C++:成员不可访问-使用友元函数允许一个类修改另一个类的成员数据

C++: Member is inaccessible - using friend function to allow one class to modify another class member data

本文关键字：成员 许一个 修改 另一个 数据 函数 访问 C++ 友元      更新时间：2023-10-16

好吧，我已经到处找答案了。

我正在尝试建立一个系统，在这个系统中，对象的私有成员只能通过一个单独类的两个成员函数进行修改。

这是要修改的对象的标题内容，表.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

这是服务器。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，restaurant/h包含server.h。

我试着把这个放在桌子上。h之前：

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

但这让我更加头疼，因为编译器会吐出类似"类服务器没有成员‘seatGuests’或‘cleanTable’"的内容，我认为这是由于一些包含冲突。这就是为什么我在原始的server.h文件中声明了函数，而不是在table.h文件中。

话虽如此，我读过的这类源声明，friend函数的实现应该在数据被修改的类中，这就是我所做的。

除此之外，编译器仍然认为成员数据是不可访问的。有人看到我在这里做错了什么吗？

如果你想让我发布更多代码，请告诉我。

提前谢谢。

编辑

对延迟发布编译器输出表示歉意。请参阅以下内容：

> 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

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？server包括table，table包括server，所以我们有循环依赖关系。为了绕过这一点，table应该包括server，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 &);
};

循环依赖关系和友元声明对我来说似乎是一个糟糕的设计。你可能需要重新思考这个设计，而不是使用前向声明来解决这个问题。找到一种方法来避免循环，避免朋友声明我的答案是变通办法，而不是解决方案

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