virtual assignment operator C++

Question

Assignment Operator in C++ can be made virtual. Why is it required? Can we make other operators virtual too?

Touter

Answer 1

The assignment operator is not required to be made virtual.

The discussion below is about operator=, but it also applies to any operator overloading that takes in the type in question, and any function that takes in the type in question.

The below discussion shows that the virtual keyword does not know about a parameter's inheritance in regards to finding a matching function signature. In the final example it shows how to properly handle assignment when dealing with inherited types.

---

Virtual functions don't know about parameter's inheritance:

A function's signature needs to be the same for virtual to come into play. So even though in the following example, operator= is made virtual, the call will never act as a virtual function in D, because the parameters and return value of operator= are different.

The function B::operator=(const B& right) and D::operator=(const D& right) are 100% completely different and seen as 2 distinct functions.

class B
{
public:
  virtual B& operator=(const B& right)
  {
    x = right.x;
    return *this;
  }

  int x;

};

class D : public B
{
public:
  virtual D& operator=(const D& right)
  {
    x = right.x;
    y = right.y;
    return *this;
  }
  int y;
};

---

Default values and having 2 overloaded operators:

You can though define a virtual function to allow you to set default values for D when it is assigned to variable of type B. This is even if your B variable is really a D stored into a reference of a B. You will not get the D::operator=(const D& right) function.

In the below case, an assignment from 2 D objects stored inside 2 B references... the D::operator=(const B& right) override is used.

//Use same B as above

class D : public B
{
public:
  virtual D& operator=(const D& right)
  {
    x = right.x;
    y = right.y;
    return *this;
  }


  virtual B& operator=(const B& right)
  {
    x = right.x;
    y = 13;//Default value
    return *this;
  }

  int y;
};


int main(int argc, char **argv) 
{
  D d1;
  B &b1 = d1;
  d1.x = 99;
  d1.y = 100;
  printf("d1.x d1.y %i %i\n", d1.x, d1.y);

  D d2;
  B &b2 = d2;
  b2 = b1;
  printf("d2.x d2.y %i %i\n", d2.x, d2.y);
  return 0;
}

Prints:

d1.x d1.y 99 100
d2.x d2.y 99 13

Which shows that D::operator=(const D& right) is never used.

Without the virtual keyword on B::operator=(const B& right) you would have the same results as above but the value of y would not be initialized. I.e. it would use the B::operator=(const B& right)

---

One last step to tie it all together, RTTI:

You can use RTTI to properly handle virtual functions that take in your type. Here is the last piece of the puzzle to figure out how to properly handle assignment when dealing with possibly inherited types.

virtual B& operator=(const B& right)
{
  const D *pD = dynamic_cast<const D*>(&right);
  if(pD)
  {
    x = pD->x;
    y = pD->y;
  }
  else
  {
    x = right.x;
    y = 13;//default value
  }

  return *this;
}

Summary

Brian, I have found some strange behavior represented in this question: #969732. Do you have any ideas? – Rebba

I understand your arguments about the usage of virtual, but in your final piece you use 'const D *pD = dynamic_cast<const D*>(&right);', which doesn't seem correct to put in the base class. Can you explain? – Haugh

@Jake88: That isn't in the base class. It is in the derived class's override of the virtual operator= first declared in the base class. – Spinose

The easiest way to disambiguate the issue is to make the derived class's copy-assignment operator marked as "override" then the code won't compile which proves your guesses about the 2 operators (= from base and derived ) are different: class Derived : public Base{ Derived& operator=(const Derived&)override{return *this;}}; Now Derived' = operator causes the compiler search for the corresponding member in its base and of course it fails which generates an error. – Doggoned

Although we can use = polymorphically, it doesn't make sense because the derived class version must has the same signature which means it should take a reference to base not to derived: struct D : B{D& operator=(const B&)override{return *this;}}; although it compiles, it needs to cast that reference to base to derived. – Doggoned

Answer 2

It depends on the operator.

The point of making an assignment operator virtual is to allow you from the benefit of being able to override it to copy more fields.

So if you have an Base& and you actually have a Derived& as a dynamic type, and the Derived has more fields, the correct things are copied.

However, there is then a risk that your LHS is a Derived, and the RHS is a Base, so when the virtual operator runs in Derived your parameter is not a Derived and you have no way of getting fields out of it.

