I have a solid understanding of most OOP theory but the one thing that confuses me a lot is virtual destructors.
I thought that the destructor always gets called no matter what and for every object in the chain.
When are you meant to make them virtual and why?
This question is related to
c++
polymorphism
shared-ptr
virtual-destructor
Virtual base class destructors are "best practice" - you should always use them to avoid (hard to detect) memory leaks. Using them, you can be sure all destructors in the inheritance chain of your classes are beeing called (in proper order). Inheriting from a base class using virtual destructor makes the destructor of the inheriting class automatically virtual, too (so you do not have to retype 'virtual' in the inheriting class destructor declaration).
I thought it would be beneficial to discuss the "undefined" behavior, or at least the "crash" undefined behavior that may occur when deleting through a base class(/struct) without a virtual destructor, or more precisely no vtable. The code below list a few simple structs (the same would be true for classes).
#include <iostream>
using namespace std;
struct a
{
~a() {}
unsigned long long i;
};
struct b : a
{
~b() {}
unsigned long long j;
};
struct c : b
{
~c() {}
virtual void m3() {}
unsigned long long k;
};
struct d : c
{
~d() {}
virtual void m4() {}
unsigned long long l;
};
int main()
{
cout << "sizeof(a): " << sizeof(a) << endl;
cout << "sizeof(b): " << sizeof(b) << endl;
cout << "sizeof(c): " << sizeof(c) << endl;
cout << "sizeof(d): " << sizeof(d) << endl;
// No issue.
a* a1 = new a();
cout << "a1: " << a1 << endl;
delete a1;
// No issue.
b* b1 = new b();
cout << "b1: " << b1 << endl;
cout << "(a*) b1: " << (a*) b1 << endl;
delete b1;
// No issue.
c* c1 = new c();
cout << "c1: " << c1 << endl;
cout << "(b*) c1: " << (b*) c1 << endl;
cout << "(a*) c1: " << (a*) c1 << endl;
delete c1;
// No issue.
d* d1 = new d();
cout << "d1: " << d1 << endl;
cout << "(c*) d1: " << (c*) d1 << endl;
cout << "(b*) d1: " << (b*) d1 << endl;
cout << "(a*) d1: " << (a*) d1 << endl;
delete d1;
// Doesn't crash, but may not produce the results you want.
c1 = (c*) new d();
delete c1;
// Crashes due to passing an invalid address to the method which
// frees the memory.
d1 = new d();
b1 = (b*) d1;
cout << "d1: " << d1 << endl;
cout << "b1: " << b1 << endl;
delete b1;
/*
// This is similar to what's happening above in the "crash" case.
char* buf = new char[32];
cout << "buf: " << (void*) buf << endl;
buf += 8;
cout << "buf after adding 8: " << (void*) buf << endl;
delete buf;
*/
}
I'm not suggesting whether you need virtual destructors or not, though I think in general it's a good practice to have them. I'm just pointing out the reason you may end up with a crash if your base class(/struct) does not have a vtable and your derived class(/struct) does and you delete an object via a base class(/struct) pointer. In this case, the address you pass to the heap's free routine is invalid and thus the reason for the crash.
If you run the above code you'll see clearly when the issue occurs. When the this pointer of the base class(/struct) is different from the this pointer of the derived class(/struct) you're going to run into this problem. In the sample above, struct a and b don't have vtables. structs c and d do have vtables. Thus an a or b pointer to a c or d object instance will be fixed up to account for the vtable. If you pass this a or b pointer to delete it will crash due to the address being invalid to the heap's free routine.
If you plan to delete derived instances which have vtables from base class pointers, you need to ensure the base class has a vtable. One way to do that is to add a virtual destructor, which you might want anyway to properly clean up resources.
I think the core of this question is about virtual methods and polymorphism, not the destructor specifically. Here is a clearer example:
class A
{
public:
A() {}
virtual void foo()
{
cout << "This is A." << endl;
}
};
class B : public A
{
public:
B() {}
void foo()
{
cout << "This is B." << endl;
}
};
int main(int argc, char* argv[])
{
A *a = new B();
a->foo();
if(a != NULL)
delete a;
return 0;
}
Will print out:
This is B.
Without virtual it will print out:
This is A.
