C calling base class constructors

Question

include  lt iostream gt   include  lt stdio h gt   using namespace std      Base class class Shape       public        void setWidth int w                   width   w                void setHeight int h                   height   h                Shape               printf  creating shape  n                  Shape int h int w               height   h           width   w           printf  creatig shape with attributes n                protected        int width        int height         Derived class class Rectangle  public Shape      public        int getArea                     return  width   height                  Rectangle                printf  creating rectangle  n                  Rectangle int h int w               printf  creating rectangle with attributes  n         height   h           width   w              int main void       Rectangle Rect      Rect setWidth 5      Rect setHeight 7       Rectangle  square   new Rectangle 5 5         Print the area of the object     cout  lt  lt   Total area     lt  lt  Rect getArea    lt  lt  endl      return 0      The output of the program is given below  creating shape  creating rectangle  creating shape  creating rectangle with attributes  Total area  35   When constructing both the derived class objects I see that it is always the default constructor of the base class that is called first  Is there a reason for this  Is this the reason why languages like python insist on explicit calls of base class constructors rather than implicit calls like C

User · Answer

Why the base class  default constructor is called  Turns out it s not always be the case  Any constructor of the base class  with different signatures  can be invoked from the derived class  constructor  In your case  the default constructor is called because it has no parameters so it s default   When a derived class is created  the order the constructors are called is always Base -  Derived in the hierarchy  If we have   class A      class B   A       class C   B       C c    When c is create  the constructor for A is invoked first  and then the constructor for B  and then the constructor for C   To guarantee that order  when a derived class  constructor is called  it always invokes the base class  constructor before the derived class  constructor can do anything else  For that reason  the programmer can manually invoke a base class  constructor in the only initialisation list of the derived class  constructor  with corresponding parameters  For instance  in the following code  Derived s default constructor will invoke Base s constructor Base  Base int i  instead of the default constructor   Derived     Base 5              If there s no such constructor invoked in the initialisation list of the derived class  constructor  then the program assumes a base class  constructor with no parameters  That s the reason why a constructor with no parameters  i e  the default constructor  is invoked

User · Answer

The default class constructor is called unless you explicitly call another constructor in the derived class   the language specifies this     Rectangle int h int w      Shape h w            Will call the other base class constructor

User · Answer

In c    compiler always ensure that functions in object hierarchy are called successfully  These functions are constructors and destructors and object hierarchy means inheritance tree    According to this rule we can guess compiler will call constructors and destructors for each object in inheritance hierarchy even if we don t implement it  To perform this operation compiler will synthesize the undefined constructors and destructors for us and we name them as a default constructors and destructors Then  compiler will call default constructor of base class and then calls constructor of derived class   In your case you don t call base class constructor but compiler does that for you by calling default constructor of base class because if compiler didn t do it your derived class which is Rectangle in your example will not be complete and it might cause disaster because  maybe you will use some member function of base class in your derived class  So for the sake of safety compiler always need all constructor calls

User · Answer

The short answer for this is   because that s what the C   standard specifies    Note that you can always specify a constructor that s different from the default  like so   class Shape       Shape            default constructor   Shape int h  int w           some custom constructor       class Rectangle   public Shape     Rectangle int h  int w    Shape h  w          you can specify which base class constructor to call      The default constructor of the base class is called only if you don t specify which one to call

User · Answer

When objects are constructed  it is always first construct base class subobject  therefore  base class constructor is called first  then call derived class constructors  The reason is that derived class objects contain subobjects inherited from base class  You always need to call the base class constructor to initialze base class subobjects  We usually call the base class constructor on derived class s member initialization list  If you do not call base class constructor explicitly  the compile will call the default constructor of base class to initialize base class subobject  However  implicit call on default constructor does not necessary work at all times  for example  if base class defines a constructor that could not be called without arguments    When objects are out of scope  it will first call destructor of derived class then call destructor of base class

User · Answer

Imagine it like this  When your sub-class inherits properties from a super-class  they don t magically appear  You still have to construct the object  So  you call the base constructor  Imagine if you class inherits a variable  which your super-class constructor initializes to an important value  If we didn t do this  your code could fail because the variable wasn t initialized

[c++] C++ calling base class constructors

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