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Showing posts from August, 2017

Binding std::function to member functions

I realized that std::function can be bound to member functions without requiring the *this object. Consider the following examples. // std::string::empty is a const function. All variables from e1 to e5 are fine. std::function<bool(std::string)> e1 = &std::string::empty; std::function<bool(std::string &)> e2 = &std::string::empty; std::function<bool(const std::string &)> e3 = &std::string::empty; std::function<bool(std::string *)> e4 = &std::string::empty; std::function<bool(const std::string *)> e5 = &std::string::empty; // std::string::push_back is not a const function. p4 and p5 don't compile. std::function<void(std::string, char)> p1 = &std::string::push_back; std::function<void(std::string &, char)> p2 = &std::string::push_back; std::function<void(std::string *, char)> p3 = &std::string::push_back; // These two don't compile because push_back is a non-const function std::funct

Avoiding intermediate copies in std::accumulate

std::accumulate makes a ton of copies internally. In fact it's 2x the size of the number of elements in the iterator range. To fix, use std::ref and std::reference_wrapper for the initial state. std::shared_ptr is also a possibility if the accumulated state must be dynamically allocated for some reason. Live code on wandbox . Update: Please see alternative solutions in the comments section. #include <iostream> #include <cstdlib> #include <algorithm> #include <vector> #include <string> #include <numeric> #include <functional> struct Vector : public std::vector<int> { Vector(std::initializer_list<int> il) : std::vector<int>(il){ std::cout << "Vector(std::initializer_list)\n"; } Vector() { std::cout << "Vector()\n"; } Vector(const Vector &v) : std::vector<int>(v) { std::cout << "Vector(const Vector &)\n"; } Vector & operator = (