[英]C++: Member is inaccessible - using friend function to allow one class to modify another class member data
好的,我已经到处寻找答案了。
我正在尝试建立一个系统,其中只能通过单独的 class 的两个成员函数来修改对象的私有成员。
这里是要修改的object的header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 文件而不是 table.h 文件中声明 function 的原因。
话虽如此,我读过的消息来源,例如state ,朋友 function 的实现应该在 class 中,其数据正在被修改,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 output 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
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,这样就足够了,对吧? 好吧, server
包括table
, table
包括server
,所以我们有循环依赖。 为了解决这个问题, table
应该包括server
并且server
应该转发 declare 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这个问题。 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。
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