Exception Handling in MFC

 

The latest version of this topic can be found at Exception Handling in MFC.

This article explains the exception-handling mechanisms available in MFC. Two mechanisms are available:

• C++ exceptions, available in MFC version 3.0 and later

• The MFC exception macros, available in MFC versions 1.0 and later

If you're writing a new application using MFC, you should use the C++ mechanism. You can use the macro-based mechanism if your existing application already uses that mechanism extensively.

You can readily convert existing code to use C++ exceptions instead of the MFC exception macros. Advantages of converting your code and guidelines for doing so are described in the article Exceptions: Converting from MFC Exception Macros.

If you have already developed an application using the MFC exception macros, you can continue using these macros in your existing code, while using C++ exceptions in your new code. The article Exceptions: Changes to Exception Macros in Version 3.0 gives guidelines for doing so.

Note

To enable C++ exception handling in your code, select Enable C++ Exceptions on the Code Generation page in the C/C++ folder of the project's Property Pages dialog box, or use the /GX compiler option. The default is /GX–, which disables exception handling.

This article covers the following topics:

• When to use exceptions

• MFC exception support

• Further reading about exceptions

When to Use Exceptions

Three categories of outcomes can occur when a function is called during program execution: normal execution, erroneous execution, or abnormal execution. Each category is described below.

• Normal execution

  The function may execute normally and return. Some functions return a result code to the caller, which indicates the outcome of the function. The possible result codes are strictly defined for the function and represent the range of possible outcomes of the function. The result code can indicate success or failure or can even indicate a particular type of failure that is within the normal range of expectations. For example, a file-status function can return a code that indicates that the file does not exist. Note that the term "error code" is not used because a result code represents one of many expected outcomes.

• Erroneous execution

  The caller makes some mistake in passing arguments to the function or calls the function in an inappropriate context. This situation causes an error, and it should be detected by an assertion during program development. (For more information on assertions, see C/C++ Assertions.)

• Abnormal execution

  Abnormal execution includes situations where conditions outside the program's control, such as low memory or I/O errors, are influencing the outcome of the function. Abnormal situations should be handled by catching and throwing exceptions.

Using exceptions is especially appropriate for abnormal execution.

MFC Exception Support

Whether you use the C++ exceptions directly or use the MFC exception macros, you will use CException Class or CException-derived objects that may be thrown by the framework or by your application.

The following table shows the predefined exceptions provided by MFC.

Exception class	Meaning
CMemoryException Class	Out-of-memory
CFileException Class	File exception
CArchiveException Class	Archive/Serialization exception
CNotSupportedException Class	Response to request for unsupported service
CResourceException Class	Windows resource allocation exception
CDaoException Class	Database exceptions (DAO classes)
CDBException Class	Database exceptions (ODBC classes)
COleException Class	OLE exceptions
COleDispatchException Class	Dispatch (automation) exceptions
CUserException Class	Exception that alerts the user with a message box, then throws a generic CException Class

Note

MFC supports both C++ exceptions and the MFC exception macros. MFC does not directly support Windows NT structured exception handlers (SEH), as discussed in Structured Exception Handling.

Further Reading About Exceptions

The following articles explain using the MFC library for exception handing:

• Exceptions: Catching and Deleting Exceptions

• Exceptions: Examining Exception Contents

• Exceptions: Freeing Objects in Exceptions

• Exceptions: Throwing Exceptions from Your Own Functions

• Exceptions: Database Exceptions

• Exceptions: OLE Exceptions

The following articles compare the MFC exception macros with the C++ exception keywords and explain how you can adapt your code:

• Exceptions: Changes to Exception Macros in Version 3.0

• Exceptions: Converting from MFC Exception Macros

• Exceptions: Using MFC Macros and C++ Exceptions

See Also

C++ Exception Handling
How Do I: Create my Own Custom Exception Classes