创建一个与多个类为友元的函数

Question

在下面的代码中，我尝试创建一个函数“patent_count”，它是“horse”、“pig”和“dog”类的友元。我可以让这个函数成为 1 个班级的朋友，但不能成为所有 3 个班级的朋友。谁能告诉我我的错误是什么？

/*******************************************************\
* Veternarian Class Problem - I need a class for each   *
* of 3 animals. Horse, Pig and Dogs                     *
\*******************************************************/

#include <cstdlib>
#include <iostream>
#include <string>

const int HORSE_KENNEL = 100; // Number of horses we can store
const int PIG_KENNEL = 100; // Number of Pigs we can store
const int DOG_KENNEL = 100; // Number of Dogs we can store

/*******************************************************\
* Class horse                                           *
*                                                       *
* Member functions                                      *
* horse_count -- Keeps track of the number of horses    *
* add_horse -- Sends data into the object               *
* next_horse -- returns data from the object            *
\*******************************************************/

// Definition of the Class
class horse {

   private:
      int horse_count;              // Variable to keep track of data
      std::string horse_data[HORSE_KENNEL]; // A Place to put the data

      // Declarations for the method Prototypes
   public:            
      // Initialize 
      horse( );

      // A Function that accepts an argument but returns nothing
      void add_horse(const std::string new_horse_data);

      // This method returns the next Horse in the queue
      std::string next_horse( );

      friend int patient_count(horse);

};


/*******************************************************\
* Method Definition - Here we flush out the prototypes  *
* outlined in the last section                          *
\*******************************************************/
inline horse::horse( )
{
   for(int i = 0; i < HORSE_KENNEL; ++i){
      horse_data[i] = "Empty Spot";
   }
   horse_count = 0; // Zero the data count
}

/*******************************************************\
* horse::add_horse -- Send data to Object               *
\*******************************************************/
inline void horse::add_horse(const std::string new_horse_data)
{
   horse_data[horse_count] = new_horse_data;
   ++horse_count;
}

/*******************************************************\
* horse::next_horse - get data from object              *
\*******************************************************/

inline std::string horse::next_horse( )
{
   // this is specifically implementing a queue
   std::string current_horse = " ";
   int target_horse = 0;
   for(int i = 0;i < HORSE_KENNEL; ++i){
      if(horse_data[i] != "Empty Spot"){
         std::cout << "Horse Number " << i << " " << horse_data[i] << std::endl; 
      }
   }
   std::cout << "Select the horse you want: "; 
   std::cin >> target_horse;

   return (horse_data[target_horse]);
}

/*******************************************************\
* Class Pig                                             *
*                                                       *
* Member functions                                      *
* pig_count -- Keeps track of the number of pigs        *
* add_pig -- Sends data into the object                 *
* next_pig -- returns data from the object              *
\*******************************************************/

// Definition of the Class
class pig {

   private:
      int pig_count;              // Variable to keep track of data
      std::string pig_data[PIG_KENNEL]; // A Place to put the data

      // Declarations for the method Prototypes
   public:            
      // Initialize 
      pig( );

      // A Function that accepts an argument but returns nothing
      void add_pig(const std::string new_pig_data);

      // This method returns the next pig in the queue
      std::string next_pig( );

      friend pig patient_count(pig);
};


/*******************************************************\
* Method Definition - Here we flush out the prototypes  *
* outlined in the last section                          *
\*******************************************************/
inline pig::pig( )
{
   for(int i = 0; i < PIG_KENNEL; ++i){
      pig_data[i] = "Empty Spot";
   }
   pig_count = 0; // Zero the data count
}

/*******************************************************\
* pig::add_pig -- Send data to Object                   *
\*******************************************************/
inline void pig::add_pig(const std::string new_pig_data)
{
   pig_data[pig_count] = new_pig_data;
   ++pig_count;
}

