Joseph S. Myers wrote: > I repeat my request from the RM Q&A for a guide for reviewers on how to > detect hidden overhead in the presence of C++ features. When will a > structure/class/union be larger than might be expected in C? When will > C++ statements involve runtime overhead that might not be expected from > equivalent C statements?
I think you're being unnecessarily fearful. A C++ structure/class/union will be larger than the same data structure written in C iff (a) it uses virtual functions, (b) it uses virtual inheritance. The former adds exactly one pointer to the very beginning of the structure, independent of the number of virtual functions, and even if there is (non-virtual) inheritance. The latter is forbidden by Ian's proposed coding standards. So, you do indeed want to avoid virtual functions in small, plentiful objects, as it can make them much bigger. Statements in the C subset of C++ have the same performance they would in C. (I'm sure someone can come up with an exception, but it will be vanishingly rare.) In C++, you can of course make what looks like simple code do something expensive; for example, you can make "a + b" be arbitrarily complex if "a" and "b" are instances of class types and you have overloaded "+". But, if you just recompile your C program as C++, it doesn't suddenly get significantly bigger or slower. -- Mark Mitchell CodeSourcery m...@codesourcery.com (650) 331-3385 x713
