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

 #ifndef ENTRANCE_H
 #define ENTRANCE_H

 #include <iostream>

 namespace lqr {

     class Entrance
     {
         unsigned int number;
         int id;
     public:
         Entrance(unsigned int number, int id);
         friend std::ostream& operator<<(std::ostream& os, const Entrance& e);
     };

 }

 #endif // ENTRANCE_H

Entrance.cpp

 #include "Entrance.h"
 #include <iostream>

 using namespace lqr;
 using namespace std;

 Entrance::Entrance(unsigned int number_val, int id_val)
     :number(number_val), id(id_val)
 {
     // do nothing
 }

 ostream& operator<<(ostream& os, const Entrance& e)
 {
     return os << "Entrance #"<< e.id << ": "<< e.number;
 }

main.cpp

#include "Entrance.h"

#include <iostream>

using namespace std;
using namespace lqr;

int main()
{
    Entrance en(, );

    cout << en << endl;

    return ;
}

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