Creating a class object in c

Question

First i am from JAVA  In java we create class object like this  Example example new Example     The Example class can have constructor or cannot have constructor  I can use the same in c   like this Example  example new Example     Where constructor is compulsory  From this tutorial http   www cplusplus com doc tutorial classes  I got that we can create object like this  Example example   Which do not require an constructor  I have two questions   What is the difference between both the way of creating class objects   If I am creating object like Example example  how to use that in an singleton class    like I usually do like this  Sample  Singleton  get sample         if  sample    NULL            sample   new Sample              return sample     Please guide me if I am wrong

User · Answer

There is two ways to make create object in c     First one is    MyClass myclass     if you don t need to call rather than default constructor     MyClass myclass 12      if you need to call constructor with parameters   Second one is    MyClass  myclass   new MyClass      if you don t need to call rather than default constructor MyClass  myclass   new MyClass 12     if you need to call constructor with parameters   In c   if you use new keyword  object will be stored in heap  it s very useful if you are using this object long time of period and if you use first method  it will be stored in stack  it can be used only short time period  Notice   if you use new keyword  remember it will return pointer value  you should declare name with    If you use second method  it doesn t delete object in the heap  you must delete by yourself using delete keyword   delete myclass

User · Answer

I can use the same in c   like this       Where constructor is compulsory  From this tutorial I got that we can create object like this       Which do not require an constructor    This is wrong  A constructor must exist in order to create an object  The constructor could be defined implicitly by the compiler under some conditions if you do not provide any  but eventually the constructor must be there if you want an object to be instantiated  In fact  the lifetime of an object is defined to begin when the constructor routine returns    From Paragraph 3 8 1 of the C  11 Standard            The lifetime of an object of type T begins when           storage with the proper alignment and size for type T is obtained  and          if the object has non-trivial initialization  its initialization is complete    Therefore  a constructor must be present      1  What is the difference between both the way of creating class objects    When you instantiate object with automatic storage duration  like this  where X is some class    X x    You are creating an object which will be automatically destroyed when it goes out of scope  On the other hand  when you do   X  x   new X      You are creating an object dynamically and you are binding its address to a pointer  This way  the object you created will not be destroyed when your x pointer goes out of scope   In Modern C    this is regarded as a dubious programming practice  although pointers are important because they allow realizing reference semantics  raw pointers are bad because they could result in memory leaks  objects outliving all of their pointers and never getting destroyed  or in dangling pointers  pointers outliving the object they point to  potentially causing Undefined Behavior when dereferenced    In fact  when creating an object with new  you always have to remember destroying it with delete   delete x    If you need reference semantics and are forced to use pointers  in C  11 you should consider using smart pointers instead   std  shared ptr lt X gt  x   std  make shared lt X gt       Smart pointers take care of memory management issues  which is what gives you headache with raw pointers  Smart pointers are  in fact  almost the same as Java or C  object references  The  almost  is necessary because the programmer must take care of not introducing cyclic dependencies through owning smart pointers      2  If i am creating object like Example example  how to use that in an singleton class    You could do something like this  simplified code    struct Example       static Example amp  instance                 static Example example          return example          private       Example           Example Example const amp     delete      Example Example amp  amp     delete      Example amp  operator    Example const amp     delete      Example amp  operator    Example amp  amp     delete

User · Answer

1 What is the difference between both the way of creating class   objects    First one is a pointer to a constructed object in heap  by new   Second one is an object that implicitly constructed   Default constructor      2 If i am creating object like Example example  how to use that in an   singleton class    It depends on your goals  easiest is put it as a member in class simply   A sample of a singleton class which has an object from Example class   class Sample        Example example   public       static inline Sample  getInstance                 if   uniqeInstance                        uniqeInstance   new Sample                    return uniqeInstance        private      Sample        virtual  Sample        Sample const Sample amp        Sample  amp operator  const Sample  amp        static Sample  uniqeInstance

User · Answer

Example example    This is a declaration of a variable named example of type Example  This will default-initialize the object which involves calling its default constructor  The object will have automatic storage duration which means that it will be destroyed when it goes out of scope   Example  example    This is a declaration of a variable named example which is a pointer to an Example  In this case  default-initialization leaves it uninitialized - the pointer is pointing nowhere in particular  There is no Example object here  The pointer object has automatic storage duration   Example  example   new Example      This is a declaration of a variable named example which is a pointer to an Example  This pointer object  as above  has automatic storage duration  It is then initialized with the result of new Example     This new expression creates an Example object with dynamic storage duration and then returns a pointer to it  So the example pointer is now pointing to that dynamically allocated object  The Example object is value-initialized which will call a user-provided constructor if there is one or otherwise initialise all members to 0   Example  example   new Example    This is similar to the previous line  The difference is that the Example object is default-initialized  which will call the default constructor of Example  or leave it uninitialized if it is not of class type    A dynamically allocated object must be deleted  probably with delete example

User · Answer

In the first case you are creating the object on the heap using new  In the second case you are creating the object on the stack  so it will be disposed of when going out of scope  In C   you ll need to delete objects on the heapexplicitly using delete when you don t Need them anymore   To call a static method from a class  do  Singleton  singleton   Singleton  get sample      in your main-function or wherever

User · Answer

First of all  both cases calls a constructor  If you write  Example  example   new Example      then you are creating an object  call the constructor and retrieve a pointer to it   If you write  Example example    The only difference is that you are getting the object and not a pointer to it  The constructor called in this case is the same as above  the default  no argument  constructor   As for the singleton question  you must simple invoke your static method by writing   Example  e   Singleton  getExample

User · Answer

Example example    Here example is an object on the stack   Example  example new Example      This could be broken into   Example  example       example new Example      Here the first statement creates a variable example which is a  pointer to Example   When the constructor is called  memory is allocated for it on the heap  dynamic allocation   It is the programmer s responsibility to free this memory when it is no longer needed   C   does not have garbage collection like java

User · Answer

1  What is the difference between both the way of creating class objects   a  pointer  Example  example new Example       you get a pointer  and when you finish it use  you have to delete it   delete example    b  Simple declaration  Example example    you get a variable  not a pointer  and it will be destroyed out of scope it was declared   2  Singleton C    This SO question may helps you

[c++] Creating a class object in c++

Examples related to c++

Examples related to class