Starting Out with C++, 3 rd Edition 1 Chapter 16 – Exceptions, Templates, and the Standard Template Library (STL) Exceptions are used to signal errors or unexpected events that occur while a program is running. We have checked for errors before by placing if statements that checked for the condition or by using assert statements.

Starting Out with C++, 3 rd Edition 2 Exceptions (cont) An exceptional condition is a value or an object that signals an error. When the error occurs, an exception is “thrown.”

Starting Out with C++, 3 rd Edition 3 Throwing an Exception Rational::Rational ( int numerator, int denominator ) { if ( denominator == 0 ) throw "ERROR: Cannot divide by zero.\n"; this->numerator = numerator; this->denominator = denominator; } // divide

Starting Out with C++, 3 rd Edition 4 Throw The line containing a throw statement is called the throw point. When a throw statement is executed, control is passed to an exception handler. When an exception is thrown by a function, the function aborts. Any type can be “thrown”

Starting Out with C++, 3 rd Edition 5 Handling an Exception An exception is handled by a try/catch construct. The try block contains a block of code that may directly or indirectly cause an exception to be thrown. The catch block contains a block of code that handles the exception.

Starting Out with C++, 3 rd Edition 6 Example Exception Handler try { Rational q = Rational( num1, num2 ); cout << q << endl; } // try catch ( char *exceptionString ) { cout << exceptionString; } // catch

Starting Out with C++, 3 rd Edition 7 Handling an Exception (cont) If divide throws an exception, the exception is caught by the catch statement. The type of the parameter to the catch statement must be the same as in the throw statement (or class from which it is derived). Execution continues after the try/catch statement, unless the catch statement does something else.

Starting Out with C++, 3 rd Edition 8 Uncaught Exceptions Ways for uncaught exceptions: 1.No catch statement with a parameter of the correct data type. 2.The exception is thrown outside a try statement. In either case, the exception causes the program to abort.

Starting Out with C++, 3 rd Edition 9 More on Exceptions Exceptions can be put in classes. This allows us to do more with them. We can have multiple exception classes. Exception classes can have parameters that allow information to be returned.

Starting Out with C++, 3 rd Edition 10 IntMain.cpp – Program (cont) in getInput… throw OutOfRange( input ); in main… try { userValue = range.getInput(); cout << "You entered " << uservalue << endl; } // try catch ( IntRange::OutOfRange ex ) { cout << "That value " << ex.value << " is out of range." << endl; } // catch cout << "End of program." << endl; } // main

Starting Out with C++, 3 rd Edition 11 Class for exceptions class BadTrait{ private: string msg; public: BadTrait(string errorMsg, int val){ char str[10]; itoa(val,str,10); // integer, string to convert to, number sys msg = errorMsg+str; } string getMsg(){ return msg;} };

Starting Out with C++, 3 rd Edition 12 How to Throw // add newEle to set only if it is a legal element Set &Set::operator+=(int newEle){ if (newEle =0) member[newEle]=true; else throw BadTrait("Error: Adding element which is out of range ", newEle); return *this; }

Starting Out with C++, 3 rd Edition 13 How to catch try{ Traits me; me+=4; me += 5;me +=11; me +=27; }catch (BadTrait b){ cout<< b.getMsg() << endl;}

Starting Out with C++, 3 rd Edition 14 Unwinding the Stack When an exception is thrown, the function immediately terminates. If that function was called by another function, then the calling function terminates as well. This process continues. This is called unwinding the stack.

Starting Out with C++, 3 rd Edition 15 Function Templates A function template is a “generic” function that can work with any data type. The programmer writes the specifications of the function, but substitutes parameters for data types. When the compiler encounters a call to the function, it generates code to handle the specific data type(s) used in the call.

Starting Out with C++, 3 rd Edition 16 Template Example We want a function that prints out the values of an array. This function should work with arrays of type int, float, char, or string.

Starting Out with C++, 3 rd Edition 17 printArray.cpp – Program #include using namespace std; template void printArray ( const T *array, const int size ) { for ( int i = 0; i < size; i++ ) {if ( !( i % 10 ) ) cout << endl; cout << array[i] << " "; } // for cout << endl; } // printArray

Starting Out with C++, 3 rd Edition 18 printArray.cpp – Program (cont) void main ( void ) { const int aSize = 5, bSize = 7, cSize = 6, dSize = 3; int a[ aSize ] = { 1, 2, 3, 4, 5 }; float b[ bSize ] = { 1.1, 2.2, 3.3, 4.4, 5.5, 6.6, 7.7 }; char c[ cSize ] = "Hello"; string d[ dSize ] = { "I", "love", "CS" }; printArray( a, aSize ); printArray( b, bSize ); printArray( c, cSize ); printArray( d, dSize ); } // main

Starting Out with C++, 3 rd Edition 19 Generic swap Suppose we wanted a generic swap. template void swap ( T &var1, T &var2 ) { T temp; temp = var1; var1 = var2; var2 = temp; } // swap

Starting Out with C++, 3 rd Edition 20 Template with Multiple Types template void swap ( T1 &var1, T2 &var2 ) {T1 temp; temp = var1; var1 = (T1)var2; var2 = (T2)temp; } // swap

Starting Out with C++, 3 rd Edition 21 Operators in Template If arithmetic or relational operators are used in templates, they must be defined for the different types of the templates. If the operators are not defined for a particular type, the compiler generates an error.

