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Why C++ does not have Virtual Constructors?
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chethanhb- Member Since Feb-2010 | Feb 25th, 2010
I am one of the person who has pricked my head in understanding the above question. Let me explain in my way...
If you have virtual function or (let me take just "virtual" keyword as of now), it means that this member function can be redefined by derived class. When we have a virtual function in a class, it will have vtable (virtual table). This vtable will have the address of the virtual function which is defined in derived.
Now the point is, how you invoke the virtual function which is defined in derived class with the help of object of the class?
Once you create an object of class, vptr (virtual pointer) will be created. Which inturn will be pointing to the base address of the vtable. So now it is clear that you can invoke the virtual funtion of derived class only with an object that has created.
Now coming to virtual constructor,
If we make constructor as virtual in base, it means that it could be redefined in derived. Keep in mind that constructor is invoked during object creation (object is not created yet. still it is in the status "creating". Object will create only after executing constructor part code). Assume you are trying to create object of the class which has virtual constructor. During this process constructor of the class will be invoked. It looks for virtual keyword. Now it tries to look for virtual constructor in derived. But not possible bcz there is no vptr and no vtable avaibale at this point of time. So, when object is not created, then there is no vptr. If no vptr for this object, then how the consturtor of derived is invoked?. No address of this construtor will available in vtable.
Hence there is no point in having virtual constructor.
I believe that i have cleared your doubt.Venkataramakrishna
- Mar 22nd, 2018
Whenever a class object is created this pointer is created just before initializing class constructor member values. If the constructor is virtual, you cannot create this pointer.
vikash tiwary
- May 11th, 2017
Virtual functions are used in order to invoke functions based on the type of object pointed by the pointer. But is not "invoked", it is called only once when it is declared. so, a constructor cannot be virtual.
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What is the difference between class and structure?
Structure: Initially (in C) a structure was used to bundle different type of data types together to perform a particular functionality. But C++ extended the structure to contain functions also. The major difference is that all declarations inside a structure are by default public. Class: Class is a successor of Structure. By default all the members inside the class are private.
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sumitv- Member Since Sep-2008 | Sep 4th, 2008
Structure does support inheritance.
try out the following code.
#include<iostream>
using namespace std;
struct Base
{
int A;
};
struct Derived:public Base
{
int B;
void display();
};
void Derived::display()
{
cout<<endl<<"A = "<<A<<endl;
}
int main()
{
Derived D;
D.A = 111;
D.display();
getchar();
return 0;
}
Try out private and protected inheritance as well. It works. :)
Regards,
SumitSajeed Ullah
- Dec 2nd, 2016
All data members of the class is by default private,
Whereas all data members of structure is by default public.Ashish
- Mar 24th, 2016
Beware, whatever is supported by a class is also supported by struct whether its inheritance, polymorphism, encapsulation, data hiding etc. etc. as of C++ 11. Only difference is, for a class default access specifier is private whereas for struct its public.
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Can we use static variables in file2 if they are defined in file1 ? If yes, then how ?
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Answered by: Kranthi Kiran
- Dec 2nd, 2006
We cannot use a 'static variable' declared in one file and use it in file 2.Reason: The purpose of 'static' keyword is to retain the value betwwen the function and to restrict thescope of the variable to the file.When a variable is declared as static it will be given memory in global area portion of the process segments with the variable mname prefixed with the file namefor Example: static int k; implies it is stored as "filename.k" in global segment.so if you try to use it in another file the compiler finds the variable prefix and flags out an error.
Vineet Srivastava
- Nov 13th, 2015
Syntatically you cannot as static is meant for one file access only. But still if you want - you can do it by passing it by reference to another file through some function.
shreekant
- Nov 5th, 2015
Nice explained by Kalayama "We cant have two storage specifiers for a single Variable declaration. If one needs to use the variable in multiple files, then he needs to declare it has extern." Its simple thing.
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What is the difference between an object and a class?
Classes and objects are separate but related concepts. Every object belongs to a class and every class contains one or more related objects. Ø A Class is static. All of the attributes of a class are fixed before, during, and after the execution of a program. The attributes of a class don't change. Ø The class to which an...
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What is the output of printf ("%d")
rahulsahu2455
- Oct 7th, 2011
It will take top element of stack which use for allocate memory for variables, so it ll print the value of last declared variable
shibshankar
- Sep 6th, 2011
It will print 0.
