Definition of constant within namespace in cpp file

Question

I've approached this strange ( for me ) effect in VS 2010. Can anyone smart shed some light on it please.

//Header.h
#include <string>
namespace MySpace {
extern const std::string SOME_CONST_STRING;
}

//Implementation.cpp
#include "Header.h"
using namespace MySpace;
const std::string SOME_CONST_STRING = "CONST_STRING_VALUE";

This causes linker to output error LNK2001: unresolved external symbol const MySpace::SOME_CONST_STRING.

However when I change Implementation.cpp like this:

//Implementation.cpp
#include "Header.h"
namespace MySpace {
const std::string SOME_CONST_STRING = "CONST_STRING_VALUE";    
}

the code builds OK.

Is it good example for prefering defining of namespace in cpp file rather than using it ?

The only difference in two versions of Implementation.cpp is using namespace MySpace; vs namespace MySpace { ... }. First one fails to link, while second links OK. — jakumate, Jul 11 '14 at 13:08
Joseph. I saw it in "coding style" questions: what is prefered way of writing it in cpp files: using namespace MySpace; vs namespace MySpace { ... }. There were few examples which one is better over the other. I'm asking if the problem above is good reason to not write using namespace MySpace; in implementation cpp files ? — jakumate, Jul 11 '14 at 13:12

score 0 · Answer 1 · answered Jul 11 '14 at 12:54

0

You can alternatively write:

const std::string MySpace::SOME_CONST_STRING = "CONST_STRING_VALUE";

That name is declared inside MySpace so you have to refer to it as such.

answered Jul 11 '14 at 12:54

Joseph Mansfield

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SOME_CONST_STRING in cpp also needs extern. – Neil Kirk Jul 11 '14 at 13:00
2

@NeilKirk It doesn't. In fact, it can't. – Joseph Mansfield Jul 11 '14 at 13:08

score 0 · Answer 2 · answered Jul 11 '14 at 12:55

0

You are missing "namespace" in your using. It should be like this:

#include "Header.h"
using namespace MySpace;
const std::string SOME_CONST_STRING = "CONST_STRING_VALUE";

answered Jul 11 '14 at 12:55

Johan Hjalmarsson

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Thanks Johan this is just a "typo" in the question. I'll edit it. – jakumate Jul 11 '14 at 12:57
SOME_CONST_STRING in cpp also needs extern. – Neil Kirk Jul 11 '14 at 13:01
Also I'm pretty sure your code doesn't work. How does it know SOME_CONST_STRING is in MySpace and not the global namespace? – Neil Kirk Jul 11 '14 at 13:01
No it doesn't, "extern" in header is sufficent. That part is OK. ;) – jakumate Jul 11 '14 at 13:03
The only difference in two versions of Implementation.cpp is using namespace MySpace; vs namespace MySpace { ... }. First one fails to link, while second links OK. – jakumate Jul 11 '14 at 13:07
@jakumate ok I've asked question here to find more info: http://stackoverflow.com/questions/24698515/is-extern-keyword-required-here-const-in-cpp-file – Neil Kirk Jul 11 '14 at 13:08

score 0 · Answer 3 · answered Jul 11 '14 at 12:58

The whole point of namespaces is that names in a particular namespace are independent of names outside that namespace. A program could have both a MySpace::SOME_CONST_STRING and a global ::SOME_CONST_STRING, and these are two completely unrelated symbols. It would be wrong for the linker to use a definition of one to satisfy a reference to the other.

When you define a variable, you have to define it in the namespace where you want it to be.

Definition of constant within namespace in cpp file

3 Answers3

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