Sessions on advanced C++ programming

A.3. Constructors and destructors

Topics

• Role of contructor
  In object-oriented languages constructors have the role to initialize newly created objects. Different styles are used including factory methods. Structures/classes without constructors can be instantiated without constructor call. If at least one constructor is defined for a class, objects must be created through one of the constructors.
• Constructor as conversion operator
  Constructors have a secondary role in C++: in can be used as conversion operator from their parametes(s) to the class type. This feature is frequently used the save coding effort. Sometime however this automatic conversion is dangerous: we can use the explicit keyword to forbid constructors as conversions.
• Creation and destruction time
  Objects in C++ could be created in many different ways: there are named automatic, free store, nonstatic member, array element, local static, global and temporary objects. The time of creation and destruction vary of this differend type of objects.
• Creation of subobjects
  Base class objects and data members are called subobjects. Subobjects are created during the initialisation process of the object, in order of inheritance declaration and attribute declaration. The initialization list of constructor is the place to provide parameters for subobjects. If initialization list contradicts the declaration order than it will be rearranged: the declaration list is the relevant.
• Creation of statics
  The global static, namespace and static member objects are created at the beginning of the program, and destroyed at its end. In practice we can take their constructor running time at the very beginning of the main function, and - in normal execution - the running time of destructor is the end of main. On abnormal termination however the destructors are not guaranteed to run. The local static variables are allocated on global store, but their construction are delayed until the controll first reaches the declaration.
• Arrays
  Array elements are created when the array object itself starts to live. The elements are constructed from the lower indices to the upper one. There might be problem if the element class has no default constructor, since no way to pass constructor parameters to array elemenst. However the copy constructor can be used to initialized the array. Be care: arrays are not polymorphic!.
• Temporaries
  Temporary objects are created in part of expressions. Their life expands until the evolution of the full expression. Most cases there is no problem with temporaries, but there might be extreme cases, where special care is needed.
• "Virtual" constructor and destructor
  Polymorphic classes most cases require virtual destructor. However there is no virtual constructor, we can simulate it with clone function.
• Objects with special storage
  We can restrict the existence of objects regarding their storage class. The solution is tricky, but sometime can be usefull the create classes with such restrictions.
• Optimizing (copy) constructor
  The "normal" way to create copy constructor and operation= is to immediatelly copy the objects. In real life implementation this would be very inefficient: too many copy operations needed. We can optimize the operations using different startegies. One of the most widely used startegy is the reference counting, which delays copy operations until it is neccessary.