In what ways do C++ exceptions slow down code when there are no exceptions thown?

Question

I have read that there is some overhead to using C++ exceptions for exception handling as opposed to, say, checking return values. I'm only talking about overhead that is incurred when no exception is thrown. I'm also assuming that you would need to implement the code that actually checks the return value and does the appropriate thing, whatever would be the equivalent to what the catch block would have done. And, it's also not fair to compare code that throws exception objects with 45 state variables inside to code that returns a negative integer for every error.

I'm not trying to build a case for or against C++ exceptions solely based on which one might execute faster. I heard someone make the case recently that code using exceptions ought to run just as fast as code based on return codes, once you take into account all the extra bookkeeping code that would be needed to check the return values and handle the errors. What am I missing?

Answer 1

There is a cost associated with exception handling on some platforms and with some compilers.

Namely, Visual Studio, when building a 32-bit target, will register a handler in every function that has local variables with non-trivial destructor. Basically, it sets up a try/finally handler.

The other technique, employed by gcc and Visual Studio targeting 64-bits, only incurs overhead when an exception is thrown (the technique involves traversing the call stack and table lookup). In cases where exceptions are rarely thrown, this can actually lead to a more efficient code, as error codes don't have to be processed.