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C++ Language-specific Answers

by Curtis Krauskopf

One strategy to use when answering a language issue question is to examine the question from various perspectives. Some perspectives to consider are:

  • the language author wants to prevent ambiguities
  • the C++ compiler vendor want to simplify the compiler's implementation
  • the code writer wants clear rules for implementing a feature
  • the maintenance programmer needs to understand existing code
  • legacy code should not be broken by new features

The best answer is one that acknowledges the tradeoffs that were made to accommodate one or more of the above groups. When reviewing your video or audio tape, give yourself extra credit for each group that you mention and their tradeoffs.

Q1) An array is instantiated with the new[] operator. Is it sufficient to delete the array using a delete operator or should a delete[] operator be used? Justify your answer.

A1) The delete[] operator should always be used. This restriction applies for both built-in types (char, int and others) and for aggregate types (structs and classes). This is required by the C++ language definition (the Annotated C++ Reference Manual).

Q2) Can I drop the [] when deleting an array of some built-in type (char, int, etc)?

A2) No. Marshall Cline's C++ FAQ Lite contains a good explanation at

Q3) What is an example of when a destructor is NOT called at the end of scope?

A3) Bjarne Stroustrup's C++ Style and Technique FAQ provides an excellent explanation at The short answer is that objects that are created on the heap are not automatically destroyed at the end of the scope that created the object.

In addition to Bjarne Stroustrup's answer, I would also add that statically created objects are also not destroyed at the end of the scope.

Q4) Explain stack unwinding.

A4) Stack unwinding occurs when an exception is thrown and control passes from a try block to a handler. Automatic objects are destroyed in the reverse order of their construction. If a destructor throws an exception during the stack unwinding process, terminate is called.

From the Borland C++ Builder help:

When an exception is thrown, the runtime library takes the thrown object, gets the type of the object, and looks upward in the call stack for a handler whose type matches the type of the thrown object. Once a handler is found, the RTL unwinds the stack to the point of the handler, and executes the handler. In the unwind process, the RTL calls destructors for all local objects in the stack frames between where the exception was thrown and where it is caught. If a destructor causes an exception to be raised during stack unwinding and does not handle it, terminate is called. Destructors are called by default, but you can switch off the default by using the -xd compiler option.

Objects that are not allocated on the stack, such as heap allocations, are not automatically released. This can cause memory leaks unless the programmer takes extra precautions to release the allocated memory when an exception is thrown. There are various ways to prevent heap-memory leaks caused by stack unwinding. One way is a user-defined garbage-collection; a second way is to specifically deallocate those resources in the exception handler.

Q5) When I write a derived class's destructor, do I need to explicitly call the destructor for my base class?

A5) Marshall Cline's C++ FAQ Lite answers this question with "No". A more explicit explanation with an example is at

Q6) Explain the difference between a class and an object.

A6) A class is a blueprint for an object. It defines how the object will be created, what data is stored, how the data can be manipulated and how the object will be destroyed. An object is an instantiation of a class. There can be multiple objects instantiated from one class. Every object has one and only one class that it was instantiated from.

Q7) Explain the difference between a struct and a class.

A7) The default members and base classes of a class are private. The default members and base classes of a struct are public.

Other than the default protection, struct and class are equivalent.

An unwritten practice amongst C++ programmers is to define a class for objects that have few or no public data members and to define a struct for objects that have few or no public methods.

Q8) What is difference between malloc()/free() and new/delete?

A8) malloc() and new both allocate space from the heap. free() and delete both release previously allocated heap space. free() should only be used with malloc'd allocations and delete should only be used with new allocations. There are two varieties of new: array allocation through a new[] operator and single object allocation through a new operator. It's the programmer's responsibility to know and track which allocation method was used in order to apply the correct deallocation: free(), delete or array delete (operator delete[]).

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