Home > Articles > Programming > C/C++

Exception Management in C++ and Python Development: Planning for the Unexpected

Stephen B. Morris digs deeper into the terrain between C++ and Python, both of which provide powerful exception-management facilities. Later maintainers of your code will thank you for implementing decent exception logic! In addition to the maintenance advantages, careful use of exception handlers can facilitate effective integration between Python and C++.
Like this article? We recommend

Like this article? We recommend

The Importance of Handling Exceptions

Exception handling is an area that programmers seem to either love or hate. I've even been surprised a few times by seeing code that swallows all exceptions, without exception! This is a lazy and hazardous way of implementing exception handlers, as illustrated in Listing 1.

Listing 1—Swallowing all exceptions. Please don't do this at home—or at work.

    aFile = open('anyOldFile.txt')
    myString = aFile.readline()
    print('File contents are: ')
except IOError as e:
    print "I/O error({0}): {1}".format(e.errno, e.strerror)
    print "Unexpected error:", sys.exc_info()[0]

Why might the code in Listing 1 be considered bad practice? Well, suppose some truly catastrophic exception occurs; for example, the runtime system runs out of memory. Listing 1 offers no way for the exception handler to cope in a graceful fashion. In other words, the last except clause in Listing 1 may be the wrong place to try to handle a more general exception.

Let's improve Listing 1 by simply removing the last except clause, as shown in Listing 2.

Listing 2—No longer swallowing all exceptions (less is more).

    aFile = open('anyOldFile.txt')
    myString = aFile.readline()
    print('File contents are: ')
except IOError as e:
    print "I/O error({0}): {1}".format(e.errno, e.strerror)

Listing 2 improves on Listing 1 by doing just one thing and doing it well; it handles the exception type IOError. If the code throws an IOError exception, the appropriate error details will be printed. An example is what occurs if the file in question doesn't exist:

I/O error(2): No such file or directory

What other exceptions might occur in Listing 2, apart from IOError? Well, any such exceptions must then be handled at a higher level; for example, in the code that calls Listing 2. On a deeper level, this can be considered an example of the Separation of Concerns design pattern. In this case, Listing 2 does some file I/O and handles any associated exceptions. Any other exceptions are the responsibility of the code that calls Listing 2. The exception-specific concerns have thereby been separated, as indicated in Listing 3.

Listing 3—Our exception-handling hierarchy.

Listing 2 code -----------> Handles IOError only
Listing 2 caller ----------> Handles all other exceptions

Viewed as a separation of concerns, the exception management strategy is distributed across the calling chain. The key requirement is to allocate only certain exception-handling responsibilities to certain code. This more sophisticated approach illustrates why the code in Listing 1 isn't very good, but is all too common.

Why isn't exception management more popular among programmers? Perhaps because it's not glamorous. Why think about possible errors in a completed piece of code, when you can just move onto the next coding task? Here's why: If you think a piece of code can't possibly fall foul of an exception state, you're probably wrong!

Writing robust exception-handlers is an opportunity to make your code stronger and more resilient. To illustrate this point, let's look again at Listing 2. Notice anything else missing from this listing? What about closing the file that we've just opened? The finally clause provides a nice mechanism for fulfilling this requirement. Let's do this with a little refactoring, as shown in Listing 4.

Listing 4—Better handling of allocated resources.

aFile = None

    aFile = open('anyOldFile.txt')
    myString = aFile.readline()
    print('File contents are: ')
except IOError as e:
    print('I/O error({0}): {1}'.format(e.errno, e.strerror))
    print 'No exception'
    if (aFile != None):
        print('File is being closed')
        print('File already closed')

The extra lines of code at the end of Listing 4 provide a degree of enhanced robustness because we're giving back our allocated file resource. With the file closed in the finally clause, we guarantee this beneficial outcome, and we avoid the problem of our code leaving files open unnecessarily. We make our code far more robust by using a language feature, and we avoid the need for complicated and error-prone mechanisms such as state variables.

