Example - 1

The following example attempts to allocate memory for a character. After the call to malloc, an if statement is used to check whether the malloc function failed.

//We do nothing so we just ignore the error.foo=malloc(sizeof(char)); //the next line checks to see if malloc failedif (foo==NULL) {}

The conditional successfully detects a NULL return value from malloc indicating a failure, however it does not do anything to handle the problem. Unhandled errors may have unexpected results and may cause the program to crash or terminate.

Instead, the if block should contain statements that either attempt to fix the problem or notify the user that an error has occurred and continue processing or perform some cleanup and gracefully terminate the program. The following example notifies the user that the malloc function did not allocate the required memory resources and returns an error code.

printf("Malloc failed to allocate memory resources");return -1;foo=malloc(sizeof(char)); //the next line checks to see if malloc failedif (foo==NULL) {}

Example - 2

In the following C++ example the method readFile() will read the file whose name is provided in the input parameter and will return the contents of the file in char string. The method calls open() and read() may result in errors if the file does not exist or does not contain any data to read. These errors will be thrown when the is_open() method and good() method indicate errors opening or reading the file. However, these errors are not handled within the catch statement. Catch statements that do not perform any processing will have unexpected results. In this case an empty char string will be returned, and the file will not be properly closed.

throw "Unable to open file " + filename;
throw "Unable to read from file " + filename;
// open input fileifstream infile;infile.open(filename);if (!infile.is_open()) {}// get length of fileinfile.seekg (0, ios::end);int length = infile.tellg();infile.seekg (0, ios::beg);// allocate memorychar *buffer = new char [length];// read data from fileinfile.read (buffer,length);if (!infile.good()) {}infile.close();return buffer;
/* bug: insert code to handle this later */try {}catch (...) {}char* readfile (char *filename) {}

The catch statement should contain statements that either attempt to fix the problem or notify the user that an error has occurred and continue processing or perform some cleanup and gracefully terminate the program. The following C++ example contains two catch statements. The first of these will catch a specific error thrown within the try block, and the second catch statement will catch all other errors from within the catch block. Both catch statements will notify the user that an error has occurred, close the file, and rethrow to the block that called the readFile() method for further handling or possible termination of the program.

throw "Unable to open file " + filename;
throw "Unable to read from file " + filename;
// open input fileifstream infile;infile.open(filename);if (!infile.is_open()) {}// get length of fileinfile.seekg (0, ios::end);int length = infile.tellg();infile.seekg (0, ios::beg);// allocate memorychar *buffer = new char [length];// read data from fileinfile.read (buffer,length);if (!infile.good()) {}infile.close();return buffer;

printf("Error: %s \n", str);infile.close();throw str;

printf("Error occurred trying to read from file \n");infile.close();throw;try {}catch (char *str) {}catch (...) {}char* readFile (char *filename) {}

Example - 3

In the following Java example the method readFile will read the file whose name is provided in the input parameter and will return the contents of the file in a String object. The constructor of the FileReader object and the read method call may throw exceptions and therefore must be within a try/catch block. While the catch statement in this example will catch thrown exceptions in order for the method to compile, no processing is performed to handle the thrown exceptions. Catch statements that do not perform any processing will have unexpected results. In this case, this will result in the return of a null String.

// initialize File and FileReader objectsFile file = new File(filename);FileReader fr = new FileReader(file);// initialize character bufferlong fLen = file.length();char[] cBuf = new char[(int) fLen];// read data from fileint iRead = fr.read(cBuf, 0, (int) fLen);// close filefr.close();retString = new String(cBuf);/* do nothing, but catch so it'll compile... */
String retString = null;try {} catch (Exception ex) {}return retString;public String readFile(String filename) {}

The catch statement should contain statements that either attempt to fix the problem, notify the user that an exception has been raised and continue processing, or perform some cleanup and gracefully terminate the program. The following Java example contains three catch statements. The first of these will catch the FileNotFoundException that may be thrown by the FileReader constructor called within the try/catch block. The second catch statement will catch the IOException that may be thrown by the read method called within the try/catch block. The third catch statement will catch all other exceptions thrown within the try block. For all catch statements the user is notified that the exception has been thrown and the exception is rethrown to the block that called the readFile() method for further processing or possible termination of the program. Note that with Java it is usually good practice to use the getMessage() method of the exception class to provide more information to the user about the exception raised.

// initialize File and FileReader objectsFile file = new File(filename);FileReader fr = new FileReader(file);// initialize character bufferlong fLen = file.length();char [] cBuf = new char[(int) fLen];// read data from fileint iRead = fr.read(cBuf, 0, (int) fLen);// close filefr.close();retString = new String(cBuf);
System.err.println ("Error: FileNotFoundException opening the input file: " + filename );System.err.println ("" + ex.getMessage() );throw new FileNotFoundException(ex.getMessage());
System.err.println("Error: IOException reading the input file.\n" + ex.getMessage() );throw new IOException(ex);
System.err.println("Error: Exception reading the input file.\n" + ex.getMessage() );throw new Exception(ex);
String retString = null;try {} catch (FileNotFoundException ex) {} catch (IOException ex) {} catch (Exception ex) {}return retString;public String readFile(String filename) throws FileNotFoundException, IOException, Exception {}