C++ 创建一个函数的朋友？

Question

所以我被要求为一所大学创建一个函数的朋友 class，我对实现有点迷茫。问题指出“添加函数 showCat(Cat&) 作为 Cat 的好友。此函数应以与 Cat::showCat() 函数相同的方式显示猫的详细信息

程序如下

#include<iostream>
#include<string>
using namespace std;

class Cat
{
private:
   string name;
   string breed;
   int age;
   static constexpr double licenseFee = 10;

public:
   void setCatData(string, string, int);
   void showCat();
   friend void showCat(const Cat& cat); //friend function declaration
   
};

void Cat::setCatData(string catName, string catBreed, int catAge)
{
   name = catName;
   breed = catBreed;
   age = catAge;
}

void Cat::showCat(const Cat& aCat) //friend function definition
{
  cout << "Cat: " << aCat.name << " is a " << aCat.breed << endl;
  cout << "The cat's age is " << aCat.age << endl;
  cout << "License fee: $" << aCat.licenseFee << endl;
}

void Cat::showCat()
{
  cout << "Cat: " << name << " is a " << breed << endl;
  cout << "The cat's age is " << age << endl;
  cout << "License fee: $" << licenseFee << endl;
}

int main()
{
     Cat myCat;
     myCat.setCatData("Tigger", "Fluffy unit", 3);
     myCat.showCat();
     
}

提供的注释并不引人注目，但这是我根据提供给我的信息所做的尝试。我对friend函数的尝试可以在猫class的public成员中的声明friend void showCat(const Cat& cat);中看到。定义可以看作

void Cat::showCat(const Cat& aCat)
{
  cout << "Cat: " << aCat.name << " is a " << aCat.breed << endl;
  cout << "The cat's age is " << aCat.age << endl;
  cout << "License fee: $" << aCat.licenseFee << endl;
}

这是我错误的根源。在 void Cat::showCat(const Cat& aCat) 线上，我得到 declaration is incompatible with "void Cat::showCat()" (declared at line 34)

我对朋友们的操作方式有点迷茫，所以如果能帮助我修复 function/program 将不胜感激 :)

Answer 1

声明

friend void showCat(const Cat& cat);

将 showCat(const Cat&) 声明为非成员函数。

所以你需要这样定义它：

// Note that this is not a member function
void showCat(const Cat& aCat)
{
    ...
}

Answer 2

而不是：

void Cat::showCat(const Cat& aCat) //friend function definition

做：

void showCat(const Cat& aCat) //friend function definition



void  showCat(const Cat& aCat) //friend function definition
{
  cout << "Cat: " << aCat.name << " is a " << aCat.breed << endl;
  cout << "The cat's age is " << aCat.age << endl;
  cout << "License fee: $" << aCat.licenseFee << endl;
}

Answer 3

可以这样定义友元函数

好友功能:

定义一个函数作为 class 的友元函数 :

:

   friend void showCat(const Cat& cat);

然后独立实现这个功能而不是作为class的成员。

       void showCat(const Cat& a)
       {
             cout << "Cat: " << a.name << " is a " << a.breed << endl;
             cout << "The cat's age is " << a.age << endl;
             cout << "License fee: $" << a.licenseFee << endl;
       }

定义一个函数作为多个函数的友元class.

 #include<iostream>
 #include<string>
 using namespace std;

 class Cat
 {
 private:
    string name;
    string breed;
    int age;
    static constexpr double licenseFee = 10;

 public:
    void setCatData(string, string, int);
    template<class A> friend void showAnimal(const A& a); //friend function declaration

 };

 class Mouse
 {
 private:
    string name;
    string breed;
    int age;
    static constexpr double licenseFee = 10;

 public:
    void setData(string, string, int);
    template<class A> friend void showAnimal(const A& a); //friend function declaration

 };
 void Cat::setCatData(string catName, string catBreed, int catAge)
 {
    name = catName;
    breed = catBreed;
    age = catAge;
 }
 void Mouse::setData(string mouseName, string mouseBreed, int mouseAge)
 {
    name = mouseName;
    breed = mouseBreed;
    age = mouseAge;
 }
 template<class A>
 void showAnimal (const A& a) //friend function definition
 {
         return;
 }
 template<>
 void showAnimal <Cat>(const Cat& a) //friend function definition
 {
   cout << "Cat: " << a.name << " is a " << a.breed << endl;
   cout << "The cat's age is " << a.age << endl;
   cout << "License fee: $" << a.licenseFee << endl;
 }

 template<>
 void showAnimal <Mouse>(const Mouse& a) //friend function definition
 {
   cout << "Cat: " << a.name << " is a " << a.breed << endl;
   cout << "The cat's age is " << a.age << endl;
   cout << "License fee: $" << a.licenseFee << endl;
 }

 int main()
 {
      Cat myCat;
      myCat.setCatData("Tigger", "Fluffy unit", 3);
      showAnimal(myCat);

      Mouse myMouse;
      myMouse.setData("Tigger", "Fluffy unit", 3);
      showAnimal(myMouse);
 }

将一个class定义为另一个class的朋友：

 #include<iostream>
 #include<string>
 using namespace std;

 class Cat
 {
 private:
    string name;
    string breed;
    int age;
    static constexpr double licenseFee = 10;

 public:
    void setCatData(string, string, int);
   friend class Show;
 };

 void Cat::setCatData(string catName, string catBreed, int catAge)
 {
    name = catName;
    breed = catBreed;
    age = catAge;
 }

class Show
{

public:

    void showAnimal(const Cat& a)
    {
          cout << "Cat: " << a.name << " is a " << a.breed << endl;
          cout << "The cat's age is " << a.age << endl;
          cout << "License fee: $" << a.licenseFee << endl;
    }
};

 int main()
 {
      Cat myCat;
      Show show;
      myCat.setCatData("Tigger", "Fluffy unit", 3);
      show.showAnimal(myCat);

 }

定义一个class的成员函数作为另一个class的友元函数：

 #include<iostream>
 #include<string>
 using namespace std;
class Cat;
class Show
{

public:

    void showAnimal(const Cat& a);
};
 class Cat
 {
 private:
    string name;
    string breed;
    int age;
    static constexpr double licenseFee = 10;

 public:
    void setCatData(string, string, int);
   friend void  Show::showAnimal(const Cat& a);
 };

 void Cat::setCatData(string catName, string catBreed, int catAge)
 {
    name = catName;
    breed = catBreed;
    age = catAge;
 }

    void Show::showAnimal(const Cat& a)
    {
          cout << "Cat: " << a.name << " is a " << a.breed << endl;
          cout << "The cat's age is " << a.age << endl;
          cout << "License fee: $" << a.licenseFee << endl;
    }

 int main()
 {
      Cat myCat;
      Show show;
      myCat.setCatData("Tigger", "Fluffy unit", 3);
      show.showAnimal(myCat);

 }

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