/*******************************************************\
* pig::next_pig - get data from object                  *
\*******************************************************/

inline std::string pig::next_pig( )
{
   // this is specifically implementing a queue
   std::string current_pig = " ";
   int target_pig = 0;
   for(int i = 0;i < PIG_KENNEL; ++i){
      if(pig_data[i] != "Empty Spot"){
         std::cout << "pig Number " << i << " " << pig_data[i] << std::endl; 
      }
   }
   std::cout << "Select the pig you want: "; 
   std::cin >> target_pig;

   return (pig_data[target_pig]);
}

/*******************************************************\
* Class dog                                             *
*                                                       *
* Member functions                                      *
* dog_count -- Keeps track of the number of dogs        *
* data_to_object -- Sends data into the object          *
* data_from_object -- returns data from the object      *
\*******************************************************/

// Definition of the Class
class dog {

  private:
    int dog_count;              // Variable to keep track of data
    std::string dog_data[DOG_KENNEL]; // A Place to put the data

  // Declarations for the method Prototypes
  public:            
    // Initialize 
    dog( );

    // A Function that accepts an argument but returns nothing
    void add_dog(const std::string new_dog_data);

    // This method returns the next dog in the queue
    std::string next_dog( );

  friend dog patient_count(dog);
};


/*******************************************************\
* Method Definition - Here we flush out the prototypes  *
* outlined in the last section                          *
\*******************************************************/
inline dog::dog( )
{
   for(int i = 0; i < DOG_KENNEL; ++i){
      dog_data[i] = "Empty Spot";
   }
   dog_count = 0; // Zero the data count
}

/*******************************************************\
* dog::add_dog -- Send data to Object                   *
\*******************************************************/
inline void dog::add_dog(const std::string new_dog_data)
{
   dog_data[dog_count] = new_dog_data;
   ++dog_count;
}

/*******************************************************\
* dog::next_dog - get data from object                  *
\*******************************************************/

inline std::string dog::next_dog( )
{
   // this is specifically implementing a queue
   std::string current_dog = " ";
   int target_dog = 0;
   for(int i = 0;i < DOG_KENNEL; ++i){
      if(dog_data[i] != "Empty Spot"){
         std::cout << "dog Number " << i << " " << dog_data[i] << std::endl; 
      }
   }
   std::cout << "Select the dog you want: "; 
   std::cin >> target_dog;

   return (dog_data[target_dog]);
}

/**************************************************\
* This function is a friend of all the animal      *
* classes and returns the total of all animals     *
* PROBLEM ******* PROBLEM *********PROBLEM *********
* When I add the other 2 classes on the next line  *
* The program stops working                        *
\**************************************************/
// int patient_count(horse target_horse) //works
int patient_count(horse target_horse, pig target_pig, dog target_dog) // Nova
{
   //  int all_animals = target_horse.horse_count; //Works
   int all_animals = target_horse.horse_count + target_pig.pig_count + target_dog.dog_count; // Nova

   return (all_animals);
}
/**************************************************\
* The Class is defined above, this section is a    *
* Small testing harness to verify that the class   *
* is doing what it was designed to do              *
\**************************************************/
int main( )
{
   int total_animals;
   horse current_horse; // Create a instance

   // Send 3 values to the object
   current_horse.add_horse("Mr Ed, 10, Male");
   current_horse.add_horse("Lightning, 4, Female");
   current_horse.add_horse("Blitz, 7, Male");

   // Call for the return of the 3 values
   std::cout << "Selected Horse  ->" << current_horse.next_horse( ) << '\n';

   pig current_pig; // Create a instance

   // Send 3 values to the object
   current_pig.add_pig("Arnold, 4, Male");
   current_pig.add_pig("Babe, 2, Female");
   current_pig.add_pig("Killer, 7, Male");

   // Call for the return of the 3 values
   std::cout << "Selected Pig  ->" << current_pig.next_pig( ) << '\n';

   dog current_dog; // Create a instance

   // Send 3 values to the object
   current_dog.add_dog("Misty, 15, Female");
   current_dog.add_dog("Tristian, 12, Male");
   current_dog.add_dog("Tempest, 11, Female");

