Menu
Is there any real reason not to make a member function virtual in C++? Of course, there's always the performance argument, but that doesn't seem to stick in most situations since the overhead of virtual functions is fairly low.
On the other hand, I've been bitten a couple of times with forgetting to make a function virtual that should be virtual. And that seems to be a bigger argument than the performance one. So is there any reason not to make member functions virtual by default?
446
15 Answers. Show activity on this post. Calling virtual functions from a constructor or destructor is dangerous and should be avoided whenever possible. All C++ implementations should call the version of the function defined at the level of the hierarchy in the current constructor and no further.
A virtual function is a member function that you expect to be redefined in derived classes. When you refer to a derived class object using a pointer or a reference to the base class, you can call a virtual function for that object and execute the derived class's version of the function.
We use virtual functions to ensure that the correct function is called for an object, regardless of the reference type used to call the function. They are basically used to achieve the runtime polymorphism and are declared in the base class by using the virtual keyword before the function.
Why are member functions not virtual by default? ¶ Δ Because many classes are not designed to be used as base classes.
One way to read your questions is "Why doesn't C++ make every function virtual by default, unless the programmer overrides that default." Without consulting my copy of "Design and Evolution of C++": this would add extra storage to every class unless every member function is made non-virtual. Seems to me this would have required more effort in the compiler implementation, and slowed down the adoption of C++ by providing fodder to the performance obsessed (I count myself in that group.)
Another way to read your questions is "Why do C++ programmers do not make every function virtual unless they have very good reasons not to?" The performance excuse is probably the reason. Depending on your application and domain, this might be a good reason or not. For example, part of my team works in market data ticker plants. At 100,000+ messages/second on a single stream, the virtual function overhead would be unacceptable. Other parts of my team work in complex trading infrastructure. Making most functions virtual is probably a good idea in that context, as the extra flexibility beats the micro-optimization.
172
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
