c++ - 创建一个函数，它是多个类的友元

Question

在下面的代码中，我试图创建一个函数“patient_count”，它是类“horse”、“pig”和“dog”的友元。我可以获得与 1 个类(class)成为 friend 的功能，但不能成为所有 3 个类(class)的 friend 。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/声明定义适当的友元函数 patient_count。

现在您有一个定义了签名的 patient_count 函数:

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

在你的各个类中声明的友元函数带有签名:

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没有用，因为它没有声明为您的任何类的友元。

请注意，在 C++ 中，函数重载(每个函数都有单独的存在)基于:

• 参数数量
• 参数序列&
• 类型 o 参数

所以这些函数中的每一个都是独立的，不一样的，需要一个单独的定义。