C++：探索std::map和std::unordered_map中的添加操作

其他 2018-11-11 23:56:59 阅读次数: 0

std::map和std::unordered_map主要提供如下几种添加操作：

  1. try_emplace ()   (C++17)
  2. emplace ()
  3. insert()
  4. [] =

下面给出一段测试代码，观察对象在添加到std::map中时，构造对象过程中会有什么区别：

#include <map>
#include <iostream>
#include <tuple>
#include <exception>
#include <string>
#include <memory>
#include <vector>

using namespace std;

class Kid {
private:
  static int cnt;
  int id;
public:
  string name;
  int age;
  Kid() {
    id = ++cnt;
    cout << "Kid()" << id << endl;
  }
  Kid(string _name, int _age) : name(_name), age(_age) {
    id = ++cnt;
    cout << "Kid(string _name, int _age)" << id << endl;
  }
  Kid(const Kid&) {
    id = ++cnt;
    cout << "Kid(const Kid&)" << id << endl;
  }
  Kid(Kid&&) {
    id = ++cnt;
    cout << "Kid(Kid&&)" << id << endl;
  }
  Kid& operator=(const Kid&) {
    cout << "operator=(const Kid&)" << id << endl;

    return *this;
  }
  bool operator< (const Kid& rhs) const
  {
    return this->age < rhs.age;
  }
};
int Kid::cnt = 0;

class People {
private:
  static int cnt;
  int id;
public:
  string name;
  int age;
  People() {
    id = ++cnt;
    cout << "People()" << id << endl;
  }
  People(string _name, int _age) : name(_name), age(_age) {
    id = ++cnt;
    cout << "People(string _name, int _age)" << id << endl;
  }
  People(const People&) {
    id = ++cnt;
    cout << "People(const People&)" << id << endl;
  }
  People(People&&) {
    id = ++cnt;
    cout << "People(People&&)" << id << endl;
  }
  People& operator=(const People&) {
    cout << "operator=(const People&)" << id << endl;
    return *this;
  }
};

int People::cnt = 0;

int main() {
  map<Kid, People> m;
  cout << "--------------------------------" << endl;
 
  {
    cout << "--------------------------------//(1)" << endl;
    Kid x1("xxl", 1213332);
    m.try_emplace(x1, "xlx", 121);
  }
  {
    cout << "--------------------------------//(2)" << endl;
    m.try_emplace(Kid("xxl", 1213332), "xlx", 121);
  }
  {
    cout << "--------------------------------//(3)" << endl;
    Kid x1("xxl", 1213332);
    People p1("xlx", 121);
    m.emplace(x1, p1);
  }
  {
    cout << "--------------------------------//(4)" << endl;
    m.emplace(Kid("xxl", 1213332), People("aaaa", 121));
  }
  {
    cout << "--------------------------------//(5)" << endl;
    m.emplace(make_pair(Kid("xxl", 1213332), People("aaaa", 121)));
  }
  {
    cout << "--------------------------------//(6)" << endl;
    m.emplace(std::piecewise_construct, std::forward_as_tuple("fffff", 999), std::forward_as_tuple("wwww", 121));
  }
  {
    cout << "--------------------------------//(7)" << endl;
    m.insert(make_pair(Kid("xxl", 1213332), People("aaaa", 121)));
  }
  {
    cout << "--------------------------------//(8)" << endl;
    m[Kid("xxl", 1213332)]= People("aaaa", 121);
  }

  cout << "--------------------------------" << endl;

  return 1;
}

VC++2017的运行结果：

--------------------------------
--------------------------------//(1)
Kid(string _name, int _age)3
Kid(const Kid&)4
People(string _name, int _age)3
--------------------------------//(2)
Kid(string _name, int _age)5
Kid(Kid&&)6
People(string _name, int _age)4
--------------------------------//(3)
Kid(string _name, int _age)7
People(string _name, int _age)5
Kid(const Kid&)8
People(const People&)6
--------------------------------//(4)
People(string _name, int _age)7
Kid(string _name, int _age)9
Kid(Kid&&)10
People(People&&)8
--------------------------------//(5)
People(string _name, int _age)9
Kid(string _name, int _age)11
Kid(Kid&&)12
People(People&&)10
Kid(Kid&&)13
People(People&&)11
--------------------------------//(6)
Kid(string _name, int _age)14
People(string _name, int _age)12
--------------------------------//(7)
People(string _name, int _age)13
Kid(string _name, int _age)15
Kid(Kid&&)16
People(People&&)14
Kid(Kid&&)17
People(People&&)15
--------------------------------//(8)
People(string _name, int _age)16
Kid(string _name, int _age)18
Kid(Kid&&)19
People()17
operator=(const People&)4
--------------------------------

