What are the rules for calling the superclass constructor

Question

What are the C   rules for calling the superclass constructor from a subclass one   For example  I know in Java  you must do it as the first line of the subclass constructor  and if you don t  an implicit call to a no-arg super constructor is assumed - giving you a compile error if that s missing

User · Accepted Answer

Base class constructors are automatically called for you if they have no argument. If you want to call a superclass constructor with an argument, you must use the subclass's constructor initialization list. Unlike Java, C++ supports multiple inheritance (for better or worse), so the base class must be referred to by name, rather than "super()".

class SuperClass
{
    public:

        SuperClass(int foo)
        {
            // do something with foo
        }
};

class SubClass : public SuperClass
{
    public:

        SubClass(int foo, int bar)
        : SuperClass(foo)    // Call the superclass constructor in the subclass' initialization list.
        {
            // do something with bar
        }
};

More info on the constructor's initialization list here and here.

User · Answer

CDerived  CDerived     CBase       iCount 0     this is the initialisation list  You can initialise member variables here too   e g  iCount    0              construct body

User · Answer

If you have a constructor without arguments it will be called before the derived class constructor gets executed   If you want to call a base-constructor with arguments you have to explicitly write that in the derived constructor like this    class base     public    base  int arg              class derived   public base     public    derived      base  number               You cannot construct a derived class without calling the parents constructor in C    That either happens automatically if it s a non-arg C tor  it happens if you call the derived constructor directly as shown above or your code won t compile

User · Answer

Everybody mentioned a constructor call through an initialization list  but nobody said that a parent class s constructor can be called explicitly from the derived member s constructor s body  See the question  Calling a constructor of the base class from a subclass  39  constructor body  for example   The point is that if you use an explicit call to a parent class or super class constructor in the body of a derived class  this is actually just creating an instance of the parent class and it is not invoking the parent class constructor on the derived object  The only way to invoke a parent class or super class constructor on a derived class  object is through the initialization list and not in the derived class constructor body  So maybe it should not be called a  superclass constructor call   I put this answer here because somebody might get confused  as I did

User · Answer

In C   there is a concept of constructor s initialization list  which is where you can and should call the base class  constructor and where you should also initialize the data members   The initialization list comes after the constructor signature following a colon  and before the body of the constructor   Let s say we have a class A    class A   public B   public    A int a  int b  int c   private    int b   c        Then  assuming B has a constructor which takes an int  A s constructor may look like this    A  A int a  int b  int c       B a   b  b   c  c     initialization list        do something     As you can see  the constructor of the base class is called in the initialization list   Initializing the data members in the initialization list  by the way  is preferable to assigning the values for b   and c  inside the body of the constructor  because you are saving the extra cost of assignment   Keep in mind  that data members are always initialized in the order in which they are declared in the class definition  regardless of their order in the initialization list   To avoid strange bugs  which may arise if your data members depend on each other  you should always make sure that the order of the members is the same in the initialization list and the class definition   For the same reason the base class constructor must be the first item in the initialization list   If you omit it altogether  then the default constructor for the base class will be called automatically   In that case  if the base class does not have a default constructor  you will get a compiler error

