Questions tagged [virtual-functions]
In object-oriented programming, a virtual function or virtual method is a function or method whose behaviour can be overridden within an inheriting class by a function with the same signature. This concept is a very important part of the polymorphism portion of object-oriented programming (OOP).
virtual-functions
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What is the difference between an abstract method and a virtual method?
What is the difference between an abstract method and a virtual method? In which cases is it recommended to use abstract or virtual methods? Which one is the best approach?
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Why do we need virtual functions in C++?
From what I've read, virtual functions are functions in the base class that you can override in its derived classes.
But earlier, when learning about basic inheritance, I was able to override base ...
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answers
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Virtual member call in a constructor
I'm getting a warning from ReSharper about a call to a virtual member from my objects constructor.
Why would this be something not to do?
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answers
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Can I call a base class's virtual function if I'm overriding it?
Say I have classes Foo and Bar set up like this:
class Foo
{
public:
int x;
virtual void printStuff()
{
std::cout << x << std::endl;
}
};
class Bar : public Foo
{...
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389
votes
15
answers
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views
Can a class member function template be virtual?
I have heard that C++ class member function templates can't be virtual. Is this true?
If they can be virtual, what is an example of a scenario in which one would use such a function?
- 3,996
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votes
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answers
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views
Why is a call to a virtual member function in the constructor a non-virtual call?
Suppose I have two C++ classes:
class A
{
public:
A() { fn(); }
virtual void fn() { _n = 1; }
int getn() { return _n; }
protected:
int _n;
};
class B : public A
{
public:
B() : A() {}
...
- 6,191
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votes
23
answers
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views
277
votes
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answers
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Is the 'override' keyword just a check for a overridden virtual method?
As far as I understand, the introduction of override keyword in C++11 is nothing more than a check to make sure that the function being implemented is the overrideing of a virtual function in the ...
- 4,701
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votes
4
answers
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Difference between virtual and abstract methods [duplicate]
Here is some code from MSDN:
// compile with: /target:library
public class D
{
public virtual void DoWork(int i)
{
// Original implementation.
}
}
public abstract class E : D
{
...
- 23.5k
268
votes
9
answers
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views
C++ "virtual" keyword for functions in derived classes. Is it necessary?
With the struct definition given below...
struct A {
virtual void hello() = 0;
};
Approach #1:
struct B : public A {
virtual void hello() { ... }
};
Approach #2:
struct B : public A {
...
- 5,083
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votes
6
answers
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views
Can you write virtual functions / methods in Java?
Is it possible to write virtual methods in Java, as one would do in C++?
Or, is there a proper Java approach which you can implement that produces similar behavior? Could I please have some examples?
- 2,163
193
votes
13
answers
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Are inline virtual functions really a non-sense?
I got this question when I received a code review comment saying virtual functions need not be inline.
I thought inline virtual functions could come in handy in scenarios where functions are called ...
- 35.1k
148
votes
15
answers
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Virtual functions and performance - C++
In my class design, I use abstract classes and virtual functions extensively. I had a feeling that virtual functions affects the performance. Is this true? But I think this performance difference is ...
- 15.4k
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votes
12
answers
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How are virtual functions and vtable implemented?
We all know what virtual functions are in C++, but how are they implemented at a deep level?
Can the vtable be modified or even directly accessed at runtime?
Does the vtable exist for all classes, ...
- 343k
131
votes
9
answers
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views
What is the performance cost of having a virtual method in a C++ class?
Having at least one virtual method in a C++ class (or any of its parent classes) means that the class will have a virtual table, and every instance will have a virtual pointer.
So the memory cost is ...
- 47.5k
129
votes
8
answers
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views
Where do "pure virtual function call" crashes come from?
I sometimes notice programs that crash on my computer with the error: "pure virtual function call".
How do these programs even compile when an object cannot be created of an abstract class?
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votes
3
answers
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views
Why is a call to a shadowing non-virtual member function in the derived class not calling the base class member function?
