Resource Management - Object Oriented Programming - Lecture Slides, Slides of Object Oriented Programming

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Object-Oriented Programming (OOP) Lecture No. 45

Resource Management

• Function acquiring a resource must properly release it
• Throwing an exception can cause resource wastage

Resource Management

• In case of exception the call to close will be ignored

First Attempt

int function1(){ try{ FILE *fileptr = fopen(“filename.txt”,“w”); fwrite(“Hello World”,1,11,fileptr); ... throw exception(); fclose(fileptr); } catch(...) { fclose(fileptr); throw; } return 0; }

Second Attempt

class FilePtr{ FILE * f; public: FilePtr(const char *name, const char * mode) { f = fopen(name, mode);} ~FilePtr() { fclose(f); } operator FILE * () { return f; } };

Example

int function1(){

FilePtr file(“filename.txt”,“w”); fwrite(“Hello World”,1,11,file); throw exception(); ... return 0;

}

Exception in Constructors

• Exception thrown in constructor cause the destructor to be called for any object built as part of object being constructed before exception is thrown
• Destructor for partially constructed object is not called

Example

class Student{ String FirstName; String SecondName; String EmailAddress; … }

• If the constructor of the SecondName throws an exception then the destructor for the First Name will be called

Example

Student::Student (String aName) : name(aName)

/The constructor of String can throw a exception/

{

...

}

Exception in Initialization List

• The programmer may want to catch the exception and perform some action to rectify the problem

Exceptions in Destructors

• Exception should not leave the destructor
• If a destructor is called due to stack unwinding, and an exception leaves the destructor then the function std::terminate() is called, which by default calls the std::abort()

Example

class Exception;

class Complex{

…

public:

~Complex(){ throw Exception(); }

};

Example

Complex::~Complex()

{

try{ throw Exception(); } catch(…){ }

}

Exception Specification

• Program can specify the list of exceptions a function is allowed to throw
• This list is also called throw list
• If we write empty list then the function wont be able to throw any exception