Here is a good discussio: http://icu-project.org/docs/papers/cpp_report/the_assignment_operator_revisited.html

Keefe

Answer 3

Brian R. Bondy wrote:

---

One last step to tie it all together, RTTI:

You can use RTTI to properly handle virtual functions that take in your type. Here is the last piece of the puzzle to figure out how to properly handle assignment when dealing with possibly inherited types.

virtual B& operator=(const B& right)
{
  const D *pD = dynamic_cast<const D*>(&right);
  if(pD)
  {
    x = pD->x;
    y = pD->y;
  }
  else
  {
    x = right.x;
    y = 13;//default value
  }

  return *this;
}

I would like to add to this solution a few remarks. Having the assignment operator declared the same as above has three issues.

The compiler generates an assignment operator that takes a const D& argument which is not virtual and does not do what you may think it does.

Second issue is the return type, you are returning a base reference to a derived instance. Probably not much of an issue as the code works anyway. Still it is better to return references accordingly.

Third issue, derived type assignment operator does not call base class assignment operator (what if there are private fields that you would like to copy?), declaring the assignment operator as virtual will not make the compiler generate one for you. This is rather a side effect of not having at least two overloads of the assignment operator to get the wanted result.

Considering the base class (same as the one from the post I quoted):

class B
{
public:
    virtual B& operator=(const B& right)
    {
        x = right.x;
        return *this;
    }

    int x;
};

The following code completes the RTTI solution that I quoted:

class D : public B{
public:
    // The virtual keyword is optional here because this
    // method has already been declared virtual in B class
    /* virtual */ const D& operator =(const B& b){
        // Copy fields for base class
        B::operator =(b);
        try{
            const D& d = dynamic_cast<const D&>(b);
            // Copy D fields
            y = d.y;
        }
        catch (std::bad_cast){
            // Set default values or do nothing
        }
        return *this;
    }

    // Overload the assignment operator
    // It is required to have the virtual keyword because
    // you are defining a new method. Even if other methods
    // with the same name are declared virtual it doesn't
    // make this one virtual.
    virtual const D& operator =(const D& d){
        // Copy fields from B
        B::operator =(d);
        // Copy D fields
        y = d.y;
        return *this;
    }

    int y;
};

This may seem a complete solution, it's not. This is not a complete solution because when you derive from D you will need 1 operator = that takes const B&, 1 operator = that takes const D& and one operator that takes const D2&. The conclusion is obvious, the number of operator =() overloads is equivalent with the number of super classes + 1.

Considering that D2 inherits D, let's take a look at how the two inherited operator =() methods look like.

class D2 : public D{
    /* virtual */ const D2& operator =(const B& b){
        D::operator =(b); // Maybe it's a D instance referenced by a B reference.
        try{
            const D2& d2 = dynamic_cast<const D2&>(b);
            // Copy D2 stuff
        }
        catch (std::bad_cast){
            // Set defaults or do nothing
        }
        return *this;
    }

    /* virtual */ const D2& operator =(const D& d){
        D::operator =(d);
        try{
            const D2& d2 = dynamic_cast<const D2&>(d);
            // Copy D2 stuff
        }
        catch (std::bad_cast){
            // Set defaults or do nothing
        }
        return *this;
    }
};

It is obvious that the operator =(const D2&) just copies fields, imagine as if it was there. We can notice a pattern in the inherited operator =() overloads. Sadly we cannot define virtual template methods that will take care of this pattern, we need to copy and paste multiple times the same code in order to get a full polymorphic assignment operator, the only solution I see. Also applies to other binary operators.

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Edit

As mentioned in the comments, the least that can be done to make life easier is to define the top-most superclass assignment operator =(), and call it from all other superclass operator =() methods. Also when copying fields a _copy method can be defined.

class B{
public:
    // _copy() not required for base class
    virtual const B& operator =(const B& b){
        x = b.x;
        return *this;
    }

    int x;
};

// Copy method usage
class D1 : public B{
private:
    void _copy(const D1& d1){
        y = d1.y;
    }

public:
    /* virtual */ const D1& operator =(const B& b){
        B::operator =(b);
        try{
            _copy(dynamic_cast<const D1&>(b));
        }
        catch (std::bad_cast){
            // Set defaults or do nothing.
        }
        return *this;
    }

    virtual const D1& operator =(const D1& d1){
        B::operator =(d1);
        _copy(d1);
        return *this;
    }

    int y;
};

class D2 : public D1{
private:
    void _copy(const D2& d2){
        z = d2.z;
    }

public:
    // Top-most superclass operator = definition
    /* virtual */ const D2& operator =(const B& b){
        D1::operator =(b);
        try{
            _copy(dynamic_cast<const D2&>(b));
        }
        catch (std::bad_cast){
            // Set defaults or do nothing
        }
        return *this;
    }

