What a Constructor Does

A constructor performs various tasks that are not visible to you as the programmer, even if you write no code for the constructor. These tasks are all associated with building a complete and correct instance of class type.

In Microsoft C++ (and some other implementations of C++), a constructor:

• Initializes the object’s virtual base pointer(s) (vbptr). This step is performed if the class is derived from virtual base classes.
  
  
• Calls base class and member constructors in the order of declaration.
  
  
• Initializes the object’s virtual function pointers (vfptr). This step is performed if the class has or inherits virtual functions. Virtual function pointers point to the class’s virtual function table (v-table) and allow correct binding of virtual function calls to code.
  
  
• Executes optional code in the body of the constructor function.

When the constructor is finished, the allocated memory is an object of a given class type. Because of the steps the constructor performs, “late binding” in the form of virtual functions can be resolved at the point of a virtual function call. The constructor has also constructed base classes and has constructed composed objects (objects included as data members). Late binding is the mechanism by which C++ implements polymorphic behavior for objects.