C++ Programing Questions And Answers-5

Q. What happens when you make call "delete this;" ?

Ans. The code has two built-in pitfalls. First, if it executes in a member function for an extern, static, or automatic object, the program will probably crash as soon as the delete statement executes. There is no portable way for an object to tell that it was instantiated on the heap, so the class cannot assert that its object is properly instantiated. Second, when an object commits suicide this way, the using program might not know about its demise. As far as the instantiating program is concerned, the object remains in scope and continues to exist even though the object did itself in. Subsequent dereferencing of the pointer can and usually does lead to disaster.

You should never do this. Since compiler does not know whether the object was allocated on the stack or on the heap, "delete this" could cause a disaster.

Q. How virtual functions are implemented C++?

Ans. Virtual functions are implemented using a table of function pointers, called the vtable.  There is one entry in the table per virtual function in the class.  This table is created by the constructor of the class.  When a derived class is constructed, its base class is constructed first which creates the vtable.  If the derived class overrides any of the base classes virtual functions, those entries in the vtable are overwritten by the derived class constructor.  This is why you should never call virtual functions from a constructor: because the vtable entries for the object may not have been set up by the derived class constructor yet, so you might end up calling base class implementations of those virtual functions

Q. What is name mangling in C++?

Ans. The process of encoding the parameter types with the function/method name into a unique name is called name mangling. The inverse process is called demangling.

For example Foo::bar(int, long) const is mangled as `bar__C3Fooil'.
For a constructor, the method name is left out. That is Foo::Foo(int, long) const is mangled as `__C3Fooil'.

Q. What is the difference between a pointer and a reference?

Ans. A reference must always refer to some object and, therefore, must always be initialized; pointers do not have such restrictions. A pointer can be reassigned to point to different objects while a reference always refers to an object with which it was initialized.

Q. How are prefix and postfix versions of operator++() differentiated?

Ans. The postfix version of operator++() has a dummy parameter of type int. The prefix version does not have dummy parameter.

Q. What is the difference between const char *myPointer and char *const myPointer?

Ans. Const char *myPointer is a non constant pointer to constant data; while char *const myPointer is a constant pointer to non constant data.

Q. How can I handle a constructor that fails?

Ans. throw an exception. Constructors don't have a return type, so it's not possible to use return codes. The best way to signal constructor failure is therefore to throw an exception.

Q. How can I handle a destructor that fails?

Ans. Write a message to a log-file. But do not throw an exception.
The C++ rule is that you must never throw an exception from a destructor that is being called during the "stack unwinding" process of another exception. For example, if someone says throw Foo(), the stack will be unwound so all the stack frames between the throw Foo() and the } catch (Foo e) { will get popped. This is called stack unwinding.
During stack unwinding, all the local objects in all those stack frames are destructed. If one of those destructors throws an exception (say it throws a Bar object), the C++ runtime system is in a no-win situation: should it ignore the Bar and end up in the } catch (Foo e) { where it was originally headed? Should it ignore the Foo and look for a } catch (Bar e) { handler? There is no good answer -- either choice loses information.
So the C++ language guarantees that it will call terminate() at this point, and terminate() kills the process. Bang you're dead.

Q. What is Virtual Destructor?

Ans. Using virtual destructors, you can destroy objects without knowing their type - the correct destructor for the object is invoked using the virtual function mechanism. Note that destructors can also be declared as pure virtual functions for abstract classes.

if someone will derive from your class, and if someone will say "new Derived", where "Derived" is derived from your class, and if someone will say delete p, where the actual object's type is "Derived" but the pointer p's type is your class.

Q. Can you think of a situation where your program would crash without reaching the breakpoint which you set at the beginning of main()?

Ans. C++ allows for dynamic initialization of global variables before main() is invoked. It is possible that initialization of global will invoke some function. If this function crashes the crash will occur before main() is entered.

Q. Name two cases where you MUST use initialization list as opposed to assignment in constructors.

Ans. Both non-static const data members and reference data members cannot be assigned values; instead, you should use initialization list to initialize them.