Starting Out with C++, 3 rd Edition 22 Class Templates Templates may also be used to create generic classes and abstract data types. Class templates allow you to create one general version of a class without having to duplicate code to handle multiple data types.

Starting Out with C++, 3 rd Edition 23 Generic Class Suppose you wanted to have a class that held a collection of objects. We want this class to work with different data types. We could create a different class for each data type, but there is a better solution. We will look at a class template.

Starting Out with C++, 3 rd Edition 24 SimpleVector.h – Program template Class SimpleVector { private: T *aptr; int arraySize; public: SimpleVector ( void ) { aptr = NULL; arraySize = 0; } ; SimpleVector ( int ); SimpleVector ( const SimpleVector & ); ~SimpleVector ( void ); int getSize ( void ) { return arraySize; }; T &operator[] ( const int & ); }; // SimpleVector

Starting Out with C++, 3 rd Edition 25 SimpleVector.cpp – Program template SimpleVector ::SimpleVector ( int s ) { arraySize = s; aptr = new T [ s ]; assert( aptr ); for ( int i = 0; i < arraySize; i++ ) *( aptr + i ) = 0; } // SimpleVector::SimpleVector template SimpleVector ::~SimpleVector ( void ) { if ( arraySize > 0 ) delete [] aptr; } // SimpleVector::~SimpleVector

Starting Out with C++, 3 rd Edition 26 Declaring Objects of Class Templates The declaration of class templates is a little different. Consider the following examples: –SimpleVector intTable ( 10 ); –SimpleVector floatTable ( 10 ); In these, the parameter inside the angle brackets replaces the T in the previous declarations.

Starting Out with C++, 3 rd Edition 27 Class Templates and Inheritance #ifndef SEARCHABLEVECTOR_H #define SEARCHABLEVECTOR_H #include “SimpleVector.h” template class SearchableVector : public SimpleVector { public: SearchableVector ( int s ) : SimpleVector ( s ) { } SearchableVector ( SearchableVector & ); SearchableVector ( SimpleVector &obj ) : SimpleVector ( obj ) { } int findItem ( T ); }; // SearchableVector

Starting Out with C++, 3 rd Edition 28 Introduction to the Standard Template Library (STL) The Standard Template Library contains many templates for useful algorithms and data structures. We’ve seen on of these earlier in the book – the vector data type.

Starting Out with C++, 3 rd Edition 29 Abstract Data Types The most important data structures in STL are containers and iterators: –Container – a class that stores data and organizes it in some fashion. –Iterator – similar to a pointer and is used to access the individual data elements in a container.

Starting Out with C++, 3 rd Edition 30 Container Classes Sequence – organizes data in a sequential fashion similar to an array. Associative – uses keys to rapidly access elements in the data structure.

Starting Out with C++, 3 rd Edition 31 Sequence Containers Container NameDescription vector An expandable array. Values may be added to or removed from the end or middle. deque Like a vector, but also allows values to be added to or removed from the front. list A doubly-linked list of data elements. Values may be inserted in or removed from any position.

Starting Out with C++, 3 rd Edition 32 Associative Containers Container NameDescription set Stores a set of keys. No duplicate values are allowed. multiset Stores a set of keys. Duplicates are allowed. map Maps a set of keys to data elements. Only one key per data element is allowed. Duplicates are not allowed. multimap Maps a set of keys to data elements. Many keys per data element are allowed. Duplicates are allowed.

Starting Out with C++, 3 rd Edition 33 Iterators Iterator TypeDescription Forward Can only move forward in a container (uses the ++ operator). Bidirectional Can move forward or backward in a container (uses the ++ and – operators). Random-Access Can move forward and backward, and can jump to a specific data element in a container. Input Can be used with cin to read information from an input device or a file. Output Can be used with cout to write information to an output device or a file.

Starting Out with C++, 3 rd Edition 34 More on Iterators Iterators are associated with containers. The type of container you have determines the type of iterator you use. For example, vectors and deques require random-access iterators. Lists, sets, multisets, maps, and multimaps require bidirectional iterators.

Starting Out with C++, 3 rd Edition 35 Algorithms Algorithms are implemented as function templates. They perform various operations on elements of containers.

Starting Out with C++, 3 rd Edition 36 Description of Some Algorithms AlgorithmDescription binary_search Performs a binary search for an object and returns true if the object is found. For example, consider binary_search(iter1, iter2, value); The statements performs a binary search in the range iter1 to iter2 looking for value count Returns the number of times a value appears in a range. For example, consider iter3 = count(iter1, iter2, value); The statements returns the number of times value is found in the range iter1 to iter2. for_each Executes a function for each element of a container. For example, consider for_each(iter1, iter2, func); The statement calls the function func for each element in the range from iter1 to iter2, passing the element as the argument.

Starting Out with C++, 3 rd Edition 37 Using Vectors – Program #include using namespace std; void doubleValue ( int &val ) { val *= 2; } // doubleValue void main ( void ) { vector numbers; vector ::iterator iter;

Starting Out with C++, 3 rd Edition 38 Using Vectors – Program (cont) for ( int x = 0; x < 10; x++ ) numbers.push_back( x ); cout << “The numbers in the vector are:\n”; for ( iter = numbers.begin(); iter != numbers.end(); iter++ ) cout << *iter << endl; cout << endl; for_each ( numbers.begin(), numbers.end(), doubleValue ); cout << “The numbers again are:\n”; for ( iter = numbers.begin(); iter != numbers.end(); iter++ ) cout << *iter << endl; cout << endl; } // main