CSC 330. An Exception is. Handling Exceptions. Error Handling. Assertions. C++ Exception. Example

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1 An Exception is CSC 330 Handling Exceptions CSC330 OO Software Design 1 An unusual, often unpredictable event, detectable by software or hardware, that requires special processing; also, in C++, a variable or class object that represents an exceptional event. An exception handler is a section of program code that is executed when a particular exception occurs. CSC330 OO Software Design 2 Assertions Error Handling CSC330 OO Software Design 3 CSC330 OO Software Design 4 C++ Exception An exception occurs when an unexpected error or unpredictable behaviors happened on your program not caused by the operating system itself. These exceptions are handled by code which is outside the normal flow of control and it needs an emergency exit. C++ has incorporated three operators to help us handle these situations:, throw and catch. CSC330 OO Software Design 5 Example buff = new char[1024]; if(buff == 0) throw "Memory allocation failure!"; //catch what is thrown... catch(char* strg) cout<<"exception raised: "<<strg<<endl; CSC330 OO Software Design 6

2 Syntax The -block: compound-statement handler-list handler-list here The throw-expression: throw expression The handler: catch (exception-declaration) compound-statement exception-declaration: type-specifier-list here Block A block is a group of C++ statements, enclosed in curly braces, that might cause an exception. This grouping restricts the exception handlers to the exceptions generated within the block. Each block may have one or more associated catch blocks. If no exception is thrown during execution of the guarded section, the catch clauses that follow the block are not executed. Execution continues at the statement after the last catch clause following the block in which the exception was thrown. If an exception is thrown during execution of the guarded section or in any routine the guarded section calls either directly or indirectly such as functions, an exception object will be created from the object created by the throw operand. At this point, the compiler looks for a catch clause in a higher execution context that can handle an exception of the type thrown or a catch handler that can handle any type of exception. The compoundstatement after the keyword is the guarded section of code. CSC330 OO Software Design 7 CSC330 OO Software Design 8 throw Statement The throw statement is used to throw an exception and its value to a matching catch exception handler. A regular throw consists of the keyword throw and an expression. The result type of the expression determines which catch block receives control. Within a catch block, the current exception and value may be re-thrown simply by specifying the throw keyword alone that is without the expression. The throw is syntactically similar to the operand of a return statement but here, it returns to the catch handler. CSC330 OO Software Design 9 The throw Statement Throw: to signal the fact that an exception has occurred; also called raise ThrowStatement throw Expression CSC330 OO Software Design 10 Raising an Exception Raising an Exception cont d CSC330 OO Software Design 11 CSC330 OO Software Design 12

3 catch Block A catch block is a group of C++ statements that are used to handle a specific thrown exception. One or more catch blocks, or handlers, should be placed after each block. A catch block is specified by: 1. The keyword catch 2. A catch parameter, enclosed in parentheses (), which corresponds to a specific type of exception that may be thrown by the block 3. A group of statements, enclosed in curly braces, whose purpose is to handle the exception catch Block cont d The compound-statement after the catch keyword is the exception handler, and catches or handles the exception thrown by the throw-expression. The exception-declaration statement part indicates the type of exception the clause handles. The type can be any valid data type, including a C++ class. If the exception-declaration statement part is just an ellipsis (...) such as, catch(...) Then, the catch clause will handle any type of exception, including C exceptions and system or application generated exceptions such as divide by zero, memory protection and floating-point violations. Such a handler must be the last handler for its block acting as default catch. CSC330 OO Software Design 13 CSC330 OO Software Design 14 catch Block cont d The catch handlers are examined in order of their appearance following the block. If no appropriate handler is found, the next dynamically enclosing block is examined. This process continues until the outermost enclosing block is examined if there are more than one block. The catch block must go right after the block without any line of codes between them. The order in which catch handlers appear is important, because handlers for a given block are examined in order of their appearance. For example, it is an error to place the handler for a base class before the handler for a derived class. catch Block cont d After a matching catch handler is found, subsequent handlers are not examined. That is why an ellipsis catch, catch(...) handler must be the last handler for its block. Besides that, catch may be overloaded so that it can accept different types as parameters. In that case the catch block executed is the one that matches the type of the exception sent through the parameter of throw. CSC330 OO Software Design 15 CSC330 OO Software Design 16 Example #include <iostream.h> int main() //declare char pointer char* buff; // block... //allocate storage for char object... buff = new char[1024]; //do a test, if allocation fails... if (buff == 0) throw "Memory allocation failure!"; //if allocation successful, display the following message, bypass the catch block... else cout<<sizeof(buff)<<" Byte successfully allocated!"<<endl; //if allocation fails, catch the type... //display message... catch(char* strg) cout<<"exception raised: "<<strg<<endl; return 0; CSC330 OO Software Design 17 Output CSC330 OO Software Design 18