Let's assume this scenario in Visual C++ 2010:
#include <iostream>
using namespace std;
struct Base {
void Display() {
cout << "Base: Non-virtual display." << ...
- 1,406
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votes
8
answers
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views
How to implement virtual methods in Python?
I know virtual methods from PHP or Java.
How can they be implemented in Python?
Or have I to define an empty method in an abstract class and override it?
- 14.3k
119
votes
6
answers
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views
Why are C# interface methods not declared abstract or virtual?
C# methods in interfaces are declared without using the virtual keyword, and overridden in the derived class without using the override keyword.
Is there a reason for this? I assume that it is just ...
- 180k
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votes
12
answers
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views
When should you not use virtual destructors?
Is there ever a good reason to not declare a virtual destructor for a class? When should you specifically avoid writing one?
- 6,950
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votes
10
answers
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views
Why C# implements methods as non-virtual by default?
Unlike Java, why does C# treat methods as non-virtual functions by default? Is it more likely to be a performance issue rather than other possible outcomes?
I am reminded of reading a paragraph from ...
- 9,163
105
votes
10
answers
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views
Safely override C++ virtual functions
I have a base class with a virtual function and I want to override that function in a derived class. Is there some way to make the compiler check if the function I declared in the derived class ...
- 226k
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votes
3
answers
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C++ virtual function return type
Is it possible for an inherited class to implement a virtual function with a different return type (not using a template as return)?
- 10.9k
79
votes
5
answers
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views
virtual assignment operator C++
Assignment Operator in C++ can be made virtual. Why is it required? Can we make other operators virtual too?
- 5,739
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votes
1
answer
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views
Should I default virtual destructors?
I have an abstract class that is declared as follow:
class my_type {
public:
virtual ~my_type() = default;
virtual void do_something() = 0;
};
Is it considered good practice to declare the ...
- 19.9k
76
votes
8
answers
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views
Should I mark all methods virtual?
In Java you can mark method as final to make it impossible to override.
In C# you have to mark method as virtual to make it possible to override.
Does it mean that in C# you should mark all methods ...
- 3,915
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votes
5
answers
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views
Does final imply override?
As I understand it, the override keyword states that a given declaration implements a base virtual method, and the compilation should fail if there is no matching base method found.
My understanding ...
- 22.1k
74
votes
11
answers
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views
What's the point of a final virtual function?
Wikipedia has the following example on the C++11 final modifier:
struct Base2 {
virtual void f() final;
};
struct Derived2 : Base2 {
void f(); // ill-formed because the virtual function ...
- 260k
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votes
2
answers
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views
Can we have a static virtual functions? If not, then WHY? [duplicate]
Possible Duplicate:
C++ static virtual members?
Can we have a static virtual functions? If not, then WHY?
class X
{
public:
virtual static void fun(){} // Why we cant have static virtual ...
- 1,877
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votes
7
answers
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views
Overriding public virtual functions with private functions in C++
Is there is any reason to make the permissions on an overridden C++ virtual function different from the base class? Is there any danger in doing so?
For example:
class base {
public:
...
- 1,500
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votes
10
answers
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views
Practical usage of virtual functions in c#
What 's the practical usage of virtual functions in c#?
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votes
4
answers
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C++ header file and function declaration ending in "= 0"
I have the following code inside the .h file and I'm not sure what does the assignment statement do and how is it called properly?
virtual void yield() = 0;
I thought that the function returns a ...
- 601
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votes
11
answers
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views
Why not have all the functions as virtual in C++?
I know that virtual functions have an overhead of dereferencing to call a method. But I guess with modern architectural speed it is almost negligible.
Is there any particular reason why all ...
- 6,575
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votes
7
answers
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Use-cases of pure virtual functions with body?
I recently came to know that in C++ pure virtual functions can optionally have a body.
What are the real-world use cases for such functions?
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votes
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answers
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views
What is the first (int (*)(...))0 vtable entry in the output of g++ -fdump-class-hierarchy?
