Ambiguous call to overloaded function

先看看我们的测试程序：

$ cat -n namespace.cpp
1
2 #include <cassert>
3 #include <cstdio>
4 #include <cstdlib>
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：

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。