C++：成员不可访问 - 使用友元函数允许一个类修改另一个类成员数据

Question

好的，我已经到处寻找答案了。

我正在尝试建立一个系统，其中对象的私有成员只能通过单独类的两个成员函数进行修改。

这里是要修改的对象的表头内容，table.h：

#ifndef TABLE_H
#define TABLE_H

#include "party.h"

/*
Seats a party for their dining experience at the restaurant.
*/
class table
{
    friend void server::seatGuests(const table &, const party &);
    friend void server::cleanTable(const table &);

private:
    const unsigned int cap_; //guest capacity (shouldn't change)
    const unsigned int id_;  //table id (shouldn't change)
    mutable bool isTaken_;   //flag (changeable by friend functions)
    party *const group_;     //party seated at the table (starts off as null)

public:
    table::table(const unsigned int id, const unsigned int cap = 4); //ctor
    inline const unsigned int table::getCap() const { return cap_; }
    inline const unsigned int table::getId() const { return id_; }
    inline const bool table::isTaken() const { return isTaken_; }
    inline const party *const table::getParty() const { return group_; };
};

void server::seatGuests(const table &t, const party &p)
{
    t.isTaken_ = false;
}

#endif // TABLE_H

这里是 server.h：

class server : public employee
{
private:
public:
    explicit server::server();
    void server::seatGuests(const table &, const party &);
    void server::cleanTable(const table &);
};

...

我收到以下错误： member "table::isTaken_" (declared at line 17) is inaccessible

server.h 文件间接包含在 table.h 中，因为 party.h 包含 restaurant.h，其中包含 server.h。

我之前试过把它放在 table.h 中：

class server;
namespace server {
   void server::seatGuests(const table &, const party &);
   void server::cleanTable(const table &);
}

但这只是让我更头疼，因为编译器会吐出类似“类服务器没有成员'seatGuests'或'cleanTable'”之类的东西，我认为这是由于一些包含冲突。这个冲突是我在原始 server.h 文件中而不是在 table.h 文件中声明函数的原因。

话虽如此，我读过的诸如this one 之类的消息表明，友元函数的实现应该在其数据被修改的类中，这就是我所做的。

除此之外，编译器仍然认为成员数据不可访问。谁能看到我在这里做错了什么？

如果您希望我发布更多代码，请告诉我。

提前致谢。

编辑

对于延迟发布编译器输出表示歉意。请看下面：

> Executing task: clang++ -Wall -std=c++17 -stdlib=libc++ -o main.out /Users/ajm/Projects/restaurant/cpp/main/main.cpp -I/Users/ajm/Projects/restaurant/cpp/main/ -I/Users/ajm/Projects/restaurant/cpp/restaurant/ -I/Users/ajm/Projects/restaurant/cpp/people/ -I/Users/ajm/Projects/restaurant/cpp/people/workers/ -I/Users/ajm/Projects/restaurant/cpp/data/ --debug <

In file included from /Users/ajm/Projects/restaurant/cpp/main/main.cpp:1:
In file included from /Users/ajm/Projects/restaurant/cpp/main/simulation.h:4:
In file included from /Users/ajm/Projects/restaurant/cpp/restaurant/restaurant.h:4:
In file included from /Users/ajm/Projects/restaurant/cpp/restaurant/tablespace.h:4:
In file included from /Users/ajm/Projects/restaurant/cpp/restaurant/table.h:4:
In file included from /Users/ajm/Projects/restaurant/cpp/people/workers/server.h:4:
In file included from /Users/ajm/Projects/restaurant/cpp/people/workers/employee.h:5:
In file included from /Users/ajm/Projects/restaurant/cpp/main/../data/job.h:4:
/Users/ajm/Projects/restaurant/cpp/people/party.h:25:16: error: unknown type name 'restaurant'
    void start(restaurant &spot);
               ^
In file included from /Users/ajm/Projects/restaurant/cpp/main/main.cpp:1:
In file included from /Users/ajm/Projects/restaurant/cpp/main/simulation.h:4:
In file included from /Users/ajm/Projects/restaurant/cpp/restaurant/restaurant.h:4:
In file included from /Users/ajm/Projects/restaurant/cpp/restaurant/tablespace.h:4:
/Users/ajm/Projects/restaurant/cpp/restaurant/table.h:8:11: error: redefinition of 'server' as different kind of
      symbol
namespace server
          ^
/Users/ajm/Projects/restaurant/cpp/restaurant/table.h:7:7: note: previous definition is here
class server;
      ^
/Users/ajm/Projects/restaurant/cpp/restaurant/table.h:10:23: error: unknown type name 'table'
void seatGuests(const table &t, const party *const p);
                      ^
/Users/ajm/Projects/restaurant/cpp/restaurant/table.h:11:23: error: unknown type name 'table'
void cleanTable(const table &t);
                      ^
/Users/ajm/Projects/restaurant/cpp/restaurant/table.h:13:31: error: unknown type name 'table'
void server::seatGuests(const table &t, const party *const p)
                              ^
/Users/ajm/Projects/restaurant/cpp/restaurant/table.h:18:31: error: unknown type name 'table'
void server::cleanTable(const table &t)
                              ^
/Users/ajm/Projects/restaurant/cpp/restaurant/table.h:42:25: error: out-of-line declaration of 'seatGuests' does not
      match any declaration in 'server'
    friend void server::seatGuests(const table &t, const party *const p);
                        ^~~~~~~~~~
/Users/ajm/Projects/restaurant/cpp/restaurant/table.h:43:25: error: out-of-line declaration of 'cleanTable' does not
      match any declaration in 'server'
    friend void server::cleanTable(const table &t);
                        ^~~~~~~~~~
8 errors generated.
The terminal process terminated with exit code: 1

Answer 1

server::seatGuests 和 server::cleanTable 的声明必须出现在 table 中的 friend 声明之前。 #include "server.h" 出现在 table 的声明之前，所以应该足够了吧？好吧，server 包括 table 和 table 包括 server 所以我们有循环依赖。为了解决这个问题，table 应该包含server 并且server 应该转发声明table。

// table.h

#include "party.h"

class table
{
    friend void server::seatGuests(const table &, const party &);
    friend void server::cleanTable(const table &);
};



// server.h

class table;
class party;

class server : public employee
{
public:
    explicit server();
    void seatGuests(const table &, const party &);
    void cleanTable(const table &);
};

循环依赖和友元声明对我来说似乎是糟糕的设计。您可能需要重新考虑设计，而不是使用前向声明来解决此问题。找到一种避免循环并避免友元声明的方法。 我的回答是解决方法，而不是解决方案。