What is the performance cost of having a virtual method in a C++ class?

I ran some timings on a 3ghz in-order PowerPC processor. On that architecture, a virtual function call costs 7 nanoseconds longer than a direct (non-virtual) function call.

So, not really worth worrying about the cost unless the function is something like a trivial Get()/Set() accessor, in which anything other than inline is kind of wasteful. A 7ns overhead on a function that inlines to 0.5ns is severe; a 7ns overhead on a function that takes 500ms to execute is meaningless.

The big cost of virtual functions isn't really the lookup of a function pointer in the vtable (that's usually just a single cycle), but that the indirect jump usually cannot be branch-predicted. This can cause a large pipeline bubble as the processor cannot fetch any instructions until the indirect jump (the call through the function pointer) has retired and a new instruction pointer computed. So, the cost of a virtual function call is much bigger than it might seem from looking at the assembly... but still only 7 nanoseconds.

Edit: Andrew, Not Sure, and others also raise the very good point that a virtual function call may cause an instruction cache miss: if you jump to a code address that is not in cache then the whole program comes to a dead halt while the instructions are fetched from main memory. This is always a significant stall: on Xenon, about 650 cycles (by my tests).

However this isn't a problem specific to virtual functions because even a direct function call will cause a miss if you jump to instructions that aren't in cache. What matters is whether the function has been run before recently (making it more likely to be in cache), and whether your architecture can predict static (not virtual) branches and fetch those instructions into cache ahead of time. My PPC does not, but maybe Intel's most recent hardware does.

My timings control for the influence of icache misses on execution (deliberately, since I was trying to examine the CPU pipeline in isolation), so they discount that cost.

It depends. :) (Had you expected anything else?)

Once a class gets a virtual function, it can no longer be a POD datatype, (it may not have been one before either, in which case this won't make a difference) and that makes a whole range of optimizations impossible.

std::copy() on plain POD types can resort to a simple memcpy routine, but non-POD types have to be handled more carefully.

Construction becomes a lot slower because the vtable has to be initialized. In the worst case, the difference in performance between POD and non-POD datatypes can be significant.

In the worst case, you may see 5x slower execution (that number is taken from a university project I did recently to reimplement a few standard library classes. Our container took roughly 5x as long to construct as soon as the data type it stored got a vtable)

Of course, in most cases, you're unlikely to see any measurable performance difference, this is simply to point out that in some border cases, it can be costly.

However, performance shouldn't be your primary consideration here. Making everything virtual is not a perfect solution for other reasons.

Allowing everything to be overridden in derived classes makes it much harder to maintain class invariants. How does a class guarantee that it stays in a consistent state when any one of its methods could be redefined at any time?

Making everything virtual may eliminate a few potential bugs, but it also introduces new ones.

If you need the functionality of virtual dispatch, you have to pay the price. The advantage of C++ is that you can use a very efficient implementation of virtual dispatch provided by the compiler, rather than a possibly inefficient version you implement yourself.

However, lumbering yourself with the overhead if you don't needx it is possibly going a bit too far. And most classesare not designed to be inherited from - to create a good base class requires more than making its functions virtual.

There is definitely measurable overhead when calling a virtual function - the call must use the vtable to resolve the address of the function for that type of object. The extra instructions are the least of your worries. Not only do vtables prevent many potential compiler optimizations (since the type is polymorphic the compiler) they can also thrash your I-Cache.

Of course whether these penalties are significant or not depends on your application, how often those code paths are executed, and your inheritance patterns.

In my opinion though, having everything as virtual by default is a blanket solution to a problem you could solve in other ways.

Perhaps you could look at how classes are designed/documented/written. Generally the header for a class should make quite clear which functions can be overridden by derived classes and how they are called. Having programmers write this documentation is helpful in ensuring they are marked correctly as virtual.

I would also say that declaring every function as virtual could lead to more bugs than just forgetting to mark something as virtual. If all functions are virtual everything can be replaced by base classes - public, protected, private - everything becomes fair game. By accident or intention subclasses could then change the behavior of functions that then cause problems when used in the base implementation.