basics/initperf.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 <string>
#include <iostream>
#include <cstdlib>   // for std::atoi()
#include <chrono>
#include <map>

//********************************************************
// the 3 approaches to initialize a Person
//********************************************************

namespace classic {
  class Person {
   private:
    std::string first;  // first name
    std::string last;   // last name
   public:
    Person(const std::string& f, const std::string& l)
     : first{f}, last{l} {
    }
    //...
  };
} // namespace classic


namespace all {
  class Person {
   private:
    std::string first;  // first name
    std::string last;   // last name
   public:
    Person(const std::string& f, const std::string& l)
     : first{f}, last{l} {
    }
    Person(const std::string& f, std::string&& l)
     : first{f}, last{std::move(l)} {
    }
    Person(std::string&& f, const std::string& l)
     : first{std::move(f)}, last{l} {
    }
    Person(std::string&& f, std::string&& l)
     : first{std::move(f)}, last{std::move(l)} {
    }
    Person(const char* f, const char* l)
     : first{f}, last{l} {
    }
    Person(const char* f, const std::string& l)
     : first{f}, last{l} {
    }
    Person(const char* f, std::string&& l)
     : first{f}, last{std::move(l)} {
    }
    Person(const std::string& f, const char* l)
     : first{f}, last{l} {
    }
    Person(std::string&& f, const char* l)
     : first{std::move(f)}, last{l} {
    }
    //...
  };
} // namespace all


namespace valmove {
  class Person {
   private:
    std::string first;  // first name
    std::string last;   // last name
   public:
    Person(std::string f, std::string l)
     : first{std::move(f)}, last{std::move(l)} {
    }
    //...
  };
} // namespace valmove


//********************************************************
// measure functions (with long strings and SSO)
//********************************************************

// measure num initializations of whatever is currently defined as Person:
template<typename PersonT>
std::chrono::nanoseconds measure(int num)
{
  std::chrono::nanoseconds totalDur{0};
  for (int i = 0; i < num; ++i) {
    std::string fname = "a firstname a bit too long for SSO";
    std::string lname = "a lastname a bit too long for SSO";

    // measure how long it takes to create 3 Persons in different ways:
    auto t0 = std::chrono::steady_clock::now();
      PersonT p1{"a firstname too long for SSO", "a lastname too long for SSO"};
      PersonT p2{fname, lname};
      PersonT p3{std::move(fname), std::move(lname)};
    auto t1 = std::chrono::steady_clock::now();
    totalDur += t1 - t0;
  }
  return totalDur;
}

// same using short string so that strings don't allocate memory:
template<typename PersonT>
std::chrono::nanoseconds measureSSO(int num)
{
  std::chrono::nanoseconds totalDur{0};
  for (int i = 0; i < num; ++i) {
    std::string fname = "short first";
    std::string lname = "short last";

    // measure how long it takes to create 3 Persons in different ways:
    auto t0 = std::chrono::steady_clock::now();
      PersonT p1{"a firstname", "a lastname"};
      PersonT p2{fname, lname};
      PersonT p3{std::move(fname), std::move(lname)};
    auto t1 = std::chrono::steady_clock::now();
    totalDur += t1 - t0;
  }
  return totalDur;
}


//********************************************************
// main():
//********************************************************

int main(int argc, const char** argv)
{
  int num = 1000;
  if (argc > 1) {
    num = std::atoi(argv[1]);
  }

  // a few iterations to make the CPU hot:
  measure<classic::Person>(num / 10);
  measureSSO<all::Person>(num / 10);

  std::map<std::string, std::chrono::duration<double,std::milli>> data;
  int numLoops = 10;
  for (int i = 0; i < numLoops; ++i) {
    // measure:
    data["a: classic:       "] += measure<classic::Person>(num/numLoops);
    data["b: all:           "] += measure<all::Person>(num/numLoops);
    data["c: valmove:       "] += measure<valmove::Person>(num/numLoops);
    data["d: classicSSO:    "] += measureSSO<classic::Person>(num/numLoops);
    data["e: allSSO:        "] += measureSSO<all::Person>(num/numLoops);
    data["f: valmoveSSO:    "] += measureSSO<valmove::Person>(num/numLoops);
  }
  for (const auto& d : data) {
    std::cout << "  " << d.first << d.second.count() / numLoops << "ms\n";
  }
}