And now you should understand when to use virtual destructors.
struct Base {
virtual void f() {}
virtual ~Base() {}
};
struct Derived : Base {
void f() override {}
~Derived() override {}
};
Base* base = new Derived;
base->f(); // calls Derived::f
base->~Base(); // calls Derived::~Derived
Virtual destructor call is no different from any other virtual function call.
For base->f(), the call will be dispatched to Derived::f(), and it's the same for base->~Base() - its overriding function - the Derived::~Derived() will be called.
Same happens when destructor is being called indirectly, e.g. delete base;. The delete statement will call base->~Base() which will be dispatched to Derived::~Derived().
If you are not going to delete object through a pointer to its base class - then there is no need to have a virtual destructor. Just make it protected so that it won't be called accidentally:
// library.hpp
struct Base {
virtual void f() = 0;
protected:
~Base() = default;
};
void CallsF(Base& base);
// CallsF is not going to own "base" (i.e. call "delete &base;").
// It will only call Base::f() so it doesn't need to access Base::~Base.
//-------------------
// application.cpp
struct Derived : Base {
void f() override { ... }
};
int main() {
Derived derived;
CallsF(derived);
// No need for virtual destructor here as well.
}
Virtual keyword for destructor is necessary when you want different destructors should follow proper order while objects is being deleted through base class pointer. for example:
Base *myObj = new Derived();
// Some code which is using myObj object
myObj->fun();
//Now delete the object
delete myObj ;
If your base class destructor is virtual then objects will be destructed in a order(firstly derived object then base ). If your base class destructor is NOT virtual then only base class object will get deleted(because pointer is of base class "Base *myObj"). So there will be memory leak for derived object.
Declare destructors virtual in polymorphic base classes. This is Item 7 in Scott Meyers' Effective C++. Meyers goes on to summarize that if a class has any virtual function, it should have a virtual destructor, and that classes not designed to be base classes or not designed to be used polymorphically should not declare virtual destructors.
Virtual base class destructors are "best practice" - you should always use them to avoid (hard to detect) memory leaks. Using them, you can be sure all destructors in the inheritance chain of your classes are beeing called (in proper order). Inheriting from a base class using virtual destructor makes the destructor of the inheriting class automatically virtual, too (so you do not have to retype 'virtual' in the inheriting class destructor declaration).
A basic definition about virtual is it determines if a member function of a class can be over-ridden in its derived classes.
A class's D-tor is called basically at the end of the scope, but there is a problem, for example when we define an instance on the Heap (dynamic allocation), we should delete it manually.
As soon as the instruction get executed, the base class destructor get called, but not for the derived one.
A Pratical example is when, in control field, you have to manipulate effectors, actuators.
At the end of the scope, if the destructor of one of the power elements (Actuator), isn't called, there will be fatal consequences.
#include <iostream>
class Mother{
public:
Mother(){
std::cout<<"Mother Ctor"<<std::endl;
}
virtual~Mother(){
std::cout<<"Mother D-tor"<<std::endl;
}
};
class Child: public Mother{
public:
Child(){
std::cout<<"Child C-tor"<<std::endl;
}
~Child(){
std::cout<<"Child D-tor"<<std::endl;
}
};
int main()
{
Mother *c = new Child();
delete c;
return 0;
}
Also be aware that deleting a base class pointer when there is no virtual destructor will result in undefined behavior. Something that I learned just recently:
How should overriding delete in C++ behave?
I've been using C++ for years and I still manage to hang myself.
Make the destructor virtual whenever your class is polymorphic.
I think the core of this question is about virtual methods and polymorphism, not the destructor specifically. Here is a clearer example:
class A
{
public:
A() {}
virtual void foo()
{
cout << "This is A." << endl;
}
};
class B : public A
{
public:
B() {}
void foo()
{
cout << "This is B." << endl;
}
};
int main(int argc, char* argv[])
{
A *a = new B();
a->foo();
if(a != NULL)
delete a;
return 0;
}
Will print out:
This is B.
Without virtual it will print out:
This is A.
And now you should understand when to use virtual destructors.
Declare destructors virtual in polymorphic base classes. This is Item 7 in Scott Meyers' Effective C++. Meyers goes on to summarize that if a class has any virtual function, it should have a virtual destructor, and that classes not designed to be base classes or not designed to be used polymorphically should not declare virtual destructors.
