Comparison of in-place vs return-by-value

test.cpp

// This is a bad comparison as the data gets written back to the source array,
// so in-place has an advantage here.

// Compile with 
//
// clang++ --std=c++11 -stdlib=libstdc++ -static -lstdc++ test.cpp

// On my machine it prints:

// When running unoptimized:
//
// finished generating data
// finished Normal in: 32193ms
// finished Inline in: 25776ms

// With -O2
//
// finished generating data
// finished Normal in: 6875ms
// finished Inline in: 3042ms



#include <algorithm>
#include <vector>
#include <iostream>
#include <chrono>
#include <ctime>

std::string gen_random(const int len) {
    static const char alphanum[] =
        "0123456789"
        "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        "abcdefghijklmnopqrstuvwxyz"
        " /_.";

    auto s = std::string{};
    s.resize(len);

    for (int i = 0; i < len; ++i) {
        s[i] = alphanum[rand() % (sizeof(alphanum) - 1)];
    }

    return s;
}

void preallocate(size_t n, std::vector<std::string>& out ) {
  out.reserve(n);
  for(size_t i=0; i < n; ++i) {
    out.emplace_back(gen_random(20));
  }
}

std::string sanitize_filename(std::string filename)
{
  using std::begin;
  using std::end;
  std::replace(begin(filename), end(filename), '/', '_');
  return filename;
}

void sanitize(std::string& filename)
{
  using std::begin;
  using std::end;
  std::replace(begin(filename), end(filename), '/', '_');
}


template <class Pred>
void run_with_time(const std::string& text, Pred pred) {
    auto start = std::chrono::system_clock::now();

    pred();

    auto end = std::chrono::system_clock::now();

    auto elapsed_seconds = std::chrono::duration_cast<std::chrono::microseconds>( end - start );

    std::cout << "finished " << text
              << " in: " << elapsed_seconds.count() << "ms\n";
}

int main()
{

  const auto example_count = 100000;
  auto srcVec = std::vector<std::string>{};
  preallocate(example_count, srcVec);
  auto src2Vec = std::vector<std::string>{};
  preallocate(example_count, src2Vec);
  std::cout << "finished generating data\n";

  run_with_time( "Normal", [&](){
      for (auto& el : srcVec) {
        el = sanitize_filename(el);
      }
      });

  run_with_time( "in-place", [&](){
      for (auto& el : src2Vec) {
        sanitize(el);
      }
      });

}