When does C++ perform better than C ?

Virtual functions could be faster than a lot of switch/case.

On the other hand, often the performance dépends more on the algorithm than the actual language. And with librairies like STL, it is often easier to write efficient algorithm.

For example, if you have a bad algorithm written in C++ and you have the choice between rewritting the algorithm for a better one or convert code to C but keep same algorithm, you might get an improvement of an order of magnitude in the first case while it be just noticeable in the second case...

Thus given that development time is not infinite, you might be able to create a more performant or complete application in the same amount of time using a more high level language.

Theorically one could wrte the most performant program in assembler... but adding the time constraint into the equation, often it won't make any sense.

Another case where C++ code might be more performant is when libraries that are used are better than the code you would be able to write in a reasonable amount of time...

And by the virtue of templates, C++ is able to do a lot of optimizations at compile time...

It depends.

For smaller projects, it's generally best to use whatever language the programmers feel most comfortable with, and only check on performance if you realize there is an actual problem. You can mix and match C with C++, so if you believe you can get better performance switching from one to the other for just one source code module, or even one function, you can.

For larger projects (measured in years of development rather than weeks or months), C++ is usually better, because it gives you better tools for structuring your application. This also means that it's often easier to pinpoint the performance bottlenecks, and thus to improve performance.

There is one catch of C++ however that you need to be vigilant about: virtual functions and polymorphism come at a cost that can not be programmed away after the fact! If you design a C++ class that you know will be allocated a lot at runtime, to the point where it may be relevant to performance, that class should better not have virtual functions! If they have, construction and destruction will take significantly longer! In one particular case I've seen an application slow down by up to 20% just because I replaced a basic struct and a couple of related functions with a class hierarchy: it wasn't the reimplementation of the functions that caused the bottleneck, but the additional time needed to set up and clean the virtual function tables!

That said, C++ can also occasionally be faster than C, even if it's the same programmer at work: the compilers are exceptionally good at optimizing code, but the C++ compiler has an easier job at this, because it has lot more information to work with! In fact the compilers are so good that programmers are advised to not manually perform trivial (or even not so trivial) optimizations: doing so might result in obscuring relevant information that the compiler would need to find an even better optimization!

After reading some other solutions and the comments on my own, I feel inclined to provide yet another, somewhat different solutionstatement:

1. Don't bother with the question of performance until you've
a) actually written a program,
b) ran it,
c) detected an actual performance problem, and
d) found the cause of the performance problem.

2. Fix it by whatever means are most appropriate.

In doing so, you'll save a lot of time on premature performance coniderations that in the end don't actually matter anyway, and have a lot more time on your hands to focus on the relevant parts of your optimization.

When it comes to choosing a language, performance shouldn't be the deciding factor. There are always other aspects that are much more important in this decision.

Well written C++ shouldn't perform much worse than well written C. That's all we can say, in my opinion.