爲什麼以下函數可以在不聲明爲朋友或成員函數的情況下訪問類成員？

Question

頭文件：

// pe10-8arr.h -- header file for a simple list class 

#ifndef SIMPLEST_ 
#define SIMPLEST_ 

// program-specific declarations 
const int TSIZE = 45;  // size of array to hold title 
struct film 
{ 
    char title[TSIZE]; 
    int rating; 
}; 

// general type definitions 
typedef struct film Item; 

const int MAXLIST = 10; 
class simplist 
{ 
private: 
    Item items[MAXLIST]; 
    int count; 
public: 
    simplist(void); 
bool isempty(void); 
bool isfull(void); 
    int itemcount(); 
bool additem(Item item); 
    void transverse(void (*pfun)(Item &item)); 
}; 

#endif 

代碼使用報頭：

#include "pe10-8arr.h" 

simplist::simplist(void) 
{ 
    count = 0; 
} 

bool simplist::isempty(void) 
{ 
    return count == 0; 
} 

bool simplist::isfull(void) 
{ 
    return count == MAXLIST; 
} 

int simplist::itemcount() 
{ 
    return count; 
} 
bool simplist::additem(Item item) 
{ 
    if (count == MAXLIST) 
     return false; 
    else 
     items[count++] = item; 
    return true; 
} 

void simplist::transverse(void (*pfun)(Item &item)) 
{ 
    for (int i = 0; i < count; i++) 
     (*pfun)(items[i]); 
} 

#include <iostream> 
#include <cstdlib>   // prototype for exit() 
#include "pe10-8arr.h"  // simple list class declaration 
           // array version 
void showmovies(Item &item); // to be used by transverse() 

int main(void) 
{ 
    using namespace std; 
    simplist movies;  // creates an empty list 
    Item temp; 

    if (movies.isfull()) // invokes isfull() member function 
    { 
     cout << "No more room in list! Bye!\n"; 
     exit(1); 
    } 
    cout << "Enter first movie title:\n"; 
    while (cin.getline(temp.title,TSIZE) && temp.title[0] != '\0') 
    { 
     cout << "Enter your rating <0-10>: "; 
     cin >> temp.rating; 
     while(cin.get() != '\n') 
      continue; 
     if (movies.additem(temp) == false) 
     { 
      cout << "List already is full!\n"; 
      break; 
     } 
     if (movies.isfull()) 
     { 
      cout << "You have filled the list.\n"; 
      break; 
     } 
     cout << "Enter next movie title (empty line to stop):\n"; 
    } 
    if (movies.isempty()) 
     cout << "No data entered. "; 
    else 
    { 
     cout << "Here is the movie list:\n"; 
     movies.transverse(showmovies); 
    } 
    cout << "Bye!\n"; 
    return 0; 
} 

void showmovies(Item &item) 
{ 
     std::cout << "Movie: " << item.title << " Rating: " 
      << item.rating << std::endl; 

} 

以上只是編譯和成功運行的代碼。誰能告訴我爲什麼函數showmovies()可以使用引用訪問simplist的項目成員，而不被聲明爲好友函數或成員函數？

Answer 1

該功能是交給Item對象（通過引用），它不知道或關心它來自哪裏。函數的調用者實際上是從完整對象內部拉動對象的函數，但transverse通過成爲類的成員具有訪問權。

Answer 2

結構成員的默認可見性爲public。該功能只使用Item，而不是simplist。

Answer 3

showMovies以Item爲參數。它不在乎它是否是會員。

Item是struct，其字段爲public，除非另有聲明。

Answer 4

內simplist：

void simplist::transverse(void (*pfun)(Item &item)) 
{ 
    for (int i = 0; i < count; i++) 
     (*pfun)(items[i]); 
} 

要調用從橫向（一個simplist的成員）showmovies，這樣你就可以訪問所有的類屬性，無論知名度。