Also be aware that deleting a base class pointer when there is no virtual destructor will result in undefined behavior. Something that I learned just recently:
How should overriding delete in C++ behave?
I've been using C++ for years and I still manage to hang myself.
when you need to call derived class destructor from base class. you need to declare virtual base class destructor in base class.
If you use shared_ptr(only shared_ptr, not unique_ptr), you don't have to have the base class destructor virtual:
#include <iostream>
#include <memory>
using namespace std;
class Base
{
public:
Base(){
cout << "Base Constructor Called\n";
}
~Base(){ // not virtual
cout << "Base Destructor called\n";
}
};
class Derived: public Base
{
public:
Derived(){
cout << "Derived constructor called\n";
}
~Derived(){
cout << "Derived destructor called\n";
}
};
int main()
{
shared_ptr<Base> b(new Derived());
}
output:
Base Constructor Called
Derived constructor called
Derived destructor called
Base Destructor called
Make the destructor virtual whenever your class is polymorphic.
What is a virtual destructor or how to use virtual destructor
A class destructor is a function with same name of the class preceding with ~ that will reallocate the memory that is allocated by the class. Why we need a virtual destructor
See the following sample with some virtual functions
The sample also tell how you can convert a letter to upper or lower
#include "stdafx.h"
#include<iostream>
using namespace std;
// program to convert the lower to upper orlower
class convertch
{
public:
//void convertch(){};
virtual char* convertChar() = 0;
~convertch(){};
};
class MakeLower :public convertch
{
public:
MakeLower(char *passLetter)
{
tolower = true;
Letter = new char[30];
strcpy(Letter, passLetter);
}
virtual ~MakeLower()
{
cout<< "called ~MakeLower()"<<"\n";
delete[] Letter;
}
char* convertChar()
{
size_t len = strlen(Letter);
for(int i= 0;i<len;i++)
Letter[i] = Letter[i] + 32;
return Letter;
}
private:
char *Letter;
bool tolower;
};
class MakeUpper : public convertch
{
public:
MakeUpper(char *passLetter)
{
Letter = new char[30];
toupper = true;
strcpy(Letter, passLetter);
}
char* convertChar()
{
size_t len = strlen(Letter);
for(int i= 0;i<len;i++)
Letter[i] = Letter[i] - 32;
return Letter;
}
virtual ~MakeUpper()
{
cout<< "called ~MakeUpper()"<<"\n";
delete Letter;
}
private:
char *Letter;
bool toupper;
};
int _tmain(int argc, _TCHAR* argv[])
{
convertch *makeupper = new MakeUpper("hai");
cout<< "Eneterd : hai = " <<makeupper->convertChar()<<" ";
delete makeupper;
convertch *makelower = new MakeLower("HAI");;
cout<<"Eneterd : HAI = " <<makelower->convertChar()<<" ";
delete makelower;
return 0;
}
From the above sample you can see that the destructor for both MakeUpper and MakeLower class is not called.
See the next sample with the virtual destructor
#include "stdafx.h"
#include<iostream>
using namespace std;
// program to convert the lower to upper orlower
class convertch
{
public:
//void convertch(){};
virtual char* convertChar() = 0;
virtual ~convertch(){}; // defined the virtual destructor
};
class MakeLower :public convertch
{
public:
MakeLower(char *passLetter)
{
tolower = true;
Letter = new char[30];
strcpy(Letter, passLetter);
}
virtual ~MakeLower()
{
cout<< "called ~MakeLower()"<<"\n";
delete[] Letter;
}
char* convertChar()
{
size_t len = strlen(Letter);
for(int i= 0;i<len;i++)
{
Letter[i] = Letter[i] + 32;
}
return Letter;
}
private:
char *Letter;
bool tolower;
};
class MakeUpper : public convertch
{
public:
MakeUpper(char *passLetter)
{
Letter = new char[30];
toupper = true;
strcpy(Letter, passLetter);
}
char* convertChar()
{
size_t len = strlen(Letter);
for(int i= 0;i<len;i++)
{
Letter[i] = Letter[i] - 32;
}
return Letter;
}
virtual ~MakeUpper()
{
cout<< "called ~MakeUpper()"<<"\n";
delete Letter;
}
private:
char *Letter;
bool toupper;
};
int _tmain(int argc, _TCHAR* argv[])
{
convertch *makeupper = new MakeUpper("hai");
cout<< "Eneterd : hai = " <<makeupper->convertChar()<<" \n";
delete makeupper;
convertch *makelower = new MakeLower("HAI");;
cout<<"Eneterd : HAI = " <<makelower->convertChar()<<"\n ";
delete makelower;
return 0;
}
The virtual destructor will call explicitly the most derived run time destructor of class so that it will be able to clear the object in a proper way.