    // Same body for other superclass arguments
    /* virtual */ const D2& operator =(const D1& d1){
        // Conversion to superclass reference
        // should not throw exception.
        // Call base operator() overload.
        return D2::operator =(dynamic_cast<const B&>(d1));
    }

    // The current class operator =()
    virtual const D2& operator =(const D2& d2){
        D1::operator =(d2);
        _copy(d2);
        return *this;
    }

    int z;
};

There is no need for a set defaults method because it would receive only one call (in the base operator =() overload). Changes when copying fields are done in one place and all operator =() overloads are affected and carry their intended purpose.

Thanks sehe for the suggestion.

Poona

I think preventing default generated copy constructors is probably easiest. D& operator=(D const&) = delete;. If you must have it copy-assignable, then at least relay implementation to the virtual method for the base case. Very quickly this becomes a candidate for the Cloneable pattern, so you can use private virtuals as in GotW18 as well as being less confusing. In other words, polymorphic classes don't mesh well with value semantics. Never will. The code shows that hiding is hard. The onus is entirely upon the developer... – Geffner

That isn't sufficient because if I delete D's operator =(const D&) I won't be able to do stuff like D d1, d2; d1 = d2; – Poona

Erm. Isn't that what I said? I said, it would easiest. Over 60% of the comment text dealt with the case 'if you must have it copy-assignable'... :) – Geffner

Yea, my bad. Calling the base oprator =() does simplify things. – Poona

Answer 4

virtual assignment is used in below scenarios:

//code snippet
Class Base;
Class Child :public Base;

Child obj1 , obj2;
Base *ptr1 , *ptr2;

ptr1= &obj1;
ptr2= &obj2 ;

//Virtual Function prototypes:
Base& operator=(const Base& obj);
Child& operator=(const Child& obj);

case 1: obj1 = obj2;

In this virtual concept doesn't play any role as we call operator= on Child class.

case 2&3: *ptr1 = obj2;
                  *ptr1 = *ptr2;

Here assignment won't be as expected. Reason being operator= is called on Base class instead.

It can be rectified using either:
1) Casting

dynamic_cast<Child&>(*ptr1) = obj2;   // *(dynamic_cast<Child*>(ptr1))=obj2;`
dynamic_cast<Child&>(*ptr1) = dynamic_cast<Child&>(*ptr2)`

2) Virtual concept

Now by simply using virtual Base& operator=(const Base& obj) won't help as signatures are different in Child and Base for operator=.

We need to add Base& operator=(const Base& obj) in Child class along with its usual Child& operator=(const Child& obj) definition. Its important to include later definition, as in the absence of that default assignment operator will be called.(obj1=obj2 might not give desired result)

Base& operator=(const Base& obj)
{
    return operator=(dynamic_cast<Child&>(const_cast<Base&>(obj)));
}

case 4: obj1 = *ptr2;

In this case compiler looks for operator=(Base& obj) definition in Child as operator= is called on Child. But since its not present and Base type can't be promoted to child implicitly, it will through error.(casting is required like obj1=dynamic_cast<Child&>(*ptr1);)

If we implement according to case2&3, this scenario will be taken care of.

As it can be seen virtual assignment makes call more elegant in case of assignments using Base class pointers/reference .

Can we make other operators virtual too? Yes

Dun

Thank you for this answer. I found it precise and clear, which helped me solve the problem to my friend's c++ assignment. :) – Haugh

In your sample code for (2), wouldn't it make more sense to use dynamic_cast<const Child &>(obj) instead of dynamic_cast<Child&>(const_cast<Base&>(obj))? – Athirst

Promotion is for builtin types (short to int...). – Does

Answer 5

It's required only when you want to guarantee that classes derived from your class get all of their members copied correctly. If you aren't doing anything with polymorphism, then you don't really need to worry about this.

I don't know of anything that would prevent you from virtualizing any operator that you want--they're nothing but special case method calls.

This page provides an excellent and detailed description of how all this works.

Clinkerbuilt

There are a few errors on that page. The code which he uses as an example of slicing doesn't actually slice. And that's ignoring the fact that the assigned is illegal anyway (const/non-const mismatch). – Edmonton