namespaces question

[sergey akifiev]

hi. i've got dumb question about namespaces. currently I'm learning C++ / refreshing C skills with "Thinking in C++" by Bruce Eckel. In chapter 10 there is a task:
# Repair the problem in OverridingAmbiguity.cpp, first with scope resolution, then instead with a using declaration that forces the compiler to choose one of the identical function names.

all right. here is the code:
NamespaceMath.h :

Code: Select all

#ifndef NAMESPACEMATH_H
#define NAMESPACEMATH_H
#include "NamespaceInt.h"
namespace Math {
  using namespace Int;
  Integer a, b;
  Integer divide(Integer, Integer);
  // ...
} 
#endif // NAMESPACEMATH_H ///:~

NamespaceOverriding2.h :

Code: Select all

#ifndef NAMESPACEOVERRIDING2_H
#define NAMESPACEOVERRIDING2_H
#include "NamespaceInt.h"
namespace Calculation {
  using namespace Int;
  Integer divide(Integer, Integer);
  // ...
} 
#endif // NAMESPACEOVERRIDING2_H ///:~

and finally OverridingAmbiguity.cpp :

Code: Select all

#include "NamespaceMath.h"
#include "NamespaceOverriding2.h"
void s() {
  using namespace Math;
  using namespace Calculation;
  // Everything's ok until:
  Math::divide(1, 2); // Ambiguity
}
int main() {}

when compiling, i have next errors:

Code: Select all

cd /home/sergey/work/c++/tasks/c10/
make -k OverridingAmbiguity
g++ -oOverridingAmbiguity OverridingAmbiguity.o
OverridingAmbiguity.o(.text+0x50): In function `s()':
: undefined reference to `Math::divide(Int::Integer, Int::Integer)'
collect2: ld returned 1 exit status
make: *** [OverridingAmbiguity]  1

Compilation exited abnormally with code 2 at Sun Jan 16 12:48:45

WHY??? i have Math::divide() defined and declared in header!!!
pls help, or my braing just blows away.

PS sorry for bad english :-)

[Kojack]

Has Math::divide actually been implemented in a cpp file somewhere? And it exactly matched the prototype in the header? (not a capital D)

The linking error means it's compiled ok, it knows which divide to call, it just can't find the actual implementation of the divide.

[sergey akifiev]

OMG! how stupid and unattentive i was.
thanks a lot. adding `{}` in *.h in divide()'s definition resolves trouble.

[Emmeran]

btw why don't u jsut use classes???

[sergey akifiev]

what do you mean? i'm just doing exercises.

[Emmeran]

i meant this:

Code: Select all

class Int
{
    static float divide(int a, int b) { return a / b; }
};

printf("10 divided by 3 is: %f\n", Int::divide(10, 3));

[haffax]

Emmeran this code is only of academic use, to learn about namespaces. No need to improve it.

[Kencho]

Anyways, using namespaces is better for large applications, as you define independence between large modules. Namespaces are the same as Packages in Java and UML.