Or visit the link
A basic definition about virtual is it determines if a member function of a class can be over-ridden in its derived classes.
A class's D-tor is called basically at the end of the scope, but there is a problem, for example when we define an instance on the Heap (dynamic allocation), we should delete it manually.
As soon as the instruction get executed, the base class destructor get called, but not for the derived one.
A Pratical example is when, in control field, you have to manipulate effectors, actuators.
At the end of the scope, if the destructor of one of the power elements (Actuator), isn't called, there will be fatal consequences.
#include <iostream>
class Mother{
public:
Mother(){
std::cout<<"Mother Ctor"<<std::endl;
}
virtual~Mother(){
std::cout<<"Mother D-tor"<<std::endl;
}
};
class Child: public Mother{
public:
Child(){
std::cout<<"Child C-tor"<<std::endl;
}
~Child(){
std::cout<<"Child D-tor"<<std::endl;
}
};
int main()
{
Mother *c = new Child();
delete c;
return 0;
}
when you need to call derived class destructor from base class. you need to declare virtual base class destructor in base class.
Any class that is inherited publicly, polymorphic or not, should have a virtual destructor. To put another way, if it can be pointed to by a base class pointer, its base class should have a virtual destructor.
If virtual, the derived class destructor gets called, then the base class constructor. If not virtual, only the base class destructor gets called.
I like to think about interfaces and implementations of interfaces. In C++ speak interface is pure virtual class. Destructor is part of the interface and expected to implemented. Therefore destructor should be pure virtual. How about constructor? Constructor is actually not part of the interface because object is always instantiated explicitly.
Any class that is inherited publicly, polymorphic or not, should have a virtual destructor. To put another way, if it can be pointed to by a base class pointer, its base class should have a virtual destructor.
If virtual, the derived class destructor gets called, then the base class constructor. If not virtual, only the base class destructor gets called.
Declare destructors virtual in polymorphic base classes. This is Item 7 in Scott Meyers' Effective C++. Meyers goes on to summarize that if a class has any virtual function, it should have a virtual destructor, and that classes not designed to be base classes or not designed to be used polymorphically should not declare virtual destructors.
Virtual keyword for destructor is necessary when you want different destructors should follow proper order while objects is being deleted through base class pointer. for example:
Base *myObj = new Derived();
// Some code which is using myObj object
myObj->fun();
//Now delete the object
delete myObj ;
If your base class destructor is virtual then objects will be destructed in a order(firstly derived object then base ). If your base class destructor is NOT virtual then only base class object will get deleted(because pointer is of base class "Base *myObj"). So there will be memory leak for derived object.
Make the destructor virtual whenever your class is polymorphic.
To be simple, Virtual destructor is to destruct the resources in a proper order, when you delete a base class pointer pointing to derived class object.
