Chapter 17

Templates and ExceptionsPart 2Dale/Weems/Headington

An Exception is 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.

The throw Statement Throw: to signal the fact that an exception has occurred; also called raise Throw Statement

throw Expression

Throw examples (1)throw 5;string str = Invalid customer age;throw str;class SalaryError{ };SalaryError sal;throw sal;

Throw examples (2)Use of anonymous (unnamed) objectclass SalaryError{};throw SalaryError();A wrong waythrow SalaryError;

The try-catch Statementtry Blockcatch (FormalParameter) Blockcatch (FormalParameter) BlockTryCatchStatementHow one part of the program catches and processes the exception that another part of the program throws.FormalParameterDataType [VariableName]

Example of a try-catch Statementtry{ // 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 ){

try-catch Continuedcout

Execution of try-catch No statements throw an exception Statement following entire try-catch statementA statement throwsan exceptionException HandlerStatements to deal with exception are executedControl moves directly to exception handler

Selecting an Exception HandlerThe computer: matches data types of throw clause with the data types in the catch clauses Searches in a north-to-south order Selects first handler whose data type matches that of the thrown exception Ellipse parameters are a wild card and catch all. Place the catch all handler last.

Selecting an Exception HandlerWhat constitutes a match?Suppose an object of type T is thrown.A catch block that takes an argument of type C is a match if:T and C are the same type.C adds a reference qualifies (e.g. int can be caught by int&)C is a base class of publicly derived class T

Note: matching process more restrictive than for function calls. Ex: int cant be caught by float or char

More on Selecting Exception Handlers The parameters name is needed only if statements in the body of the exception handler use that variable. It is a good idea to use only user-defined classes (and structs) as exception types, one type per exception descriptive identifiers

An exampleclass SalaryError// Exception class{};class BadRange// Exception class{};if ( condition )throw SalaryError();if ( condition )throw BadRange();

Nonlocal Exception Handlers It is more common for the throw to occur inside a function that is called from within a try-clause than for the throw to be located within the try-catch statement

Throwing an Exception to be Caught by the Calling Codevoid Func3(){ try { Func4(); } catch ( ErrType ) { } }

void Func4() { if ( error ) throw ErrType(); }

NormalreturnFunctioncallReturn fromthrown exception

No ErrType handler

ErrType handler

No ErrType handler

No ErrType handler

No ErrType handler

throw ErrType();

ImmediatereturnImmediatereturnImmediatereturnFunc1No ErrType handler

No ErrType handler

No ErrType handler

No ErrType handler

No ErrType handler

throw ErrType();

ImmediatereturnProgramterminatesimmediatelyFunc2Func3Func4mainmainImmediate returnFunction Func1 has a handler for ErrTypeNo function has a handler for ErrTypeFunc1Func2Func3Func4Passing an Exception up the Chain of Function Calls

Re-Throwing an ExceptionThe throw expression is optional.throw;Re-throwing an exception in C++ allows partial exception handling.

An example (1)void WriteToFile( parameters ){ // Open a file for outputtry{while ( condition ){DoSomething( arguments ); // May throw a BadData exception // Write to output file}}

An example (2)catch ( BadData ){// Write massage to output file and // close itthrow;// Re-throw the exception}// Continue processing// and close the output file}

A Solution// quotient.cpp -- Quotient program#include #include using namespace std;int Quotient( int, int );class DivByZero // Exception class{};int main(){ int numer; // Numerator int denom; // Denominator

cout > numer >> denom; while (cin) { try { cout

int Quotient( /* in */ int numer, // The numerator /* in */ int denom ) // The denominator{ if (denom == 0) throw DivByZero(); return numer / denom;}

Standard Exceptions Exceptions Thrown by the Languagenew, dynamic_cast, typeid, exception specificationExceptions Thrown by Standard Library RoutinesFacilities inherited from the C languageFacilities designed specifically for C++

Exception thrown by newfloat* arr;try{arr = new float[50000];}catch ( bad_alloc ){cout

C library routine set errnoSimulate throwing exceptions from C library routinesclass CLibErr// Our own exception class{};void SomeFunc( parameters ){errno = 0;C_lib_routine( arguments );if (errno != 0)throw CLibErr();}

Exceptions thrown by C++ libraries (1)Exceptions thrown by string classesvoid SomeFunc( parameters ){string s1, s2;try{s2 = s1.substr(pos, len);// May throw out_of_range()s1 = s1 + s1 + s2;// May throw length_error()}

Exceptions thrown by C++ libraries (2)Exceptions thrown by string classescatch ( out_of_range ){cout

Dividing by ZEROApply 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??}

The end of Chapter 17 Part 2