C ++名称空间错误

Question

I'm running mingw-w64 on Windows 7 Pro 64-bit.

I'm getting errors when I try the code in a namespace, but it works fine without the namespace. The code is:

//foo.h

#ifndef FOO_H
#define FOO_H

namespace joke {

    extern "C" int fooStack[10];
    extern "C" int *fooPtr;
    extern "C" void fooInit();
    extern "C" void fooPush(int iVar);
    extern "C" int fooPop();

}

#endif // FOO_H

---

// foo.cpp

#include "foo.h"

using namespace joke;

int fooStack[10];
int *fooPtr;

void fooInit() {
    fooPtr = &fooStack[0] + 9;
}

void fooPush(int iVar) {
    *fooPtr = iVar;
    fooPtr -= 1;
}

int fooPop() {
    int oVar;
    fooPtr += 1;
    oVar = *fooPtr;
}

---

// bar.cpp

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

using namespace std;
using namespace joke;

int inVar = 0;
int outVar = 0;

void Report() {
    int i;
    cout << "Pointer:   " << fooPtr << endl;
    cout << "Stack:     ";
    for (i = 0; i < 10; i++) {
        cout << fooStack[i] << " ";
    }
    cout << endl;
    cout << "inVar:     " << inVar << endl;
    cout << "outVar:    " << outVar << endl << endl;
}

int main() {
    int i;
    fooInit();
    cout << endl;
    cout << "Initializing" << endl;
    Report();
    for (i = 0; i < 8; i++) {
        inVar = (i * 34) + 34;
        cout << "set inVar" << endl;
        Report();
        fooPush(inVar);
        cout << "Push inVar" << endl;
        Report();
    }
    for (i = 0; i < 8; i++) {
        outVar = fooPop();
        cout << "Pop outVar" << endl;
        Report();
    }
    return 0;
}

The makefile:

all:

    g++ -S foo.cpp

    g++ -S bar.cpp

    g++ -c foo.s

    g++ -c bar.s

    g++ -o FB foo.o bar.o

As I said, this all works fine if the namespace specifications are removed or commented-out. With the namespace specifications in place, the results are:

c:\work\gccWork\FooTest>make
g++ -S foo.cpp
foo.cpp: In function 'void fooInit()':
foo.cpp:11:2: error: reference to 'fooPtr' is ambiguous
  fooPtr = &fooStack[0] + 9;
  ^
foo.cpp:8:6: note: candidates are: int* fooPtr
  int *fooPtr;
       ^
In file included from foo.cpp:3:0:
foo.h:9:18: note:                 int* joke::fooPtr
  extern "C" int *fooPtr;
                  ^
foo.cpp:11:12: error: reference to 'fooStack' is ambiguous
  fooPtr = &fooStack[0] + 9;
            ^
foo.cpp:7:5: note: candidates are: int fooStack [10]
 int fooStack[10];
     ^
In file included from foo.cpp:3:0:
foo.h:8:17: note:                 int joke::fooStack [10]
  extern "C" int fooStack[10];
                 ^
foo.cpp: In function 'void fooPush(int)':
foo.cpp:15:3: error: reference to 'fooPtr' is ambiguous
  *fooPtr = iVar;
   ^
foo.cpp:8:6: note: candidates are: int* fooPtr
 int *fooPtr;
      ^
In file included from foo.cpp:3:0:
foo.h:9:18: note:                 int* joke::fooPtr
  extern "C" int *fooPtr;
                  ^
foo.cpp:16:2: error: reference to 'fooPtr' is ambiguous
  fooPtr -= 1;
  ^
foo.cpp:8:6: note: candidates are: int* fooPtr
 int *fooPtr;
      ^
In file included from foo.cpp:3:0:
foo.h:9:18: note:                 int* joke::fooPtr
  extern "C" int *fooPtr;
                  ^
foo.cpp: In function 'int fooPop()':
foo.cpp:21:2: error: reference to 'fooPtr' is ambiguous
  fooPtr += 1;
  ^
foo.cpp:8:6: note: candidates are: int* fooPtr
 int *fooPtr;
      ^
In file included from foo.cpp:3:0:
foo.h:9:18: note:                 int* joke::fooPtr
  extern "C" int *fooPtr;
                  ^
foo.cpp:22:10: error: reference to 'fooPtr' is ambiguous
  oVar = *fooPtr;
          ^
foo.cpp:8:6: note: candidates are: int* fooPtr
 int *fooPtr;
      ^
In file included from foo.cpp:3:0:
foo.h:9:18: note:                 int* joke::fooPtr
  extern "C" int *fooPtr;
                  ^
make: *** [all] Error 1

I've looked through various c++ textbooks and references, as well as several references and forums here online. I don't see any problems with the syntax.

What am I missing?

Answer 1

You are asking for C linkage. But C has no namespace concept. That seems like a conflict of interest to me.

Answer 2

fooPtr is declared both globally (in the unnamed namespace ) and in namespace joke . They are different entities.

The using namespace joke then tells the compiler to use names from within joke as candidates to match identifiers

The names ::fooPtr and joke::fooPtr are both visible to the compiler when the code has a statement fooPtr = &fooStack[0] + 9 . The using namespace joke means that both ::fooPtr and joke::fooPtr are valid matches. There is no basis to to prefer one over the other, so the compiler rejects the code due to ambiguity.