#include<iostream>
using namespace std;
class B{
public:
B(){
cout<<"B()\n";
}
virtual ~B(){
cout<<"~B()\n";
}
};
class D: public B{
public:
D(){
cout<<"D()\n";
}
~D(){
cout<<"~D()\n";
}
};
int main(){
B *b = new D();
delete b;
return 0;
}
OUTPUT:
B()
D()
~D()
~B()
==============
If you don't give ~B() as virtual. then output would be
B()
D()
~B()
where destruction of ~D() is not done which leads to leak
What is a virtual destructor or how to use virtual destructor
A class destructor is a function with same name of the class preceding with ~ that will reallocate the memory that is allocated by the class. Why we need a virtual destructor
See the following sample with some virtual functions
The sample also tell how you can convert a letter to upper or lower
#include "stdafx.h"
#include<iostream>
using namespace std;
// program to convert the lower to upper orlower
class convertch
{
public:
//void convertch(){};
virtual char* convertChar() = 0;
~convertch(){};
};
class MakeLower :public convertch
{
public:
MakeLower(char *passLetter)
{
tolower = true;
Letter = new char[30];
strcpy(Letter, passLetter);
}
virtual ~MakeLower()
{
cout<< "called ~MakeLower()"<<"\n";
delete[] Letter;
}
char* convertChar()
{
size_t len = strlen(Letter);
for(int i= 0;i<len;i++)
Letter[i] = Letter[i] + 32;
return Letter;
}
private:
char *Letter;
bool tolower;
};
class MakeUpper : public convertch
{
public:
MakeUpper(char *passLetter)
{
Letter = new char[30];
toupper = true;
strcpy(Letter, passLetter);
}
char* convertChar()
{
size_t len = strlen(Letter);
for(int i= 0;i<len;i++)
Letter[i] = Letter[i] - 32;
return Letter;
}
virtual ~MakeUpper()
{
cout<< "called ~MakeUpper()"<<"\n";
delete Letter;
}
private:
char *Letter;
bool toupper;
};
int _tmain(int argc, _TCHAR* argv[])
{
convertch *makeupper = new MakeUpper("hai");
cout<< "Eneterd : hai = " <<makeupper->convertChar()<<" ";
delete makeupper;
convertch *makelower = new MakeLower("HAI");;
cout<<"Eneterd : HAI = " <<makelower->convertChar()<<" ";
delete makelower;
return 0;
}
From the above sample you can see that the destructor for both MakeUpper and MakeLower class is not called.
See the next sample with the virtual destructor
#include "stdafx.h"
#include<iostream>
using namespace std;
// program to convert the lower to upper orlower
class convertch
{
public:
//void convertch(){};
virtual char* convertChar() = 0;
virtual ~convertch(){}; // defined the virtual destructor
};
class MakeLower :public convertch
{
public:
MakeLower(char *passLetter)
{
tolower = true;
Letter = new char[30];
strcpy(Letter, passLetter);
}
virtual ~MakeLower()
{
cout<< "called ~MakeLower()"<<"\n";
delete[] Letter;
}
char* convertChar()
{
size_t len = strlen(Letter);
for(int i= 0;i<len;i++)
{
Letter[i] = Letter[i] + 32;
}
return Letter;
}
private:
char *Letter;
bool tolower;
};
class MakeUpper : public convertch
{
public:
MakeUpper(char *passLetter)
{
Letter = new char[30];
toupper = true;
strcpy(Letter, passLetter);
}
char* convertChar()
{
size_t len = strlen(Letter);
for(int i= 0;i<len;i++)
{
Letter[i] = Letter[i] - 32;
}
return Letter;
}
virtual ~MakeUpper()
{
cout<< "called ~MakeUpper()"<<"\n";
delete Letter;
}
private:
char *Letter;
bool toupper;
};
int _tmain(int argc, _TCHAR* argv[])
{
convertch *makeupper = new MakeUpper("hai");
cout<< "Eneterd : hai = " <<makeupper->convertChar()<<" \n";
delete makeupper;
convertch *makelower = new MakeLower("HAI");;
cout<<"Eneterd : HAI = " <<makelower->convertChar()<<"\n ";
delete makelower;
return 0;
}
The virtual destructor will call explicitly the most derived run time destructor of class so that it will be able to clear the object in a proper way.
Or visit the link
I like to think about interfaces and implementations of interfaces. In C++ speak interface is pure virtual class. Destructor is part of the interface and expected to implemented. Therefore destructor should be pure virtual. How about constructor? Constructor is actually not part of the interface because object is always instantiated explicitly.
If you use shared_ptr(only shared_ptr, not unique_ptr), you don't have to have the base class destructor virtual:
#include <iostream>
#include <memory>
using namespace std;
class Base
{
public:
Base(){
cout << "Base Constructor Called\n";
}
~Base(){ // not virtual
cout << "Base Destructor called\n";
}
};
class Derived: public Base
{
public:
Derived(){
cout << "Derived constructor called\n";
}
~Derived(){
cout << "Derived destructor called\n";
}
};
int main()
{
shared_ptr<Base> b(new Derived());
}
output:
Base Constructor Called
Derived constructor called
Derived destructor called
Base Destructor called
A virtual constructor is not possible but virtual destructor is possible. Let us experiment.......
