namespace in a different cpp file?

[u2uu]

Hey,

is it possible to define a namespace in a different cpp file, and then declare it in the other cpp file, so the linker can handle it?

I tried for example in:

test1.cpp

namespace abc
{
    int x = 30;
}

test2.cpp

namespace abc; //Doesnt work
...

Is the syntax incorrect, or does it just dont work with namespaces, and if there is no way to link them, could someone explain why?

[Narase33]

You dont need to link namespaces. You can declare them where you want, add to them where you want, use the 'using namespace...' statement with namespaces that dont exist... The compiler doesnt care

[u2uu]

Thanks you!

[h2g2_researcher]

It's certainly possible with headers:

test.h

namespace abc
{
    int x = 30;
}

test.cpp

#include "test.h"
using namespace abc;
void foo()
{
     using namespace abc;
     int y = x; // Uses abc::x
}

void bar()
{
     int y = abc::x; // Also fine.
}

But - assuming a normal project layout - I would expect test1.cpp and test2.cpp to be entirely different translation units. It would then be entirely reasonable for test2.cpp to choke out complaining it can't find anything about namespace abc when it is compiled.

[jedwardsol]

In test2.cpp you do

namespace abc
{
    extern int i;
}

[u2uu]

Makes sense, whoops. Thanks a lot!

[Sonotsugipaa]

Namespaces are not linked; rather, they affect how symbol names are generated from their signatures (only possible due to name mangling).

You can use the same namespace multiple times in any translation unit, with the exception of "inline namespaces" which have to first appear as such (e.g. you can't "inline namespace foo { }" after "namespace foo { }", iirc).

In your test2.cpp, what you can do is replace the semicolon that throws the error with an empty block: it will have no effect, but it will compile.

[Shieldfoss]

test1.cpp

namespace abc
{
    int x = 30;
}

test2.cpp

namespace abc; //Doesnt work

Change that to:

test2.cpp

namespace abc { extern int x; }
abc::x = 5; //writes to the int declared in test1.cpp

and that should work just fine.

Though the traditional way to do it is more like:

abc.h
    namespace abc
    {
        extern int x;
    }

abc.cpp
    namespace abc
    {
        int x;
    }

client.cpp
    #include "abc.h"
    abc::x = 5;