   // Call for the return of the 3 values
   std::cout << "Selected Dog  ->" << current_dog.next_dog( ) << '\n';


   // Now get the results from the friend function
   //  total_animals = patient_count(current_horse); // Works
   total_animals = patient_count(current_horse, current_pig, current_dog); // Nova
   std::cout << "Total Animals: " << total_animals << std::endl;
   return (0);
}

Answer 1

在为每个类声明它们时，您需要使用适当的签名/声明来定义适当的友元函数 patent_count。

现在，您有一个使用签名定义的 patent_count 函数：

int patient_count(horse target_horse, pig target_pig, dog target_dog);

在各个类中声明的友元函数都带有签名：

dog patient_count(dog);
int patient_count(horse);
pig patient_count(pig);

您需要为每个签名定义函数。
您使用 3 个输入参数定义的函数几乎毫无用处，因为它没有声明为您的任何类的友元。

请注意，在 C++ 中，函数是基于以下条件重载的（每个函数都有单独的存在）：

• 参数数量
• 和参数序列
• 类型 o 参数

因此，每个函数都是独立的且不相同，需要单独的定义。

Answer 2

嗯......我认为有一个很多更简单的方法来处理这个问题：

class animal { 
    static int count;

    animal() { ++count; }
    ~animal() { --count; }
};

class horse : public animal { 
    // horse stuff
};

class pig : public animal { 
    // pig stuff here
};

class dog : public animal { 
    // dog stuff here
};

int patient_count() { return animal::count; }

除此之外，你的代码似乎有一个相当基本的问题：它混淆了（例如）一个动物与一组动物。你有很多这样的想法：

dog current_dog; // Create a instance

// Send 3 values to the object
current_dog.add_dog("Misty, 15, Female");
current_dog.add_dog("Tristian, 12, Male");
current_dog.add_dog("Tempest, 11, Female");

这毫无意义。狗应该准确地代表：一只狗。它有一个名字、一种年龄、一种性别等等。上面的实际上是三只狗，而不是一只。为了代表它们，你应该有一个狗的集合——最好是像 std::vector 这样的标准集合，但是如果你不允许使用它（这可能是半合理的，因为这听起来/看起来像家庭作业）至少是一个数组。

dog dogs[10];   // an array of 10 dogs (none yet initialized though)

dogs[0] = dog("Misty, 15, female");
dogs[1] = dog("Tristian, 12, male");
dogs[2] = dog("Tempest, 11, female");

猪、牛、马等几乎都是一样的：一个动物对象应该代表一种实际的动物。动物的集合与单个动物是不同的。但请注意上面的评论：一个由 10 只狗组成的数组就是 10 只狗（尽管它们都没有名字、年龄或性别，但我们已经定义了它们，因此它们都正式存在）。这意味着当您定义数组时，patent_count 将报告存在 10 只狗，无论包含有意义数据的数量是多少。从这种方式来看，std::vector 显然是更好的选择。如果您执行以下操作：

std::vector<dog> dogs;

dogs.push_back("Misty, 15, female");
dogs.push_back("Tristian, 12, male");
dogs.push_back("Tempest, 11, female");

此时，您已经创建并存储了 3 只狗，因此如果您此时打印出 patent_count，它应该显示 3（代表创建/定义的实际狗）而不是 10 （或其他）来表示潜在动物的数量，而忽略包含有意义数据的数量。

Answer 3

在您的类中，您将它们声明为以下 3 个函数的友元（分别针对马、猪和狗类）

int patient_count(horse);
pig patient_count(pig);
dog patient_count(dog);

这些函数都不存在。唯一存在的功能是这个：

int patient_count(horse,pig,dog);

用该声明替换每个类中的朋友行。您还需要转发声明猪和狗。

Answer 4

因为

int patient_count(horse);
pig patient_count(pig);
dog patient_count(dog);

和

int patient_count(horse target_horse, pig target_pig, dog target_dog);

是病人计数的重载。最好的办法是创建一个带有 Animal_count 字段的基类动物，如下所示

/*******************************************************\
* Veternarian Class Problem - I need a class for each   *
* of 3 animals. Horse, Pig and Dogs                     *
\*******************************************************/