#include <iostream>
using namespace std;
class Base
{
public:
Base(){
cout << "Base Constructor Called\n";
}
~Base(){
cout << "Base Destructor called\n";
}
};
class Derived1: public Base
{
public:
Derived1(){
cout << "Derived constructor called\n";
}
~Derived1(){
cout << "Derived destructor called\n";
}
};
int main()
{
Base *b = new Derived1();
delete b;
}
The above code output the following:
Base Constructor Called
Derived constructor called
Base Destructor called
The construction of derived object follow the construction rule but when we delete the "b" pointer(base pointer) we have found that only the base destructor is called. But this must not happen. To do the appropriate thing, we have to make the base destructor virtual. Now let see what happens in the following:
#include <iostream>
using namespace std;
class Base
{
public:
Base(){
cout << "Base Constructor Called\n";
}
virtual ~Base(){
cout << "Base Destructor called\n";
}
};
class Derived1: public Base
{
public:
Derived1(){
cout << "Derived constructor called\n";
}
~Derived1(){
cout << "Derived destructor called\n";
}
};
int main()
{
Base *b = new Derived1();
delete b;
}
The output changed as following:
Base Constructor Called
Derived Constructor called
Derived destructor called
Base destructor called
So the destruction of the base pointer (which takes an allocation on derived object!) follows the destruction rule, i.e first the Derived, then the Base. On the other hand, there is nothing like a virtual constructor.
Declare destructors virtual in polymorphic base classes. This is Item 7 in Scott Meyers' Effective C++. Meyers goes on to summarize that if a class has any virtual function, it should have a virtual destructor, and that classes not designed to be base classes or not designed to be used polymorphically should not declare virtual destructors.
I thought it would be beneficial to discuss the "undefined" behavior, or at least the "crash" undefined behavior that may occur when deleting through a base class(/struct) without a virtual destructor, or more precisely no vtable. The code below list a few simple structs (the same would be true for classes).
#include <iostream>
using namespace std;
struct a
{
~a() {}
unsigned long long i;
};
struct b : a
{
~b() {}
unsigned long long j;
};
struct c : b
{
~c() {}
virtual void m3() {}
unsigned long long k;
};
struct d : c
{
~d() {}
virtual void m4() {}
unsigned long long l;
};
int main()
{
cout << "sizeof(a): " << sizeof(a) << endl;
cout << "sizeof(b): " << sizeof(b) << endl;
cout << "sizeof(c): " << sizeof(c) << endl;
cout << "sizeof(d): " << sizeof(d) << endl;
// No issue.
a* a1 = new a();
cout << "a1: " << a1 << endl;
delete a1;
// No issue.
b* b1 = new b();
cout << "b1: " << b1 << endl;
cout << "(a*) b1: " << (a*) b1 << endl;
delete b1;
// No issue.
c* c1 = new c();
cout << "c1: " << c1 << endl;
cout << "(b*) c1: " << (b*) c1 << endl;
cout << "(a*) c1: " << (a*) c1 << endl;
delete c1;
// No issue.
d* d1 = new d();
cout << "d1: " << d1 << endl;
cout << "(c*) d1: " << (c*) d1 << endl;
cout << "(b*) d1: " << (b*) d1 << endl;
cout << "(a*) d1: " << (a*) d1 << endl;
delete d1;
// Doesn't crash, but may not produce the results you want.
c1 = (c*) new d();
delete c1;
// Crashes due to passing an invalid address to the method which
// frees the memory.
d1 = new d();
b1 = (b*) d1;
cout << "d1: " << d1 << endl;
cout << "b1: " << b1 << endl;
delete b1;
/*
// This is similar to what's happening above in the "crash" case.
char* buf = new char[32];
cout << "buf: " << (void*) buf << endl;
buf += 8;
cout << "buf after adding 8: " << (void*) buf << endl;
delete buf;
*/
}
I'm not suggesting whether you need virtual destructors or not, though I think in general it's a good practice to have them. I'm just pointing out the reason you may end up with a crash if your base class(/struct) does not have a vtable and your derived class(/struct) does and you delete an object via a base class(/struct) pointer. In this case, the address you pass to the heap's free routine is invalid and thus the reason for the crash.
