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

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.csdn.net/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 }

其中有pivot节点，是一个快速排序？