c++ friend 遇到 namespace 无法访问 private 成员的问题

相关的文章（比较有意思，记录一下）：http://www.cnblogs.com/lx20030303/archive/2012/09/21/2696258.html

 

先看例子。如下。

尝试编译的话，错误提示：

Entrance.cpp
src\Entrance.cpp(15) : error C2248: “lqr::Entrance::id”: 无法访问 private 成员
(在“lqr::Entrance”类中声明)
C:\Users\Administrator\Desktop\tmp\include\Entrance.h(11) : 参见“lqr::E
ntrance::id”的声明
C:\Users\Administrator\Desktop\tmp\include\Entrance.h(9) : 参见“lqr::En
trance”的声明
src\Entrance.cpp(15) : error C2248: “lqr::Entrance::number”: 无法访问 private
成员(在“lqr::Entrance”类中声明)
C:\Users\Administrator\Desktop\tmp\include\Entrance.h(10) : 参见“lqr::E
ntrance::number”的声明
C:\Users\Administrator\Desktop\tmp\include\Entrance.h(9) : 参见“lqr::En
trance”的声明
NMAKE : fatal error U1077: “"D:\Program Files (x86)\Microsoft Visual Studio 12.
0\VC\BIN\cl.EXE"”: 返回代码“0x2”
Stop.

解决办法：把Entrance.cpp的第13行改为ostream& lqr::operator<<(ostream& os, const Entrance& e)

Entrance.h

 1 #ifndef ENTRANCE_H

 2 #define ENTRANCE_H

 3 

 4 #include <iostream>

 5 

 6 namespace lqr {

 7 

 8     class Entrance

 9     {

10         unsigned int number;

11         int id;

12     public:

13         Entrance(unsigned int number, int id);

14         friend std::ostream& operator<<(std::ostream& os, const Entrance& e);

15     };

16 

17 }

18 

19 #endif // ENTRANCE_H

 

Entrance.cpp

 1 #include "Entrance.h"

 2 #include <iostream>

 3 

 4 using namespace lqr;

 5 using namespace std;

 6         

 7 Entrance::Entrance(unsigned int number_val, int id_val)

 8     :number(number_val), id(id_val)

 9 {

10     // do nothing

11 }

12 

13 ostream& operator<<(ostream& os, const Entrance& e)

14 {

15     return os << "Entrance #"<< e.id << ": "<< e.number;

16 }

 

main.cpp

#include "Entrance.h"





#include <iostream>



using namespace std;

using namespace lqr;



int main()

{

    Entrance en(99, 123);

    

    cout << en << endl;

    

    return 0;

}

 

Makefile

# compiler

CC = cl

# linker

LINK = link

# libraries



# headers

HEADER_PATH = /I include

# options

EHSC = /EHsc

COMPILATION_ONLY = /c

C_OUT = /Fo:

L_OUT = /OUT:

# compiler & linker debug option, to disable debug, replace '/Zi' & '/DEBUG' with empty strings

C_DEBUG = /Zi

L_DEBUG = /DEBUG

# C_DEBUG = # L_DEBUG = # targets

bin\test.exe: bin obj obj\main.obj obj\Entrance.obj

    $(LINK) $(L_DEBUG) $(L_OUT)bin\test.exe obj\main.obj obj\Entrance.obj



obj\main.obj: src\main.cpp include\Entrance.h

    $(CC) $(C_DEBUG) $(EHSC) $(HEADER_PATH) $(COMPILATION_ONLY) src\main.cpp $(C_OUT)obj\main.obj



obj\Entrance.obj: src\Entrance.cpp include\Entrance.h

    $(CC) $(C_DEBUG) $(EHSC) $(HEADER_PATH) $(COMPILATION_ONLY) src\Entrance.cpp $(C_OUT)obj\Entrance.obj

    

# folders



obj:

    mkdir obj

    

bin:

    mkdir bin

    

# clean # bin, obj folders and pdb files



.PHONY: clean

clean:

    -rmdir /s /q bin

    -rmdir /s /q obj

    -del *.pdb