#include <cstdlib>
#include <iostream>
#include <string>

const int HORSE_KENNEL = 100; // Number of horses we can store
const int PIG_KENNEL = 100; // Number of Pigs we can store
const int DOG_KENNEL = 100; // Number of Dogs we can store


class animal{
protected:
    int animal_count;
public:
    friend int patient_count(animal);
};



/*******************************************************\
* Class horse                                           *
*                                                       *
* Member functions                                      *
* animal_count -- Keeps track of the number of horses    *
* add_horse -- Sends data into the object               *
* next_horse -- returns data from the object            *
\*******************************************************/
// Definition of the Class
class horse : public animal {

  private:
    std::string horse_data[HORSE_KENNEL]; // A Place to put the data

  // Declarations for the method Prototypes
  public:            
    // Initialize 
    horse::horse( );

    // A Function that accepts an argument but returns nothing
    void add_horse(const std::string new_horse_data);

    // This method returns the next Horse in the queue
    std::string next_horse( );


};


/*******************************************************\
* Method Definition - Here we flush out the prototypes  *
* outlined in the last section                          *
\*******************************************************/
inline horse::horse( )
{
  for(int i = 0; i < HORSE_KENNEL; ++i){
    horse_data[i] = "Empty Spot";
  }
  animal_count = 0; // Zero the data count
}

/*******************************************************\
* horse::add_horse -- Send data to Object               *
\*******************************************************/
inline void horse::add_horse(const std::string new_horse_data)
{
  horse_data[animal_count] = new_horse_data;
  ++animal_count;
}

/*******************************************************\
* horse::next_horse - get data from object              *
\*******************************************************/

inline std::string horse::next_horse( )
{
  // this is specifically implementing a queue
  std::string current_horse = " ";
  int target_horse = 0;
  for(int i = 0;i < HORSE_KENNEL; ++i){
    if(horse_data[i] != "Empty Spot"){
      std::cout << "Horse Number " << i << " " << horse_data[i] << std::endl; 
    }
  }
  std::cout << "Select the horse you want: "; 
  std::cin >> target_horse;

return (horse_data[target_horse]);
}

/*******************************************************\
* Class Pig                                             *
*                                                       *
* Member functions                                      *
* animal_count -- Keeps track of the number of pigs        *
* add_pig -- Sends data into the object                 *
* next_pig -- returns data from the object              *
\*******************************************************/

// Definition of the Class
class pig :public animal{

  private:
     // Variable to keep track of data
    std::string pig_data[PIG_KENNEL]; // A Place to put the data

  // Declarations for the method Prototypes
  public:            
    // Initialize 
    pig::pig( );

    // A Function that accepts an argument but returns nothing
    void add_pig(const std::string new_pig_data);

    // This method returns the next pig in the queue
    std::string next_pig( );
};


/*******************************************************\
* Method Definition - Here we flush out the prototypes  *
* outlined in the last section                          *
\*******************************************************/
inline pig::pig( )
{
  for(int i = 0; i < PIG_KENNEL; ++i){
    pig_data[i] = "Empty Spot";
  }
  animal_count = 0; // Zero the data count
}

/*******************************************************\
* pig::add_pig -- Send data to Object                   *
\*******************************************************/
inline void pig::add_pig(const std::string new_pig_data)
{
  pig_data[animal_count] = new_pig_data;
  ++animal_count;
}

/*******************************************************\
* pig::next_pig - get data from object                  *
\*******************************************************/

inline std::string pig::next_pig( )
{
  // this is specifically implementing a queue
  std::string current_pig = " ";
  int target_pig = 0;
  for(int i = 0;i < PIG_KENNEL; ++i){
    if(pig_data[i] != "Empty Spot"){
      std::cout << "pig Number " << i << " " << pig_data[i] << std::endl; 
    }
  }
  std::cout << "Select the pig you want: "; 
  std::cin >> target_pig;

return (pig_data[target_pig]);
}

/*******************************************************\
* Class dog                                             *
*                                                       *
* Member functions                                      *
* animal_count -- Keeps track of the number of dogs        *
* data_to_object -- Sends data into the object          *
* data_from_object -- returns data from the object      *
\*******************************************************/