For this code:
class B1{
public:
virtual void f1() {}
};
class D : public B1 {
public:
void f1() {}
};
int main () {
B1 *b1 = new B1();
D *d = new D();
return 0;
}
After ...
- 22.4k
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votes
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answers
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views
Performance penalty for working with interfaces in C++?
Is there a runtime performance penalty when using interfaces (abstract base classes) in C++?
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votes
11
answers
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views
Alternative virtual function calls implementations?
C++ supports dynamic binding through virtual mechanism. But as I understand the virtual mechanism is an implementation detail of the compiler and the standard just specifies the behaviors of what ...
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votes
3
answers
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views
Why do we need to use virtual ~A() = default; instead of virtual ~A() {} in C++11?
In Stack Overflow post Checking the object type in C++11, I have the comment:
In C++11 you'll actually want to do virtual ~A() = default; Otherwise, you'll lose the implict move constructors.
What ...
- 186k
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votes
2
answers
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views
Avoiding the overhead of C# virtual calls
I have a few heavily optimized math functions that take 1-2 nanoseconds to complete. These functions are called hundreds of millions of times per second, so call overhead is a concern, despite the ...
- 1,492
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votes
4
answers
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views
Why use virtual functions? [duplicate]
Possible Duplicate:
Can someone explain C++ Virtual Methods?
I have a question regarding to the C++ virtual functions.
Why and when do we use virtual functions? Can anyone give me a real time ...
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votes
3
answers
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views
What if I don't heed the warning "hides inherited member. To make the current member override that implementation...."
This is maybe a fine point, but it concerns the warning that the compiler issues if you do something like:
class A
{
public virtual void F() { }
}
class B : A
{
public void F() { }
}
Then ...
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votes
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answers
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Ways to detect whether a C++ virtual function has been redefined in a derived class
In brief: From a C++ base-class pointer which points to an instance of a derived class, how can one determine at run-time whether a non-pure virtual function (with an implementation in the base class) ...
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votes
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answers
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Can virtual functions be constexpr?
Can virtual functions like X::f() in the following code
struct X
{
constexpr virtual int f() const
{
return 0;
}
};
be constexpr?
- 23.2k
40
votes
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answers
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Template or abstract base class?
If I want to make a class adaptable, and make it possible to select different algorithms from the outside -- what is the best implementation in C++?
I see mainly two possibilities:
Use an abstract ...
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votes
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answers
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views
Can you cache a virtual function lookup in C++?
Say I have a virtual function call foo() on an abstract base class pointer, mypointer->foo(). When my app starts up, based on the contents of a file, it chooses to instantiate a particular concrete ...
- 21.2k
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votes
4
answers
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views
Performance of Expression.Compile vs Lambda, direct vs virtual calls
I'm curious how performant the Expression.Compile is versus lambda expression in the code and versus direct method usage, and also direct method calls vs virtual method calls (pseudo code):
var foo = ...
- 9,692
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votes
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answer
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views
How has CPU architecture evolution affected virtual function call performance?
Years ago I was learning about x86 assembler, CPU pipelining, cache misses, branch prediction, and all that jazz.
It was a tale of two halves. I read about all the wonderful advantages of the lengthy ...
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votes
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answers
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Return Type Covariance with Smart Pointers
In C++ we can do this:
struct Base
{
virtual Base* Clone() const { ... }
virtual ~Base(){}
};
struct Derived : Base
{
virtual Derived* Clone() const {...} //overrides Base::Clone
};
...
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votes
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answers
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views
What are the differences between overriding virtual functions and hiding non-virtual functions?
Given the following code fragment, what are the differences in the function calls? What is function hiding? What is function overriding? How do they relate to function overloads? What is the ...
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votes
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overriding protected internal with protected!
This is an extension for this question asked an hour ago.
We cannot modify the access modifiers, when overriding a virtual method in derived class. Consider Control class in System.Web.UI namespace
...
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