Std :: function is heavier than automatically saving a lambda function

Question

I heard that the cost is std::functionheavier than autofor working with the lambda function. efficient modern C ++ item5. I want to explain the mechanism why it std::functionuses more memory than autowith some code example. Can someone help me?

change

class Widget {
public:
  Widget(int i) : i_(i) {}
  bool operator<(const Widget& o) { return o.value() > i_; }
  int value() const { return i_; };
private:
  int i_;
  int dummy_[1024];
};

int main() {
  // performance difference between auto and std::function
  { 
    auto less1 = [](const auto& p1, const auto& p2) {
      return *p1 < *p2;
    };
    std::cout << "size of less1: " << sizeof(less1) << endl;

    function<bool(const std::unique_ptr<Widget>&,
                  const std::unique_ptr<Widget>&)>
        less2 = [](const std::unique_ptr<Widget>& p1,
                   const std::unique_ptr<Widget>& p2) {
          return *p1 < *p2;
        };
    std::cout << "size of less2: " << sizeof(less2) << endl;

    {
      // auto
      std::vector<std::unique_ptr<Widget>> ws1;
      for (auto i = 0; i < 1024*100; ++i) {
        ws1.emplace_back(new Widget(std::rand()));
      }

      auto start = std::chrono::high_resolution_clock::now();
      std::sort(ws1.begin(), ws1.end(), less1);
      auto end = std::chrono::high_resolution_clock::now();
      cout << ws1[0].get()->value() << " time: " << (end - start).count() << endl;
    }

    {
      // std::function
      // 25% slower than using auto
      std::vector<std::unique_ptr<Widget>> ws2;
      for (auto i = 0; i < 1024*100; ++i) {
        ws2.emplace_back(new Widget(std::rand()));
      }

      auto start = std::chrono::high_resolution_clock::now();
      std::sort(ws2.begin(), ws2.end(), less2);
      auto end = std::chrono::high_resolution_clock::now();
      cout << ws2[0].get()->value() << " time: " << (end - start).count() << endl;
    }
  }
  return 0;
}

from https://github.com/danielhongwoo/mec/blob/master/item5/item5.cpp

I think this code shows that usage is std::functionslower than using auto. But not memory usage. I just want to prove it with some real code.

Answer 1

std::function . , - . , ( , ) operator ().

std::function, operator(), ( ). auto a a , .