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先看看我们的测试程序：

$ cat -n namespace.cpp
 1
 2  #include  3  #include  4  #include  5  6  /* Here is our namespace Foo */  7  namespace Foo  8  {  9      void sub(void) 10     { 11         printf("Foo::sub()\\n"); 12     } 13  } 14 15  /* sub() is in global namespace. */ 16  void sub(void) 17  { 18     printf("global sub()\\n"); 19  } 20 21  int main(int argc, char *argv\[\]) 22  { 23      /* calls sub() in global namespace. */ 24      sub(); 25      /* calls sub() in namespace Foo. */ 26      Foo::sub(); 27 28      /* change search path to global and Foo. */ 29      using namespace Foo; 30 31      /* compiler cannot determine which sub() to be called. */ 32      // expect error 33      // sub(); 34      // 35 36      /* tell the compiler which one to use. */ 37      ::sub(); 38      Foo::sub(); 39 40      return 0; 41  } 42

D:\> version
Microsoft Windows Server 2003, Enterprise Edition Service Pack 1 (Build 3790)

D:\> cl /DWIN32 namespace.cpp
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 12.00.8804 for 80x86
Copyright (C) Microsoft Corp 1984-1998. All rights reserved.

namespace.cpp
Microsoft (R) Incremental Linker Version 6.00.8447
Copyright (C) Microsoft Corp 1992-1998. All rights reserved.

/out:namespace.exe
namespace.obj

D:\> namespace
global sub()
Foo::sub()
global sub()
Foo::sub()

$ uname -a
CYGWIN_NT-5.2 uniware 1.5.5(0.94/3/2) 2003-09-20 16:31 i686 unknown unknown Cygwin

$ gcc –version
gcc (GCC) 3.3.1 (cygming special)
Copyright (C) 2003 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ gcc -Wall -o namespace namespace.cpp

$ ./namespace
global sub()
Foo::sub()
global sub()
Foo::sub()

结果和VC++的一致。如果把第33行的注释去掉，则：

D:\> cl /DWIN32 namespace.cpp
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 12.00.8804 for 80x86
Copyright (C) Microsoft Corp 1984-1998. All rights reserved.

namespace.cpp
namespace.cpp(33) : error C2668: ‘sub’ : ambiguous call to overloaded function

$ gcc -Wall namespace.cpp
namespace.cpp: In function int main(int, char**)': namespace.cpp:33: error: call of overloaded sub()’ is ambiguous
namespace.cpp:17: error: candidates are: void sub()
namespace.cpp:10: error: void Foo::sub()

GCC编译的结果和VC++的类似，不过GCC的提示信息更加准确，也有助于定位问题。

人总是会犯错误的，微软的工程师也不例外，这里就是Visual C++ 6.0中的类似错误。

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Copyright 2006-2007,zedware_at_gmail_dot_com
Last modified on Monday, 2006-07-10