CODEWITHSUNDEEP
c++
TryinHard
TryinHard

Reputation: 4118

Getting the redefinition error for virtual destructor

I've the following C++ snippet:

#include <iostream>

using namespace std;

class A
{
public:
    virtual ~A()
    {
    }
};

A::~A()
{
    
}

int main(int argc, char * argv [])
{
    return 0;
}

Why am I getting these errors?:

error: redefinition of 'A::~A()' A::~A()

error: 'virtual A::~A()' previously defined here

 virtual ~A()**

Upvotes: 3

Views: 1972

Answers (4)

Marco A.
Marco A.

Reputation: 43662

There's a difference between declaration and definition. C++ follows the ODR - One Definition Rule, that is: you need to define stuff only ONCE.

  • Declaration = you let the world know something exists and how the parameters/return types are

  • Definition = you write the actual code for that function/class/object/whatever

In your code you have TWO definitions and one declaration. That violates the ODR:

#include <iostream>

using namespace std;

class A
{
public:
    virtual ~A() < - declares a virtual destructor
    { <- defines the code (nothing) for the virtual destructor
    }
};

A::~A() <- defines the code AGAIN for the virtual destructor of the class A
{

}

int main(int argc, char * argv [])
{
    return 0;
}

This is not allowed and thus the error. If the defines were:

#include <iostream>

using namespace std;

class A
{
public:
    virtual ~A()
    { 
       printf("World?");
    }
};

A::~A()
{
   printf("Hello?");
}

The compiler wouldn't know if you wanted to print "Hello?" or "World?".

The virtual keyword doesn't help here since it only says: "if a class derives from this one and has a destructor, use polymorphism". It's out-of-topic talking about polymorphism here so I'll let it go.

Upvotes: 3

Vlad from Moscow
Vlad from Moscow

Reputation: 310950

This is a definition of the destructor inside the class definition

class A
{
    public:

               virtual ~A()
                     {
                      }
    };

and this is also the destructor definition outside the class definition.

 A::~A()
{

}

So you defined the destructor twice and the compiler reports about this mistake.

You could define the destructor inside the class the following way

class A
{
    public:

               virtual ~A() = default;
};

Upvotes: 1

mathematician1975
mathematician1975

Reputation: 21351

Simply use the following in your class

virtual ~A();

instead of

virtual ~A()
             {
              }

The compiler is actually telling you exactly what the problem is here. You have two implementations - one inline in your class and another outside it here

A::~A()
{

}

you cannot have both.

Upvotes: 6

Wojtek Surowka
Wojtek Surowka

Reputation: 20993

You are defining the destructor twice - once inline in the class, once outside. You need e.g. to change the inline definition to declaration only:

virtual ~A();

Upvotes: 1

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