我该如何处理这个 C++ Multimap 问题？

Question

我是 C++ 的初学者，正在研究简单的程序，但遇到了一个让我很困惑的问题...基本上我创建了一个多重映射，它接受字符串人名的一部分和一个人对象的指针。

所以我使用的第一个函数是“Add()”函数。我基本上用名字和年龄构造了一个新人，然后设置一个指向该对象的指针。然后我将名称和对象作为一对插入到多重映射中。

问题发生在我运行 find() 函数时...由于我是初学者，我很确定我正在做一些非常简单但非常愚蠢的事情。我认为您不需要运行代码，因为它非常简单。感谢所有帮助。谢谢！

我的可运行文件：

#include "Person.h"
#include <map>
using namespace std;

multimap<string, Person*> personMap;
multimap<string, Person*>::iterator counter;


void add()
{
  string name;
  int age;
  cout << "Please enter the name of the person: ",  cin >> name;
  cout << "Please enter the age of the person: ", cin >> age;
  Person generatedPerson(name, age);

  // Avoid dynamic class pointers for memory optimization.
  // Avoided: Person* pointer = new Person(name, age).
  Person *pointer = &generatedPerson;
  personMap.insert({name, pointer});

}

void find()
{
  string personToBeFound;
  cout << "Who do you wish to find, friend: ", cin >> personToBeFound;


  for (counter = personMap.begin(); counter != personMap.end(); counter++)
  {
    if (counter->first == personToBeFound)
    {
      cout << "\nName: " << counter->first << " Age: " << counter->second->getAge() << endl;
    }
    else if (counter->first != personToBeFound)
    {
      cout << "Error 404, person does not exist..." << endl;
    }
  }
}

// Experimental....
int main(int argc, char* argv[])
{
  int menuChoice = -1;

  while (menuChoice != 0)
  {
    cout << "\nPlease enter: "
            "\n1 - to add a person "
            "\n2 - to find a person"
            "\n0 - to quit\n" << endl;
    cin >> menuChoice;
    switch(menuChoice)
    {
      case 1: add();
        break;
      case 2: find();
        break;
      case 0: menuChoice = 0;
    }
  }
}

头文件：

#ifndef PERSON_H
#define PERSON_H

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

class Person {
 public:
  // Constructors

  /**
   * Create a Person with the given name and age.
   *
   * @param name name of the person
   * @param age age of the person - defaults to 0
   */
  Person(const std::string& name, unsigned short age = 0);


  // No explicit destructor necessary


  // Mutators

  /**
   * Set the name attribute
   *
   * @param name name of the person
   */
  void setName(const std::string& name);

  /**
   * Set the age attribute
   *
   * @param age age of the person
   */
  void setAge(unsigned short age);

  /**
   * Increment the age attribute
   */
  void growOlder();

  /**
   * Add a person to our list of children
   *
   * @param child Person to add as a child
   */
  void addChild(const Person& child);


  // Accessors

  /**
   * @return the Person's name
   */
  const std::string& getName() const;

  /**
   * @return the Person's age
   */
  unsigned short getAge() const;

  /**
   * @return a list of this Person's children
   */
  const std::vector<const Person *>& getChildren() const;

  /**
   * Define the ostream's << operator as a friend of this class
   * to allow this object to be printed to an output stream
   *
   * @param output the stream to print to
   * @param p the Person to print
   *
   * @return the output stream printed to
   */
  friend std::ostream& operator<< (std::ostream& output, const Person& p);


 private:

  // 0-arg Constructor
  Person();

  // Private attributes
  std::string _name;
  unsigned short _age;
  std::vector<const Person *> _children;

}; // Person

#endif

方法定义：

#include "Person.h"

Person::Person(const std::string& name, unsigned short age) :
    _name(name) , _age(age) {
}


void Person::setName(const std::string& name) {
  _name = name;
}


void Person::setAge(unsigned short age) {
  _age = age;
}


void Person::growOlder() {
  _age++;
}


void Person::addChild(const Person& child) {
  _children.push_back(&child);
}


const std::string& Person::getName() const {
  return _name;
}


unsigned short Person::getAge() const {
  return _age;
}


const std::vector<const Person *>& Person::getChildren() const {
  return _children;
}


std::ostream& operator<< (std::ostream& output, const Person& aPerson) {
  // Print our attributes to the output stream
  return output << "Name: '" << aPerson._name << "', Age: " << aPerson._age;
}

Answer 1

您好@1201ProgramAlarm 的评论是绝对正确的，但是，我自己想，我将如何去“拯救”程序（a）以最小的努力，（b）获得正确的版本和（c）非常与原始版本相比。

所以这是一个新的main.cc 版本（Person.h、Person.cc 无需更改）：

#include "Person.h"
#include <map>

using namespace std;

multimap <string, Person *> personMap;
multimap <string, Person *>::iterator counter;


void add()
{
    string name;
    int    age;

    cout << "Please enter the name of the person: ", cin >> name;
    cout << "Please enter the age  of the person: ", cin >> age;

    personMap.insert({ name, new Person(name, age) });
}


void find()
{
    string personToBeFound;

    cout << "Who do you wish to find, friend: ", cin >> personToBeFound;

    for (counter = personMap.begin(); counter != personMap.end(); counter++)
    {
        if (counter->first == personToBeFound)
        {
            cout << "\nName: " << counter->first
                 <<   " Age: " << counter->second->getAge() << endl;
        }
        else
        {
            cout << "Error 404, person does not exist..." << endl;
        }
    }
}


int main(int argc, char* argv[])
{
    int    menuChoice = -1;
    while (menuChoice != 0)
    {
        cout << "\nPlease enter: "
            "\n1 - to add a person "
            "\n2 - to find a person"
            "\n0 - to quit\n" << endl;

        cin >> menuChoice;
        switch(menuChoice)
        {
        case 1:
            add();
            break;

        case 2:
            find();
            break;

        case 0:
            menuChoice = 0;
            break;
        }
    }

    // -- How to go about clearing the heap? --

    for (counter = personMap.begin(); counter != personMap.end(); counter++)
    {
        if (counter->second) {
            delete   counter->second;
            counter->second = nullptr;
        }
    }
}

我使用g++ -o main Person.cc main.cc 编译。问候，M。

此问题后更新：

为什么我不应该只删除键而不是只删除 multimap <string, Person *> 条目的值？

回答：

我只是想展示当multimap 超出（本地）范围并因此会被@ 自动销毁时，可以采取哪些措施来防止堆上的孤立 Person 对象987654329@系统。更具体地说：

(a) 和C一样，malloc-ed 后面又需要free-ed，C++ 中的等价物是：new-ed 是什么需要@ 987654335@-ed 稍后。从长远来看，这是防止内存泄漏的最佳做法。

(b) 现在，回到我们的multimap <string, Person *>。这里，键是strings，值是指向Person 对象的指针。这些Person 指针 是在调用add() 函数时获得堆上分配的相应Person 对象 的唯一方法。

(c) 在我们的例子中，在multimap 被自动销毁后，整个程序无论如何都会停止，所以delete-ing Person 对象使用额外的for-loop 不是这里很重要，因为在相应的进程终止后，堆上剩余的 Person 对象会被操作系统进行垃圾回收。