If you run the above code you'll see clearly when the issue occurs. When the this pointer of the base class(/struct) is different from the this pointer of the derived class(/struct) you're going to run into this problem. In the sample above, struct a and b don't have vtables. structs c and d do have vtables. Thus an a or b pointer to a c or d object instance will be fixed up to account for the vtable. If you pass this a or b pointer to delete it will crash due to the address being invalid to the heap's free routine.
If you plan to delete derived instances which have vtables from base class pointers, you need to ensure the base class has a vtable. One way to do that is to add a virtual destructor, which you might want anyway to properly clean up resources.
To be simple, Virtual destructor is to destruct the resources in a proper order, when you delete a base class pointer pointing to derived class object.
#include<iostream>
using namespace std;
class B{
public:
B(){
cout<<"B()\n";
}
virtual ~B(){
cout<<"~B()\n";
}
};
class D: public B{
public:
D(){
cout<<"D()\n";
}
~D(){
cout<<"~D()\n";
}
};
int main(){
B *b = new D();
delete b;
return 0;
}
OUTPUT:
B()
D()
~D()
~B()
==============
If you don't give ~B() as virtual. then output would be
B()
D()
~B()
where destruction of ~D() is not done which leads to leak
Make the destructor virtual whenever your class is polymorphic.
A virtual constructor is not possible but virtual destructor is possible. Let us experiment.......
#include <iostream>
using namespace std;
class Base
{
public:
Base(){
cout << "Base Constructor Called\n";
}
~Base(){
cout << "Base Destructor called\n";
}
};
class Derived1: public Base
{
public:
Derived1(){
cout << "Derived constructor called\n";
}
~Derived1(){
cout << "Derived destructor called\n";
}
};
int main()
{
Base *b = new Derived1();
delete b;
}
The above code output the following:
Base Constructor Called
Derived constructor called
Base Destructor called
The construction of derived object follow the construction rule but when we delete the "b" pointer(base pointer) we have found that only the base destructor is called. But this must not happen. To do the appropriate thing, we have to make the base destructor virtual. Now let see what happens in the following:
#include <iostream>
using namespace std;
class Base
{
public:
Base(){
cout << "Base Constructor Called\n";
}
virtual ~Base(){
cout << "Base Destructor called\n";
}
};
class Derived1: public Base
{
public:
Derived1(){
cout << "Derived constructor called\n";
}
~Derived1(){
cout << "Derived destructor called\n";
}
};
int main()
{
Base *b = new Derived1();
delete b;
}
The output changed as following:
Base Constructor Called
Derived Constructor called
Derived destructor called
Base destructor called
So the destruction of the base pointer (which takes an allocation on derived object!) follows the destruction rule, i.e first the Derived, then the Base. On the other hand, there is nothing like a virtual constructor.
struct Base {
virtual void f() {}
virtual ~Base() {}
};
struct Derived : Base {
void f() override {}
~Derived() override {}
};
Base* base = new Derived;
base->f(); // calls Derived::f
base->~Base(); // calls Derived::~Derived
Virtual destructor call is no different from any other virtual function call.
For base->f(), the call will be dispatched to Derived::f(), and it's the same for base->~Base() - its overriding function - the Derived::~Derived() will be called.
Same happens when destructor is being called indirectly, e.g. delete base;. The delete statement will call base->~Base() which will be dispatched to Derived::~Derived().
If you are not going to delete object through a pointer to its base class - then there is no need to have a virtual destructor. Just make it protected so that it won't be called accidentally:
// library.hpp
struct Base {
virtual void f() = 0;
protected:
~Base() = default;
};
void CallsF(Base& base);
// CallsF is not going to own "base" (i.e. call "delete &base;").
// It will only call Base::f() so it doesn't need to access Base::~Base.
//-------------------
// application.cpp
struct Derived : Base {
void f() override { ... }
};
int main() {
Derived derived;
CallsF(derived);
// No need for virtual destructor here as well.
}
Source: Stackoverflow.com