handles problems encountered by the new operator

set_new_handler(&my_new_handler);

For safety, you can declare which exceptions a function should be able to throw

int f() throw (std::bad_alloc)
// f may only throw bad_alloc

A useful exception is out_of_range (header <stdexcept>), which is thrown by at() and by bitset<>::operator[]().

Exception handling code can slow a program down even when no exceptions happen. The code size will also increase.

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Exceptions