从以上运行结果中可以看到，只讲对象构造效率的话，效率从高到低依次为：

(6) > (2) > (4) > (5) (7) > (1) > (3) > (8)

其中(6)只调用了2次构造函数，(2)调用了2次构造函数和1次转移构造函数，(4)调用了2次构造函数和2次转移构造函数，(5)(7)调用了2次构造函数和4次转移构造函数，(1)调用了2次构造函数和1次拷贝构造函数，(3)调用了2次构造函数和2次拷贝构造函数，(8)调用了3次构造函数、1次拷贝构造函数和1次赋值函数。但对象构造效率高低还不能决定添加操作的效率，即添加操作的效率排名不一定退以上排名一样。

下面对以上几种方式分别进行代码测试，代码如下：

#include <iostream>
#include <sstream>
#include <fstream>
#include <tuple>
#include <exception>
#include <string>
#include <vector>
#include <functional>
#include <array>
#include <chrono>
#include <unordered_map>
#include <map>
#include <cstdlib>
#include <ctime>
#include <climits>

using namespace std;

class Kid_ {
private:
  static int cnt;
  int id;
public:
  string name;
  int money;
  Kid_() {
    id = ++cnt;
  }
  Kid_(string& _name, int& _money) : name(_name), money(_money) {
    id = ++cnt;
  }
  Kid_(const Kid_& k) : name(k.name), money(k.money) {
    id = ++cnt;
  }
  Kid_(Kid_&& k) : name(move(k.name)), money(move(k.money)) {
    id = ++cnt;
  }

  bool operator< (const Kid_& rhs) const {
    return this->name < rhs.name;
  }
};
int Kid_::cnt = 0;

class People_ {
private:
  static int cnt;
  int id;
public:
  string name;
  int money;
  People_() {
    id = ++cnt;
  }
  People_(string& _name, int& _money) : name(_name), money(_money) {
    id = ++cnt;
  }
  People_(const People_& p) : name(p.name), money(p.money) {
    id = ++cnt;
  }
  People_(People_&& p) : name(move(p.name)), money(move(p.money)) {
    id = ++cnt;
  }
};
int People_::cnt = 0;

struct KeyHash {
  std::size_t operator()(const Kid_& k) const
  {
    return std::hash<string>()(k.name) ^ k.money;
  }
};

struct KeyEqual {
  bool operator()(const Kid_& lhs, const Kid_& rhs) const
  {
    return lhs.name == rhs.name && lhs.money == rhs.money;
  }
};

string getRandomString(int length) {
  string ret;

  for (int i = 0; i < length; i++) {
    int r = std::rand() % 10;

    if (i == 0 && r ==0) {  
      i--;
      continue;
    }

    ret += to_string(r);
  }

  return ret;
}

void testMapAdd() {
  std::srand(std::time(0));
  int times = 1000000;

  cout << "--------------------------------//map" << endl;

  {
    long long timeUsed = 0;
    map<Kid_, People_ > m;

    cout << "--------------------------------//(1)" << endl;
    for (int i = 0; i < times; i++) {
      string s1 = getRandomString(10);
      int r1 = std::rand();
      string s2 = getRandomString(10);
      int r2 = std::rand();

      auto start = std::chrono::system_clock::now();
      Kid_ k(s1, r1);
      m.try_emplace(k, s2, r2);
      auto end = std::chrono::system_clock::now();

      timeUsed += std::chrono::duration_cast<std::chrono::nanoseconds>(end - start).count();
    }

    cout << timeUsed << ":\t" << ((float)timeUsed / times) << endl;
  }
  {
    long long timeUsed = 0;
    map<Kid_, People_ > m;

    cout << "--------------------------------//(2)" << endl;
    for (int i = 0; i < times; i++) {
      string s1 = getRandomString(10);
      int r1 = std::rand();
      string s2 = getRandomString(10);
      int r2 = std::rand();

      auto start = std::chrono::system_clock::now();
      m.try_emplace(Kid_(s1, r1), s2, r2);
      auto end = std::chrono::system_clock::now();

      timeUsed += std::chrono::duration_cast<std::chrono::nanoseconds>(end - start).count();
    }

    cout << timeUsed << ":\t" << ((float)timeUsed / times) << endl;
  }
  {
    long long timeUsed = 0;
    map<Kid_, People_ > m;

    cout << "--------------------------------//(3)" << endl;
    for (int i = 0; i < times; i++) {
      string s1 = getRandomString(10);
      int r1 = std::rand();
      string s2 = getRandomString(10);
      int r2 = std::rand();

      auto start = std::chrono::system_clock::now();
      Kid_ x1(s1, r1);
      People_ p1(s2, r2);
      m.emplace(x1, p1);
      auto end = std::chrono::system_clock::now();