User · Answer

If you simply want to pass all constructor arguments to the base-class   parent   here is a minimal example  This uses templates to forward every constructor call with 1  2 or 3 arguments to the parent class std  string  Code Live-Version  include  lt iostream gt   include  lt string gt   class ChildString  public std  string       public          template lt typename    Args gt          ChildString Args    args   std  string args                           std  cout                   lt  lt   quot  tConstructor call ChildString nArgs  quot                   lt  lt  sizeof    Args   lt  lt   quot     quot   lt  lt   this                  lt  lt  std  endl                 int main         std  cout  lt  lt   quot Check out  quot   lt  lt  std  endl      std  cout  lt  lt   quot  thttp   www cplusplus com reference string string string  quot   lt  lt  std  endl      std  cout  lt  lt   quot for available string constructors quot   lt  lt  std  endl       std  cout  lt  lt  std  endl      std  cout  lt  lt   quot Initialization  quot   lt  lt  std  endl      ChildString cs1   quot copy  2  quot         char char arr      quot from c-string  4  quot       ChildString cs2  char arr        std  string str    quot substring  3  quot       ChildString cs3  str  0  str length          std  cout  lt  lt  std  endl      std  cout  lt  lt   quot Usage  quot   lt  lt  std  endl      std  cout  lt  lt   quot  tcs1   quot   lt  lt  cs1  lt  lt  std  endl      std  cout  lt  lt   quot  tcs2   quot   lt  lt  cs2  lt  lt  std  endl      std  cout  lt  lt   quot  tcs3   quot   lt  lt  cs3  lt  lt  std  endl       return 0     Output Check out      http   www cplusplus com reference string string string  for available string constructors  Initialization      Constructor call ChildString nArgs 1   copy  2      Constructor call ChildString nArgs 1   from c-string  4      Constructor call ChildString nArgs 3   substring  3   Usage      cs1  copy  2      cs2  from c-string  4      cs3  substring  3   Update  Using Variadic Templates To generalize to n arguments and simplify         template  lt class C gt          ChildString C arg   std  string arg                        std  cout  lt  lt   quot  tConstructor call ChildString C arg    quot   lt  lt   this  lt  lt  std  endl                    template  lt class C1  class C2 gt          ChildString C1 arg1  C2 arg2   std  string arg1  arg2                        std  cout  lt  lt   quot  tConstructor call ChildString C1 arg1  C2 arg2  C3 arg3    quot   lt  lt   this  lt  lt  std  endl                    template  lt class C1  class C2  class C3 gt          ChildString C1 arg1  C2 arg2  C3 arg3   std  string arg1  arg2  arg3                        std  cout  lt  lt   quot  tConstructor call ChildString C1 arg1  C2 arg2  C3 arg3    quot   lt  lt   this  lt  lt  std  endl             to template lt typename    Args gt          ChildString Args    args   std  string args                           std  cout                   lt  lt   quot  tConstructor call ChildString nArgs  quot                   lt  lt  sizeof    Args   lt  lt   quot     quot   lt  lt   this                  lt  lt  std  endl

User · Answer

In C    the no-argument constructors for all superclasses and member variables are called for you  before entering your constructor  If you want to pass them arguments  there is a separate syntax for this called  constructor chaining   which looks like this   class Sub   public Base     Sub int x  int y      Base x   member y            Type member       If anything run at this point throws  the bases members which had previously completed construction have their destructors called and the exception is rethrown to to the caller  If you want to catch exceptions during chaining  you must use a function try block   class Sub   public Base     Sub int x  int y    try   Base x   member y             function body goes here     catch const ExceptionType  amp e        throw kaboom          Type member       In this form  note that the try block is the body of the function  rather than being inside the body of the function  this allows it to catch exceptions thrown by implicit or explicit member and base class initializations  as well as during the body of the function  However  if a function catch block does not throw a different exception  the runtime will rethrow the original error  exceptions during initialization cannot be ignored

User · Answer

If you have default parameters in your base constructor the base class will be called automatically     using namespace std   class Base       public      Base int a 1     a a          protected      int  a      class Derived   public Base     public    Derived         void printit     cout  lt  lt   a  lt  lt  endl        int main        Derived d     d printit       return 0      Output is  1

User · Answer

Nobody mentioned the sequence of constructor calls when a class derives from multiple classes  The sequence is as mentioned while deriving the classes

User · Answer

The only way to pass values to a parent constructor is through an initialization list   The initilization list is implemented with a   and then a list of classes and the values to be passed to that classes constructor   Class2  Class2 string id    Class1 id             Also remember that if you have a constructor that takes no parameters on the parent class  it will be called automatically prior to the child constructor executing

[c++] What are the rules for calling the superclass constructor?

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