13.31 对HasPtr的vector使用sort，查看何时使用了swap

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本文详细解析了自定义HasPtr类在C++ STL容器中的行为，包括构造、赋值、比较和排序过程。通过具体代码示例，展示了HasPtr类与vector容器的交互，解释了为何会出现多次赋值和复制构造调用，以及std::swap的执行情况。

HasPtr.h

#include <iostream>
#include <string>

class HasPtr {
public:
   HasPtr()
      : ps(new std::string()), i() {
      std::cout << "Default constructor execute" << std::endl;
   }
   HasPtr(const std::string &s, int a)
      : ps(new std::string(s)), i(a) {
      std::cout << "Sting with int constructor execute" << std::endl;
   }
   HasPtr(const std::string &s = std::string())
      : ps(new std::string(s)), i(std::stoi(s)) {
      std::cout << "String constructor execute" << std::endl;
   }
   HasPtr(const HasPtr &obj)
      : ps(new std::string(*obj.ps)), i(obj.i) {
      std::cout << "Copy constructor execute" << std::endl;
   }
   HasPtr & operator=(const HasPtr &rhs) {
      std::cout << "Assign execute" << std::endl;
      ps = new std::string(*rhs.ps);
      i = rhs.i;
      return *this;
   }
   ~HasPtr() {
      delete ps;
   }

   std::string get_str() const {
      return *ps;
   }

   int get_i() const {
      return i;
   }

   bool operator<(const HasPtr obj) const {
      std::cout << "Operator < execute" << std::endl;
      return i < obj.i;
   }

   friend void swap(HasPtr &lhs, HasPtr &rhs) {
      std::cout << "HasPtr::swap function execute" << std::endl;
      std::swap(lhs.ps, rhs.ps);
      std::swap(lhs.i, rhs.i);
   }

private:
   std::string *ps;
   int i;
};

main.cpp

// Demo.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include <iostream>
#include <fstream>
#include <algorithm>
#include <string>
#include <vector>
#include <list>
#include <map>
#include <unordered_map>
#include <numeric>
#include <iterator>
#include <memory>
#include "HasPtr.h"

using namespace std;

int main()
{
   std::vector<HasPtr> v;
   v.push_back(std::to_string(10));
   v.push_back(std::to_string(5));
   v.push_back(std::to_string(7));
   v.push_back(std::to_string(3));
   v.push_back(std::to_string(2));
   v.push_back(std::to_string(9));

   std::cout << "==========List the elements==========" << std::endl;
   for (const auto &i : v) {
      std::cout << i.get_str() << " ";
   }
   std::cout << std::endl;
   std::cout << "=====================================" << std::endl;

   sort(v.begin(), v.end());

   std::cout << "==========List the elements==========" << std::endl;
   for (const auto &i : v) {
      std::cout << i.get_str() << " ";
   }
   std::cout << std::endl;
   std::cout << "=====================================" << std::endl;

   printf("_MSC_VER : %d \n", _MSC_VER);
   printf("_MSC_FULL_VER : %d \n", _MSC_FULL_VER);
   printf("_MSC_BUILD : %d \n", _MSC_BUILD);

#ifdef _MSVC_LANG
   printf("_MSVC_LANG : C++%d \n", (_MSVC_LANG / 100) % 2000);
#endif

   return 0;

   return 0;
}

执行结果：

String constructor execute
Copy constructor execute
String constructor execute
Copy constructor execute
Copy constructor execute
String constructor execute
Copy constructor execute
Copy constructor execute
Copy constructor execute
String constructor execute
Copy constructor execute
Copy constructor execute
Copy constructor execute
Copy constructor execute
String constructor execute
Copy constructor execute
Copy constructor execute
Copy constructor execute
Copy constructor execute
Copy constructor execute
String constructor execute
Copy constructor execute
==========List the elements==========
10 5 7 3 2 9
=====================================
Copy constructor execute
Copy constructor execute
Operator < execute
Copy constructor execute
Operator < execute
Assign execute
Assign execute
Copy constructor execute
Copy constructor execute
Operator < execute
Copy constructor execute
Operator < execute
Copy constructor execute
Operator < execute
Assign execute
Copy constructor execute
Operator < execute
Assign execute
Copy constructor execute
Copy constructor execute
Operator < execute
Copy constructor execute
Operator < execute
Assign execute
Assign execute
Assign execute
Assign execute
Copy constructor execute
Copy constructor execute
Operator < execute
Copy constructor execute
Operator < execute
Assign execute
Assign execute
Assign execute
Assign execute
Assign execute
Copy constructor execute
Copy constructor execute
Operator < execute
Copy constructor execute
Operator < execute
Copy constructor execute
Operator < execute
Assign execute
Copy constructor execute
Operator < execute
Assign execute
==========List the elements==========
2 3 5 7 9 10
=====================================

这个执行过程很奇怪，前两次的string consructor和copy constructor我可以理解，后面多次的assign和copy我就没法理解了。同时，swap也没有执行过。我觉得可能标准库用了std::swap().