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What is abstraction?
Abstraction is of the process of hiding unwanted details from the user.
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What is polymorphism? Explain with an example?
"Poly" means "many" and "morph" means "form". Polymorphism is the ability of an object (or reference) to assume (be replaced by) or become many different forms of object. Example: function overloading, function overriding, virtual functions. Another example can be a plus ‘+’ sign, used for adding two integers or for using it to concatenate two strings.
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What is a class?
Class is a user-defined data type in C++. It can be created to solve a particular kind of problem. After creation the user need not know the specifics of the working of a class.
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Answered by: hirma
- Sep 18th, 2005
class is a userdefined datatype which consists of attributes(datamembers) and behavior(member functions).Bydefault the members of a class is private.
Nikita
- Jan 23rd, 2018
Class is a user define data type which contains the variable and function which r of two type private and public where the public class members can be accessed from any other class but the private cla...
Atul
- Dec 18th, 2017
Class is same as structure in C, but in Class we have functions
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How to create an object for a class that hasa constructor defined under privete access specifier
Prasad2008
- Jun 10th, 2008
There was little error in the code in my previous answer. We have to define a public static member function for this purpose. Here is the codeclass sample{sample* p;sample();public:static s...
Prasad2008
- Jun 10th, 2008
We have to define a public static member function for this purposeclass sample{sample* p;sample();public:static sample genObject();};sample* sample::genObject(){p=new sample;return p;}void main(){sample *p1=sample::genObject();}
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What is the difference between c&c++?
pammi_11
- May 31st, 2008
c is a procedural language. where as c++ is object oriented programming language.
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Inheritance
Which one of the following statements regarding C++ class inheritance is FALSE?
a. Inheritance promotes generic design and code reuse.b. Struct cannot be inherited in C++.
c. C++ supports multiple inheritance. d. Inheritance is a mechanism through which a subclass inherits the properties and behavior of its superclass.abhimanipal
- Feb 21st, 2010
The above answers are wrong. structs can be inherited in C++ . The only point to note is that for structs the defualt mode of inheritance for structs is public but for classes its private
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Why size of an empty function is 5?
rahilchoudhary
- Sep 28th, 2010
There is no function to calculate the size of a function.And this 5 value of an empty function is a myth.Certain working programs can even have size of 4.Regards
ajrobb
- Sep 22nd, 2010
It isn't.#include <iostream>void f() {}int main() { std::cout << sizeof f() << 'n'; // fails to compile on g++ because return is void std::cout << sizeo...
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What are Virtual Functions? How to implement virtual functions in "C"
kamranahmad
- May 26th, 2008
Hi here i have more simplified example Hope this will give you clear view and check this linkhttp://www.codersource.net/published/view/325/virtual_functions_in.aspx#include <stdio.h>#include <...
SomGollakota
- Apr 21st, 2008
Sukesh, Your response is essentially correct if we are talking about pure virtual functions. However, when we talk about virtual functions, then we perhaps need to make a distinction between regu...
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What is out put of C++ code?
cclass base
{
public:
virtual void display(int i = 10)
{
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The following code results in class eg { void ml( ) {int i = 6; for {int i=5: i< 10;i ++ } { if{i==6} continue; system.out.println {i} ; } } ; } 1. syntax / compilation error 2. 5 6 7 8 9 3. 6 7 8 9 4. 5 7 8 9
talktome.ruchi
- Sep 23rd, 2011
The correct answer is option 4, since the scope of both the i's are different so we can declare it multiple times
anabarai
- Jul 4th, 2011
If you see the for for block:
for {int i=5: i< 10;i ++ }
you will see that it has a semi colon after the int i=5 which results in a syntax error. so the answer would be 1. syntax/compilation error -
Can destructor be private?
NikunjSingh
- Feb 23rd, 2012
Destrcutors can be made private to stop user from creating objects on stack. One can still create objects on stack using a static function as member function. But if intention is not to let user crea...
mehulgala177
- Sep 22nd, 2011
YESSSS destructors can be private. Infact this concept is used in Singleton design pattern. So, outsiders cannot delete object of that class. It has to b deleted from the class itself. I know...I...
C++ Interview Questions
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