I was looking for the fastest way to popcount large arrays of data. I encountered a very weird effect: Changing the loop variable from unsigned to uint64_t made the performance drop by 50% on my PC. The Benchmark #include <iostream> #include <chrono> #include <x86intrin.h> int main(int argc, char* argv[]) { using namespace std; if (argc … Read more
Locked. This question and its answers are locked because the question is off-topic but has historical significance. It is not currently accepting new answers or interactions. I am the developer of some family tree software (written in C++ and Qt). I had no problems until one of my customers mailed me a bug report. The … Read more
Why does this bit of code, const float x[16] = { 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6}; const float z[16] = {1.123, 1.234, 1.345, 156.467, 1.578, 1.689, 1.790, 1.812, 1.923, 2.034, 2.145, 2.256, 2.367, 2.478, 2.589, 2.690}; float y[16]; for (int i = 0; i … Read more
I always mess up how to use const int*, const int * const, and int const * correctly. Is there a set of rules defining what you can and cannot do? I want to know all the do’s and all don’ts in terms of assignments, passing to the functions, etc. 2 21 Read it backwards … Read more
What are undefined reference/unresolved external symbol errors? What are common causes and how to fix/prevent them? 3 37
What is a lambda expression in C++11? When would I use one? What class of problem do they solve that wasn’t possible prior to their introduction? A few examples, and use cases would be useful. 10 The problem C++ includes useful generic functions like std::for_each and std::transform, which can be very handy. Unfortunately they can … Read more
Is if (a < 901) faster than if (a <= 900)? Not exactly as in this simple example, but there are slight performance changes on loop complex code. I suppose this has to do something with generated machine code in case it’s even true. 1 14 No, it will not be faster on most architectures. … Read more
I have a solid understanding of most OOP theory but the one thing that confuses me a lot is virtual destructors. I thought that the destructor always gets called no matter what and for every object in the chain. When are you meant to make them virtual and why? 1 19 Virtual destructors are useful … Read more
I’m coming from a Java background and have started working with objects in C++. But one thing that occurred to me is that people often use pointers to objects rather than the objects themselves, for example this declaration: Object *myObject = new Object; rather than: Object myObject; Or instead of using a function, let’s say … Read more
One of the most interesting projects I’ve worked on in the past couple of years was a project about image processing. The goal was to develop a system to be able to recognize Coca-Cola ‘cans’ (note that I’m stressing the word ‘cans’, you’ll see why in a minute). You can see a sample below, with … Read more