      timeUsed += std::chrono::duration_cast<std::chrono::nanoseconds>(end - start).count();
    }

    cout << timeUsed << ":\t" << ((float)timeUsed / times) << endl;
  }
  {
    long long timeUsed = 0;
    map<Kid_, People_ > m;

    cout << "--------------------------------//(4)" << endl;
    for (int i = 0; i < times; i++) {
      string s1 = getRandomString(10);
      int r1 = std::rand();
      string s2 = getRandomString(10);
      int r2 = std::rand();

      auto start = std::chrono::system_clock::now();
      m.emplace(Kid_(s1, r1), People_(s2, r2));
      auto end = std::chrono::system_clock::now();

      timeUsed += std::chrono::duration_cast<std::chrono::nanoseconds>(end - start).count();
    }

    cout << timeUsed << ":\t" << ((float)timeUsed / times) << endl;
  }
  {
    long long timeUsed = 0;
    map<Kid_, People_ > m;

    cout << "--------------------------------//(5)" << endl;
    for (int i = 0; i < times; i++) {
      string s1 = getRandomString(10);
      int r1 = std::rand();
      string s2 = getRandomString(10);
      int r2 = std::rand();

      auto start = std::chrono::system_clock::now();
      m.emplace(make_pair(Kid_(s1, r1), People_(s2, r2)));
      auto end = std::chrono::system_clock::now();

      timeUsed += std::chrono::duration_cast<std::chrono::nanoseconds>(end - start).count();
    }

    cout << timeUsed << ":\t" << ((float)timeUsed / times) << endl;
  }
  {
    long long timeUsed = 0;
    map<Kid_, People_ > m;

    cout << "--------------------------------//(6)" << endl;
    for (int i = 0; i < times; i++) {
      string s1 = getRandomString(10);
      int r1 = std::rand();
      string s2 = getRandomString(10);
      int r2 = std::rand();

      auto start = std::chrono::system_clock::now();
      m.emplace(std::piecewise_construct, std::forward_as_tuple(s1, r1), std::forward_as_tuple(s2, r2));
      auto end = std::chrono::system_clock::now();

      timeUsed += std::chrono::duration_cast<std::chrono::nanoseconds>(end - start).count();
    }

    cout << timeUsed << ":\t" << ((float)timeUsed / times) << endl;
  }
  {
    long long timeUsed = 0;
    map<Kid_, People_ > m;

    cout << "--------------------------------//(8)" << endl;
    for (int i = 0; i < times; i++) {
      string s1 = getRandomString(10);
      int r1 = std::rand();
      string s2 = getRandomString(10);
      int r2 = std::rand();

      auto start = std::chrono::system_clock::now();
      m[Kid_(s1, r1)] = People_(s2, r2);
      auto end = std::chrono::system_clock::now();

      timeUsed += std::chrono::duration_cast<std::chrono::nanoseconds>(end - start).count();
    }

    cout << timeUsed << ":\t" << ((float)timeUsed / times) << endl;
  }
  /////////////////////////////////////////////////////////////////////////////////////////
  cout << "--------------------------------//unordered_map" << endl;

  {
    long long timeUsed = 0;
    unordered_map<Kid_, People_, KeyHash, KeyEqual> m(3 * times);

    cout << "--------------------------------//(1)" << endl;
    for (int i = 0; i < times; i++) {
      string s1 = getRandomString(10);
      int r1 = std::rand();
      string s2 = getRandomString(10);
      int r2 = std::rand();

      auto start = std::chrono::system_clock::now();
      Kid_ k(s1, r1);
      m.try_emplace(k, s2, r2);
      auto end = std::chrono::system_clock::now();

      timeUsed += std::chrono::duration_cast<std::chrono::nanoseconds>(end - start).count();
    }

    cout << timeUsed << ":\t" << ((float)timeUsed / times) << endl;
  }
  {
    long long timeUsed = 0;
    unordered_map<Kid_, People_, KeyHash, KeyEqual> m(3 * times);

    cout << "--------------------------------//(2)" << endl;
    for (int i = 0; i < times; i++) {
      string s1 = getRandomString(10);
      int r1 = std::rand();
      string s2 = getRandomString(10);
      int r2 = std::rand();

      auto start = std::chrono::system_clock::now();
      m.try_emplace(Kid_(s1, r1), s2, r2);
      auto end = std::chrono::system_clock::now();

      timeUsed += std::chrono::duration_cast<std::chrono::nanoseconds>(end - start).count();
    }

    cout << timeUsed << ":\t" << ((float)timeUsed / times) << endl;
  }
  {
    long long timeUsed = 0;
    unordered_map<Kid_, People_, KeyHash, KeyEqual> m(3 * times);