Running Python Example Code

One of Python's many merits is its lightweight nature. It's pretty straightforward to get started coding in Python—no need to install (at least initially) and learn to use complex IDEs such as Eclipse. Instead, you can simply run up a Python console. In Linux, this is as simple as running the python command, which results in something similar to Listing 5.

Listing 5—The Python console.

Python 2.7.3 (default, Sep 26 2013, 20:08:41) 

[GCC 4.6.3] on linux2

Type "help", "copyright", "credits" or "license" for more information.


Here's how to get the Python host version:
>>> import sys

>>> sys.version

'2.7.3 (default, Sep 26 2013, 20:08:41) \n[GCC 4.6.3]'


Obviously, as code examples become more complex, copying-and-pasting the code into a console gets a bit clunky. But the console approach is useful for getting started or for testing code snippets.

So what about C++ exception management?

Handling Exceptions with C++

C++ also has a powerful exception-management facility. Listing 6 shows a simple example of C++ exception management. Here I attempt to force a memory-allocation exception called bad_alloc by allocating a pathologically large array of integers.

Listing 6—C++ exception management.

#include <iostream>
#include <exception>

using namespace std;

int main () {
      int* myarray = NULL;

      if (myarray) {
            cout << "Not NULL" << endl;
      } else {
            cout << "Is NULL" << endl;

            int* myarray = new int[1000000000];
      catch (exception& e)
            cout << "Standard exception: " << e.what() << endl;
            if (myarray) {
                  cout << "Not NULL" << endl;
            } else {
                  cout << "Is NULL" << endl;

      if(myarray) {
            cout << "Deleting myarray" << endl;
            delete [] myarray;

      cout << "Returning" << endl;

      return 0;

In Listing 6, I've also inserted some code to determine if and when the allocation occurs, via the call to new(). Listing 7 shows a run of the code where the exception occurs.

Listing 7—A C++ exception handler in action.

Standard exception: std::bad_alloc

Notice in Listing 7 that the exception handler has been invoked, and the type of exception is indeed bad_alloc. Naturally, this means that the runtime system was unable to provide the massive array I requested.

Of course, not everyone likes exception handlers. A veteran C++ programmer probably wouldn't use a try-catch block here. Instead, a pro would most likely just check that the call to new() returns a non-NULL pointer:

if(myarray) { // The myarray is not NULL if call to new() succeeded

Programming professionals also might dislike exception handlers because C++ exception checking potentially consumes valuable resources. For mere mortals, however, the use of the exception mechanism still has its merits. Aside from bad_alloc, the other exception types are as follows:

  • bad_exception is thrown by certain dynamic exception-specifiers.
  • bad_typeid is thrown by typeid.
  • bad_function_call is thrown by an empty function object.
  • bad_weak_ptr is thrown by shared_ptr when passed a bad weak_ptr.

Generic exceptions such as logic_error and runtime_error are another option, which programmers can use for application-specific purposes. Using runtime_error is potentially a good step in the direction of exception-centric development, where any exceptions get reported to a central repository.

Catching Exceptions by Reference in C++

The exception that can be caught by the handler in Listing 6 is a bad_alloc. The bad_alloc exception is itself derived from the standard base-class exception, so bad_alloc can be caught this way because it's part of the object graph. This works because capturing by reference in turn captures all related classes—which is good news because we can also capture other exceptions. If we capture an exception and don't know what to do with it, we can just rethrow it for handling at a higher level.

C++ and RAII (Resource Acquisition Is Initialization)

You might have noticed that the C++ example doesn't include the finally clause in its exception structure. The normal "finally" semantics are intended to be provided by a design feature of C++ called Resource Acquisition Is Initialization (RAII). Coupled with a feature called stack unwinding, RAII provides a model for exception-safe C++ code. RAII is a nice approach because it facilitates the exception-safe release of resources such as files, memory, allocated objects, and so on. RAII allows for appropriate destructor code to be invoked automatically as part of the exception management infrastructure.

RAII mechanisms are facilitated by some of the elements of C++ 11, specifically the smart pointer classes and mutex locks. Of course, the latter are geared toward multithreaded C++ programming; I described them to some extent in my article "C++ 11 Memory Management."

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020