4 Example: Multiple catch //exception: multiple catch blocks #include <iostream.h> #include <stdlib.h> int main () char * teststr; teststr = new char [10]; //test, if memory allocation fails then, throws this error to the matching catch... if (teststr == NULL) throw "Allocation failure"; for (int i=0; i<=15; i++) //another test, if n>9, throw this error to the respective catch.. if (i>9) throw i; teststr[i]='z'; cout<<"teststr["<<i<<"] = "<<teststr[i]<<endl; Example: Multiple catch cont d //catch the error if, i > 9, by displaying some error message... catch (int j) cout<<"the exception: "; cout<<"index "<<j<<" is out of range"<<endl; //catch the error if, allocation fail for *teststr by displaying some error... catch (char * strg) cout<<"the exception: "<<strg<<endl; system("pause"); return 0; CSC330 OO Software Design 19 CSC330 OO Software Design 20 Output Handling an Exception CSC330 OO Software Design 21 CSC330 OO Software Design 22 The -catch Statement How one part of the program catches and processes the exception that another part of the program throws. TryCatchStatement Block catch (FormalParameter) Block catch (FormalParameter) FormalParameter DataType VariableName CSC330 OO Software Design 23 Example of a -catch Statement // Statements that process personnel data and may throw // exceptions of type int, string, and SalaryError catch ( int ) // Statements to handle an int exception catch ( string s ) CSC330 OO Software Design 24

5 -catch Continued cout << s << endl; // Prints "Invalid customer age" // More statements to handle an age error catch ( SalaryError ) // Statements to handle a salary error Execution of -catch A statement throws an exception Control moves directly to exception handler Exception Handler No statements throw an exception Statements to deal with exception are executed CSC330 OO Software Design 25 Statement following entire -catch statement CSC330 OO Software Design 26 Standard Exception Hierarchies Standard Exceptions The C++ exception class serves as the base class for all exceptions thrown by certain expressions and by the Standard C++ Library. CSC330 OO Software Design 27 CSC330 OO Software Design 28 CSC330 OO Software Design 29 CSC330 OO Software Design 30

6 Exceptions Example Exceptions Example cont d CSC330 OO Software Design 31 CSC330 OO Software Design 32 Dividing by ZERO Apply what you know: int Quotient( /* in */ int numer, // The numerator /* in */ int denom ) // The denominator if (denom!= 0) return numer / denom; else // What to do?? CSC330 OO Software Design 33 A Solution // quotient.cpp -- Quotient program #include<iostream> #include <string> using namespace std; int Quotient( int, int ); class DivByZero // Exception class ; int main() int numer; // Numerator int denom; // Denominator CSC330 OO Software Design 34 cout << "Enter numerator and denominator: "; cin >> numer >> denom; while (cin) cout << "Their quotient: " << Quotient(numer, denom) << endl; catch ( DivByZero ) cout << "*** Denominator can't be 0" << endl; cout << "Enter numerator and denominator: "; cin >> numer >> denom; return 0; CSC330 OO Software Design 35 int Quotient( /* in */ int numer, // The numerator /* in */ int denom ) // The denominator if (denom == 0) throw DivByZero(); return numer / denom; CSC330 OO Software Design 36