一个相关问题：https://ask.youkuaiyun.com/questions/846424
stackoverflow上的提问和回答：https://stackoverflow.com/questions/58281051/stl-sort-a-vector-by-customer-operator-why-should-i-define-the-operator

用以验证执行过程的程序：

#include <algorithm>
#include <iostream>
#include <string>
#include <vector>

class Test {

  private:

    std::string str;
    int i;

  public:

    Test()
    {
      std::cout << "Test::Test()\n";
    }

    Test(const std::string &str): str(str), i(stoi(str))
    {
      std::cout << "Test::Test(\"" << str << "\")\n";
    }

    Test(int i): str(std::to_string(i)), i(i)
    {
      std::cout << "Test::Test(" << i << ")\n";
    }

    Test(const Test &test): str(test.str), i(test.i)
    {
      std::cout << "Test::Test(const Test &test " << test << ")\n";
    }

    Test& operator=(const Test &test)
    {
      std::cout << "*this " << *this << " = test " << test << '\n';
      str = test.str; i = test.i;
      return *this;
    }

    ~Test()
    {
      std::cout << "Test::~Test() " << *this << '\n';
    }

    bool operator<(const Test &test) const
    {
      std::cout << "*this " << *this << " < test " << test
        << ": " << (i < test.i ? "true" : "false") << '\n';
      return i < test.i;
    }

    friend std::ostream& operator<<(std::ostream&, const Test&);

};

std::ostream& operator<<(std::ostream &out, const Test &test)
{
  return out << "{ str: \"" << test.str << "\", i: " << test.i << " }";
}

template <typename T>
std::ostream& operator<<(std::ostream &out, const std::vector<T> &vec)
{
  const char *sep = "";
  for (const T &elem : vec) out << sep << elem, sep = ", ";
  return out;
}

#define DEBUG(...) std::cout << #__VA_ARGS__ << ";\n"; __VA_ARGS__ 

int main()
{
  DEBUG(std::vector<Test> test = { 1, 3, 2 });
  DEBUG(std::cout << "test[]: " << test << '\n');
  DEBUG(std::sort(test.begin(), test.end()));
  DEBUG(std::cout << "test[]: " << test << '\n');
}

执行结果：

g++ -std=c++17 -O2 -Wall -pedantic -pthread main.cpp && ./a.out
std::vector<Test> test = { 1, 3, 2 };
Test::Test(1)
Test::Test(3)
Test::Test(2)
Test::Test(const Test &test { str: "1", i: 1 })
Test::Test(const Test &test { str: "3", i: 3 })
Test::Test(const Test &test { str: "2", i: 2 })
Test::~Test() { str: "2", i: 2 }
Test::~Test() { str: "3", i: 3 }
Test::~Test() { str: "1", i: 1 }
std::cout << "test[]: " << test << '\n';
test[]: { str: "1", i: 1 }, { str: "3", i: 3 }, { str: "2", i: 2 }
std::sort(test.begin(), test.end());
*this { str: "3", i: 3 } < test { str: "1", i: 1 }: false
Test::Test(const Test &test { str: "3", i: 3 })
*this { str: "3", i: 3 } < test { str: "1", i: 1 }: false
*this { str: "3", i: 3 } = test { str: "3", i: 3 }
Test::~Test() { str: "3", i: 3 }
*this { str: "2", i: 2 } < test { str: "1", i: 1 }: false
Test::Test(const Test &test { str: "2", i: 2 })
*this { str: "2", i: 2 } < test { str: "3", i: 3 }: true
*this { str: "2", i: 2 } = test { str: "3", i: 3 }
*this { str: "2", i: 2 } < test { str: "1", i: 1 }: false
*this { str: "3", i: 3 } = test { str: "2", i: 2 }
Test::~Test() { str: "2", i: 2 }
std::cout << "test[]: " << test << '\n';
test[]: { str: "1", i: 1 }, { str: "2", i: 2 }, { str: "3", i: 3 }
Test::~Test() { str: "1", i: 1 }
Test::~Test() { str: "2", i: 2 }
Test::~Test() { str: "3", i: 3 }