C++ Exception handling (Error handling)

Exception handling explained in this section is one of the recent incorporations to the ANSI-C++ standard. If you use a C++ compiler that is not adapted to this standard it is possible that you cannot use this feature.

During the development of a program, it may be some cases where we do not have the certainty that a piece of the code is going to work right, either because it accesses to resources that do not exist or because it gets out of an expected range, etc...

This type of anomalous situations are included in what we consider exceptions and C++ has recently incorporated three new operators to help us to handle these situations: try, throw and catch.

Its form of use is the following one: 

try {
  // code to be tried
            throw exception;
}
catch (type  exception)
{
  // code to be executed in case of exception
}
And its operation:
- The code within the try block is executed normally. In case that an exception takes place, this code must use throw keyword and a parameter to throw an exception. The type of the parameter details the exception and can be of any valid type.
- If an exception has taken place, that is to say, if it has been executed a throw instruction within the try block, the catch block is executed receiving as parameter the exception passed by throw.

For example: 

// exceptions
                                #include <iostream.h>
                                int main () {
                                  char myarray[10];
                                  try
                                  {
                                    for (int n=0; n<=10; n++)
                                    {
                                      if (n>9) throw "Out of range";
                                      myarray[n]='z';
                                    }
                                  }
                                  catch (char * str)
                                  {
                                    cout << "Exception: " << str << endl;
                                  }
                                  return 0;
                                }
Exception: Out of range

In this example, if within the n loop, n gets to be more than 9 an exception is thrown, since myarray[n] would in that case point to a non-trustworthy memory address. When throw is executed, the try block finalizes right away and every object created within the try block is destroyed. After that the control is passed to the corresponding catch block (that is only executed in these cases). Finally the program continues right after the catch block, in this case: return 0; .

The syntax used by throw is similar to the one of return : Only one parameter that is not needed to be enclosed between parenthesis.

The catch block must go right after the try block without including any code line among them. The parameter that catch accepts can be of any valid type. Even more, catch can be overloaded so that it can accept different types as parameters. In that case the catch block executed is the one that matches with the type of the exception sent (the parameter of throw): 

// exceptions: multiple catch blocks
                                #include <iostream.h>
                                int main () {
                                  try
                                  {
                                    char * mystring;
                                    mystring = new char [10];
                                    if (mystring == NULL) throw "Allocation failure";
                                    for (int n=0; n<=100; n++)
                                    {
                                      if (n>9) throw n;
                                      mystring[n]='z';
                                    }
                                  }
                                  catch (int i)
                                  {
                                    cout << "Exception: ";
                                    cout << "index " << i << " is out of range" << endl;
                                  }
                                  catch (char * str)
                                  {
                                    cout << "Exception: " << str << endl;
                                  }
                                  return 0;
                                   }
Exception: index 10 is out of range

In this case there is a possibility that happen, at least, two different exceptions:

  1. That the required block of 10 characters cannot be assigned (something rare, but possible): in this case an exception is thrown that will be catched by catch (to char * str).
  2. That the maximum index for mystring is exceeded: in this case the exception thrown will be catched by catch (int i) , since parameter is an integer number.

We can also define a catch block that captures all the exceptions independently of the type used in the call to throw. For that we have to write three points instead of the parameter type and name accepted by catch: 

try {
            // code here
}
catch (...) {
  cout << "Exception occurred";
}

It is also possible to nest try-catch blocks within more external try blocks. In these cases, we have the possibility that an internal catch block forwards the exception received to the external level, for that it is used the expression throw; with no arguments. For example: 

try {
  try {
            // code here
  }
  catch (int n) {
      throw;
  }
}
catch (...) {
  cout << "Exception occurred";
}

Exceptions not catched

If an exception is not catched by any catch statement because there is no catch statement with a matching type, the special function terminate will be called.

This function is generally defined so that it terminates the current process immediately showing an "Abnormal termination" error message. Its format is:

void terminate();

 

Standard exceptions

Some functions of the standard C++ language library send exceptions that can be captured if we include them within a try block. These exceptions are sent with a class derived from std::exception as type. This class (std::exception) is defined in the C++ standard header file <exception> and serves as pattern for the standard hierarchy of exceptions:
exception
bad_alloc (lanzada por new)
bad_cast (lanzada por dynamic_cast si falla con un tipo referenciado)
bad_exception (lanzada cuando una excepción no coincide con ningún catch)
bad_typeid (lanzada por typeid)
logic_error
domain_error
invalid_argument
length_error
out_of_range
runtime_error
overflow_error
range_error
underflow_error
ios_base::failure (Lanzada por ios::clear)
Because this is a class hierarchy, if you include a catch block to capture any of the exceptions of this hierarchy using the argument by reference (i.e. adding an ampersand & after the type) you will also capture all the derived ones (rules of inheritance in C++).

The following example catches an exception of type bad_typeid (derived from exception) that is generated when requesting information about the type pointed by a null pointer: 

// standard exceptions
#include <iostream>
#include <exception>
#include <typeinfo>
using namespace std;
class A {virtual f() {}; };
int main () {
  try {
    A * a = NULL;
    typeid (*a);
  }
  catch (std::exception& e)
  {
    cout << "Exception: " << e.what();
  }
  return 0;
}
Exception: Attempted typeid of NULL pointer

You can use the classes of standard hierarchy of exceptions to throw your exceptions or derive new classes from it.