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Assignment operator & inheritance

The Comet · 2013-11-20T09:04:45

Consider the following code: #include <iostream> class Base { protected: Base() {} Base( const Base& that ) { *this = that; } ~Base() {} public: Base& operator=( const Base& that ) { std::cout << "base assignment" << std::endl; return *this; } }; class Derived : public Base { public: Derived() {} Derived( const Derived& that ) : Base( that ) { *this = that; } ~Derived() {} Derived& operator=( const Base& that ) { std::cout << "derived assignment" << std::endl; return *this; } }; int main() { Derived test1, test2; std::cout << "derived = derived" << std::endl; test1 = test2; std::cout << std::endl; std::cout << "base = derived" << std::endl; *static_cast<Base*>( &test1 ) = test2; std::cout << std::endl; std::cout << "derived = base" << std::endl; test1 = *static_cast<Base*>( &test2 ); std::cout << std::endl; std::cout << "base = base" << std::endl; *static_cast<Base*>( &test1 ) = *static_cast<Base*>( &test2 ); return 0; } A few questions: I noticed that operator overloads aren't overridden like normal methods. For instance, if I were to declare virtual Base& operator=( const Base& that ) in the base class, and try to override that in the derived class with Base& operator=( const Base& that ), then were to assign two derived objects, the base overload is still called rather than the derived overload. However, if I assign a base to derived, derived to base, or base to base, the operator overload in the derived class is called. Can someone explain this behavior? Taking the example from question 1., if I remove the "virtual" keyword from the base declaration of the overload, it behaves differently. Only in the case of assigning a base object to a derived object is the derived operator called. In all other cases, the base operator is called. Can someone explain this behavior? Is there a way for both overloads (in the base and derived class) to be called when an assignment occurs? Thanks!

General and Gameplay Programming Programming

Started by TheComet November 18, 2013 10:16 AM

10 comments, last by Hodgman 10 years, 5 months ago

TheComet

3,927

Author

November 20, 2013 08:49 AM

If the derived class "Node" did require an explicit overload, it would be for an assignment operator that takes an argument of the derived type, not the base type. Or perhaps I didn't fully understand your situation?

No you're right, there are probably not many cases where a derived type is assigned to a base type. I was simply being hypothetical.

As a side note, the standard way to do that is with pure virtual functions. If there's no other function that needs to be pure then make the destructor pure virtual. A class that people inherit from should generally have a virtual destructor anyway - you can run in to trouble when deleting an instance of the class if it's not.

I didn't know you could make the destructor pure virtual, thanks for the tip!

"I would try to find halo source code by bungie best fps engine ever created, u see why call of duty loses speed due to its detail." -- GettingNifty

Hodgman

52,718

November 20, 2013 09:04 AM

Most of the time, I wouldn't thing that assignment would make sense with polymorphic types.

If B and C are implementations of the A interface, what does it mean to ask a B object to take on the values from a C object? They've got different internal representations, so can't be assigned...

I'd recommend looking into the noncopyable class (or other ways of disabling assignment) -- many polymorphic types are probably naturally non-copyable.
I actually make pretty much all of my classes non-copyable to begin with, until I actually need to be able to clone them ;)

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Assignment operator & inheritance

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