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?
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.
The following post is mostly opinion, but here goes:
Object oriented design is three things, and encapsulation (information hiding) is the first of these things. If a class design is not solid on this, then the rest doesn't really matter very much.
It has been said before that "inheritance breaks encapsulation" (Alan Snyder '86) A good discussion of this is present in the group of four design pattern book. A class should be designed to support inheritance in a very specific manner. Otherwise, you open the possibility of misuse by inheritors.
I would make the analogy that making all of your methods virtual is akin to making all your members public. A bit of a stretch, I know, but that's why I used the word 'analogy'
Stroustrup, the designer of the language, says:
Because many classes are not designed to be used as base classes. For example, see class complex.
Also, objects of a class with a virtual function require space needed by the virtual function call mechanism - typically one word per object. This overhead can be significant, and can get in the way of layout compatibility with data from other languages (e.g. C and Fortran).
See The Design and Evolution of C++ for more design rationale.
As you are designing your class hierarchy, it may make sense to write a function that should not be overridden. One example is if you are doing the "template method" pattern, where you have a public method that calls several private virtual methods. You would not want derived classes to override that; everyone should use the base definition.
There is no "final" keyword, so the best way to communicate to other developers that a method should not be overridden is to make it non-virtual. (other than easily ignored comments)
At the class level, making the destructor non-virtual communicates that the class should not be used as a base class, such as the STL containers.
Making a method non-virtual communicates how it should be used.
The Non-Virtual Interface idiom makes use of non-virtual methods. For more information please refer to Herb Sutter "Virtuality" article.
http://www.gotw.ca/publications/mill18.htm
And comments on the NVI idiom:
http://www.parashift.com/c++-faq-lite/strange-inheritance.html#faq-23.3 http://accu.org/index.php/journals/269 [See sub-section]
I'll ignore performance and memory cost, because I have no way to measure them for the "in general" case...
Classes with virtual member functions are non-POD. So if you want to use your class in low-level code which relies on it being POD, then (among other restrictions) any member functions must be non-virtual.
Examples of things you can portably do with an instance of a POD class:
Other things people have mentioned that I agree with:
Many classes are not designed for inheritance. Making their methods virtual would be misleading, since it implies child classes might want to override the method, and there shouldn't be any child classes.
Many methods are not designed to be overridden: same thing.
Also, even when things are intended to be subclassed / overridden, they aren't necessarily intended for run-time polymorphism. Very occasionally, despite what OO best practice says, what you want inheritance for is code reuse. For example if you're using CRTP for simulated dynamic binding. So again you don't want to imply your class will play nicely with runtime polymorphism by making its methods virtual, when they should never be called that way.
In summary, things which are intended to be overridden for runtime polymorphism should be marked virtual, and things which don't, shouldn't. If you find that almost all your member functions are intended to be virtual, then mark them virtual unless there's a reason not to. If you find that most of your member functions are not intended to be virtual, then don't mark them virtual unless there's a reason to do so.
It's a tricky issue when designing a public API, because flipping a method from one to the other is a breaking change, so you have to get it right first time. But you don't necessarily know before you have any users, whether your users are going to want to "polymorph" your classes. Ho hum. The STL container approach, of defining abstract interfaces and banning inheritance entirely, is safe but sometimes requires users to do more typing.
There are several reasons.
First, performance: Yes, the overhead of a virtual function is relatively low seen in isolation. But it also prevents the compiler from inlining, and that is a huge source of optimization in C++. The C++ standard library performs as well as it does because it can inline the dozens and dozens of small one-liners it consists of. Additionally, a class with virtual methods is not a POD datatype, and so a lot of restrictions apply to it. It can't be copied just by memcpy'ing, it becomes more expensive to construct, and takes up more space. There are a lot of things that suddenly become illegal or less efficient once a non-POD type is involved.
And second, good OOP practice. The point in a class is that it makes some kind of abstraction, hides its internal details, and provides a guarantee that "this class will behave so and so, and will always maintain these invariants. It will never end up in an invalid state". That is pretty hard to live up to if you allow others to override any member function. The member functions you defined in the class are there to ensure that the invariant is maintained. If we didn't care about that, we could just make the internal data members public, and let people manipulate them at will. But we want our class to be consistent. And that means we have to specify the behavior of its public interface. That may involve specific customizability points, by making individual functions virtual, but it almost always also involves making most methods non-virtual, so that they can do the job of ensuring that the invariant is maintained. The non-virtual interface idiom is a good example of this: http://www.gotw.ca/publications/mill18.htm
Third, inheritance isn't often needed, especially not in C++. Templates and generic programming (static polymorphism) can in many cases do a better job than inheritance (runtime polymorphism). Yes, you sometimes still need virtual methods and inheritance, but it is certainly not the default. If it is, you're Doing It Wrong. Work with the language, rather than try to pretend it was something else. It's not Java, and unlike Java, in C++ inheritance is the exception, not the rule.