lib/contmove.cpp

The following code example is taken from the book
C++ Move Semantics - The Complete Guide by Nicolai M. Josuttis, Leanpub, 2020
The code is licensed under a Creative Commons Attribution 4.0 International License.

---

// raw code

#include <iostream>
#include <cstdlib>   // for std::atoi()
#include <chrono>
#include <array>
#include <vector>
#include <algorithm>

template<int numElems, typename T>
void test(int numLoops, const T& initialVal)
{
  // initialize durations:
  using millisecondsDouble = std::chrono::duration<double, std::milli>;
  millisecondsDouble arrCopy{0}, arrMove{0}, vecCopy{0}, vecMove{0};

  // declare source objects:
  std::array<T, numElems> arr;
  std::vector<T> vec;

  // loop several times over:
  for (int i = 0; i < numLoops; ++i) {

    // (re-)initialize source objects:
    std::fill(arr.begin(), arr.end(), initialVal);
    vec.resize(numElems);
    std::fill(vec.begin(), vec.end(), initialVal);

    // measure copy and move for arrays:
    auto t0 = std::chrono::steady_clock::now();
    auto arr1{arr};             // copies all elements/values
    auto t1 = std::chrono::steady_clock::now();
    auto arr2{std::move(arr)};  // moves all elements/values
    auto t2 = std::chrono::steady_clock::now();
    arrCopy += t1 - t0;
    arrMove += t2 - t1;
   
    // measure copy and move for vectors:
    auto t3 = std::chrono::steady_clock::now();
    auto vec1{vec};             // copies all elements/values
    auto t4 = std::chrono::steady_clock::now();
    auto vec2{std::move(vec)};  // moves elements/values as a whole
    auto t5 = std::chrono::steady_clock::now();
    vecCopy += t4 - t3;
    vecMove += t5 - t4;

    // disable warning about destinations never used:
    if (arr1[0] != arr2[0]) std::cout << "oops" << '\n';
    if (vec1[0] != vec2[0]) std::cout << "oops" << '\n';
  }

  // print measured durations:
  std::cout << " array copy:  " << arrCopy.count() << '\n';
  std::cout << "       move:  " << arrMove.count() << '\n';
  std::cout << " vector copy: " << vecCopy.count() << '\n';
  std::cout << "        move: " << vecMove.count() << '\n';
}

int main(int argc, char** argv)
{
  int numLoops = 1'000;
  if (argc > 1) {
    numLoops = std::atoi(argv[1]);
  }

  std::cout << "double:\n";
  test<10'000>(numLoops, 42.7);

  std::cout << "short string:\n";
  test<10'000>(numLoops, std::string{"short"});

  std::cout << "long string:\n";
  test<10'000>(numLoops, std::string{"a very long string to avoid small string optimization"});
}