// Definition of the Class
class dog :public animal {

  private:
    std::string dog_data[DOG_KENNEL]; // A Place to put the data

  // Declarations for the method Prototypes
  public:            
    // Initialize 
    dog::dog( );

    // A Function that accepts an argument but returns nothing
    void add_dog(const std::string new_dog_data);

    // This method returns the next dog in the queue
    std::string next_dog( );


};


/*******************************************************\
* Method Definition - Here we flush out the prototypes  *
* outlined in the last section                          *
\*******************************************************/
inline dog::dog( )
{
  for(int i = 0; i < DOG_KENNEL; ++i){
    dog_data[i] = "Empty Spot";
  }
  animal_count = 0; // Zero the data count
}

/*******************************************************\
* dog::add_dog -- Send data to Object                   *
\*******************************************************/
inline void dog::add_dog(const std::string new_dog_data)
{
  dog_data[animal_count] = new_dog_data;
  ++animal_count;
}

/*******************************************************\
* dog::next_dog - get data from object                  *
\*******************************************************/

inline std::string dog::next_dog( )
{
  // this is specifically implementing a queue
  std::string current_dog = " ";
  int target_dog = 0;
  for(int i = 0;i < DOG_KENNEL; ++i){
    if(dog_data[i] != "Empty Spot"){
      std::cout << "dog Number " << i << " " << dog_data[i] << std::endl; 
    }
  }
  std::cout << "Select the dog you want: "; 
  std::cin >> target_dog;

return (dog_data[target_dog]);
}

/**************************************************\
* This function is a friend of all the animal      *
* classes and returns the total of all animals     *
* PROBLEM ******* PROBLEM *********PROBLEM *********
* When I add the other 2 classes on the next line  *
* The program stops working                        *
\**************************************************/
// int patient_count(horse target_horse) //works
int patient_count(animal target_animal) // Nova
{
    int all_animals = target_animal.animal_count;
    return (all_animals);
}
int patient_count(horse target_horse, pig target_pig, dog target_dog) // Nova
{
//  int all_animals = target_horse.animal_count; //Works
   int all_animals = patient_count(target_horse) + patient_count(target_pig) + patient_count(target_dog); // Nova

return (all_animals);
}
/**************************************************\
* The Class is defined above, this section is a    *
* Small testing harness to verify that the class   *
* is doing what it was designed to do              *
\**************************************************/
int main( )
{
  int total_animals;
  horse current_horse; // Create a instance

  // Send 3 values to the object
  current_horse.add_horse("Mr Ed, 10, Male");
  current_horse.add_horse("Lightning, 4, Female");
  current_horse.add_horse("Blitz, 7, Male");

  // Call for the return of the 3 values
  std::cout << "Selected Horse  ->" << current_horse.next_horse( ) << '\n';

  pig current_pig; // Create a instance

  // Send 3 values to the object
  current_pig.add_pig("Arnold, 4, Male");
  current_pig.add_pig("Babe, 2, Female");
  current_pig.add_pig("Killer, 7, Male");

  // Call for the return of the 3 values
  std::cout << "Selected Pig  ->" << current_pig.next_pig( ) << '\n';

  dog current_dog; // Create a instance

  // Send 3 values to the object
  current_dog.add_dog("Misty, 15, Female");
  current_dog.add_dog("Tristian, 12, Male");
  current_dog.add_dog("Tempest, 11, Female");

  // Call for the return of the 3 values
  std::cout << "Selected Dog  ->" << current_dog.next_dog( ) << '\n';


  // Now get the results from the friend function
//  total_animals = patient_count(current_horse); // Works
  total_animals = patient_count(current_horse, current_pig, current_dog); // Nova
  std::cout << "Total Animals: " << total_animals << std::endl;
return (0);
}