valarray

问题 To speed up the calculations in my library, I decided to use the std::valarray class. The documentation says: std::valarray and helper classes are defined to be free of certain forms of aliasing, thus allowing operations on these classes to be optimized similar to the effect of the keyword restrict in the C programming language. In addition, functions and operators that take valarray arguments are allowed to return proxy objects to make it possible for the compiler to optimize an expression

问题 To speed up the calculations in my library, I decided to use the std::valarray class. The documentation says: std::valarray and helper classes are defined to be free of certain forms of aliasing, thus allowing operations on these classes to be optimized similar to the effect of the keyword restrict in the C programming language. In addition, functions and operators that take valarray arguments are allowed to return proxy objects to make it possible for the compiler to optimize an expression

问题 Is it ok/safe/good practice to write the following in order to fill a valarray with the content of a vector? vector<int> myVect = {2,0,1,0,0,1,1,0,0,0}; // Any vector valarray<int> myVala ( &(myVect[0]), myVect.size() ); //Edit: as suggested by Xeo, this code looks much cleaner (C++11) valarray<int> myVala ( myVect.data(), myVect.size() ); It seems to work fine but I would like to be sure it works on any case. 回答1: Yes, this is perfectly fine. The contents of vector are guaranteed to be

问题 Is it ok/safe/good practice to write the following in order to fill a valarray with the content of a vector? vector<int> myVect = {2,0,1,0,0,1,1,0,0,0}; // Any vector valarray<int> myVala ( &(myVect[0]), myVect.size() ); //Edit: as suggested by Xeo, this code looks much cleaner (C++11) valarray<int> myVala ( myVect.data(), myVect.size() ); It seems to work fine but I would like to be sure it works on any case. 回答1: Yes, this is perfectly fine. The contents of vector are guaranteed to be

问题 Is it ok/safe/good practice to write the following in order to fill a valarray with the content of a vector? vector<int> myVect = {2,0,1,0,0,1,1,0,0,0}; // Any vector valarray<int> myVala ( &(myVect[0]), myVect.size() ); //Edit: as suggested by Xeo, this code looks much cleaner (C++11) valarray<int> myVala ( myVect.data(), myVect.size() ); It seems to work fine but I would like to be sure it works on any case. 回答1: Yes, this is perfectly fine. The contents of vector are guaranteed to be

问题 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

问题 Code: #include <valarray> #include <iostream> using namespace std; int main() { valarray<int> v0(2, 4); valarray<int> v1; v1 = v0; cout << "v0.size: " << v0.size() << endl; cout << "v1.size: " << v1.size() << endl; cout << "v0[0]: " << v0[0] << endl; cout << "v1[0]: " << v1[0] << endl; } Output: v0.size: 4 v1.size: 0 v0[0]: 2 Segmentation fault For the assignment: v1 = v0; I would think the constructor: valarray<T>& operator=( const valarray<T>& other ); should be used and according to the

问题 I like vectors a lot. They're nifty and fast. But I know this thing called a valarray exists. Why would I use a valarray instead of a vector? I know valarrays have some syntactic sugar, but other than that, when are they useful? 回答1: Valarrays (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

问题 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

问题 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