C++ performance of accessing member variables versus local variables

Question

Is it more efficient for a class to access member variables or local variables? For example, suppose you have a (callback) method whose sole responsibility is to receive dat

Answer 1

The amount of data that you will be interacting with will have a bigger influence on the execution speed than the way you represent the data in the implementation of the algorithm.

The processor does not really care if the data is on the stack or on the heap (apart from the chance that the top of the stack will be in the processor cache as peterchen mentioned) but for maximum speed, the data will have to fit into the processor's cache (L1 cache if you have more than one level of cache, which pretty much all modern processors have). Any load from L2 cache - or $DEITY forbid, main memory - will slow down the execution. So if you're processing a string that's a few hundred KB in size and chances on every invocation, the difference will not even be measurable.

Keep in mind that in most cases, a 10% speedup in a program is pretty much undetectable to the end user (unless you manage to reduce the runtime of your overnight batch from 25h back to less than 24h) so this is not worth fretting over unless you are sure and have the profiler output to back up that this particular piece of code is within the 10%-20% 'hot area' that has a major influence over your program's runtime.

Other considerations should be more important, like maintainability or other external factors. For example if the above code is in heavily multithreaded code, using local variables can make the implementation easier.