    cout << "--------------------------------//(3)" << endl;
    for (int i = 0; i < times; i++) {
      string s1 = getRandomString(10);
      int r1 = std::rand();
      string s2 = getRandomString(10);
      int r2 = std::rand();

      auto start = std::chrono::system_clock::now();
      Kid_ x1(s1, r1);
      People_ p1(s2, r2);
      m.emplace(x1, p1);
      auto end = std::chrono::system_clock::now();

      timeUsed += std::chrono::duration_cast<std::chrono::nanoseconds>(end - start).count();
    }

    cout << timeUsed << ":\t" << ((float)timeUsed / times) << endl;
  }
  {
    long long timeUsed = 0;
    unordered_map<Kid_, People_, KeyHash, KeyEqual> m(3 * times);

    cout << "--------------------------------//(4)" << endl;
    for (int i = 0; i < times; i++) {
      string s1 = getRandomString(10);
      int r1 = std::rand();
      string s2 = getRandomString(10);
      int r2 = std::rand();

      auto start = std::chrono::system_clock::now();
      m.emplace(Kid_(s1, r1), People_(s2, r2));
      auto end = std::chrono::system_clock::now();

      timeUsed += std::chrono::duration_cast<std::chrono::nanoseconds>(end - start).count();
    }

    cout << timeUsed << ":\t" << ((float)timeUsed / times) << endl;
  }
  {
    long long timeUsed = 0;
    unordered_map<Kid_, People_, KeyHash, KeyEqual> m(3 * times);

    cout << "--------------------------------//(5)" << endl;
    for (int i = 0; i < times; i++) {
      string s1 = getRandomString(10);
      int r1 = std::rand();
      string s2 = getRandomString(10);
      int r2 = std::rand();

      auto start = std::chrono::system_clock::now();
      m.emplace(make_pair(Kid_(s1, r1), People_(s2, r2)));
      auto end = std::chrono::system_clock::now();

      timeUsed += std::chrono::duration_cast<std::chrono::nanoseconds>(end - start).count();
    }

    cout << timeUsed << ":\t" << ((float)timeUsed / times) << endl;
  }
  {
    long long timeUsed = 0;
    unordered_map<Kid_, People_, KeyHash, KeyEqual> m(3 * times);

    cout << "--------------------------------//(6)" << endl;
    for (int i = 0; i < times; i++) {
      string s1 = getRandomString(10);
      int r1 = std::rand();
      string s2 = getRandomString(10);
      int r2 = std::rand();

      auto start = std::chrono::system_clock::now();
      m.emplace(std::piecewise_construct, std::forward_as_tuple(s1, r1), std::forward_as_tuple(s2, r2));
      auto end = std::chrono::system_clock::now();

      timeUsed += std::chrono::duration_cast<std::chrono::nanoseconds>(end - start).count();
    }

    cout << timeUsed << ":\t" << ((float)timeUsed / times) << endl;
  }
  {
    long long timeUsed = 0;
    unordered_map<Kid_, People_, KeyHash, KeyEqual> m(3 * times);

    cout << "--------------------------------//(8)" << endl;
    for (int i = 0; i < times; i++) {
      string s1 = getRandomString(10);
      int r1 = std::rand();
      string s2 = getRandomString(10);
      int r2 = std::rand();

      auto start = std::chrono::system_clock::now();
      m[Kid_(s1, r1)] = People_(s2, r2);
      auto end = std::chrono::system_clock::now();

      timeUsed += std::chrono::duration_cast<std::chrono::nanoseconds>(end - start).count();
    }
    
    cout << timeUsed << ":\t" << ((float)timeUsed / times) << endl;
  }
}


int main() {
   testMapAdd();

  return 0;
}

运行结果：

--------------------------------//map
--------------------------------//(1)
934167100:      934.167
--------------------------------//(2)
892646600:      892.647
--------------------------------//(3)
919973300:      919.973
--------------------------------//(4)
917704500:      917.705
--------------------------------//(5)
935676700:      935.677
--------------------------------//(6)
946283500:      946.284
--------------------------------//(8)
939161000:      939.161
--------------------------------//unordered_map
--------------------------------//(1)
269299400:      269.299
--------------------------------//(2)
278339800:      278.34
--------------------------------//(3)
269483600:      269.484
--------------------------------//(4)
273019500:      273.02
--------------------------------//(5)
274466600:      274.467
--------------------------------//(6)
267864800:      267.865
--------------------------------//(8)
273746400:      273.746

 从数值上看，这几种方式的效率相差并不大，而且每次执行的结果的排名都不完全相同。这应该是编译器做了大量的编译优化，所以执行效率都还是不错的。

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