valarray

Consider the following MCVE, where I have two value arrays where w is two times v ( try it out here ): #include <valarray> using namespace std; int main() { valarray<int> v { 1, 2, 3 }; for ([[maybe_unused]] auto x : v) {} // Ok auto w = v * 2; // Leads to failure in loop below //valarray<int> w = v * 2; // Works //auto w = v*=2; // Works //auto w = v; w *= 2; // Works for ([[maybe_unused]] auto x : w) {} // Failure here } This example fails to compile with clang and gcc at the last loop with (gcc output here): error: no matching function for call to 'begin(std::_Expr<std::__detail::_BinClos

Yes, this has been asked before , and the answer has been: valarray s (value arrays) are intended to bring some of the speed of Fortran to C++. You wouldn't make a valarray of pointers so the compiler can make assumptions about the code and optimise it better. (The main reason that Fortran is so fast is that there is no pointer type so there can be no pointer aliasing.) or: valarray is also supposed to eliminate any possibility of aliasing [...] But these answers make no sense to me. valarray and vector are class templates , and as such, they don't even exist until instantiated. And of course,

I am trying to use valarray since it is much like MATLAB while operating vector and matrices. I first did some performance check and found that valarray cannot achieve the performance declared as in the book C++ programming language by Stroustrup. The test program actually did 5 million multiplication of doubles. I thought that c = a*b would at least be comparable to the for loop double type element multiplication, but I am totally wrong. I tried on several computers and Microsoft Visual C++ 6.0 and Visual Studio 2008. By the way, I tested on MATLAB using the following code: len = 5*1024*1024;