How do I define a variable in a namespace with "using namespace"?

Question

So I have a namespace thing with extern int variables declared in a header. I'm trying to define them in a .cpp with using namespace thing; to simplify initialization, but it doesn't seem to work the way I expected when trying to define a variable in thing.cpp. What gives?

main.cpp:

#include <cstdio>
#include "thing.hpp"

int main()
{
    printf("%d\n%d\n",thing::a,thing::func());
    printf("Zero initialized array:\n");
    for(int i = 0; i < 10; i++)
        printf("%d",thing::array[i]);

    return 0;
}

thing.hpp:

#pragma once

namespace thing {
    extern int
        a,
        b,
        array[10];
    extern int func();
}

thing.cpp

#include "thing.hpp"

using namespace thing;

// I wanted to do the same thing with 'a' for all variables
int a,thing::b,thing::array[10];

int thing::func() {
    return 12345;
}

error:

/tmp/ccLbeQXP.o: In function `main':
main.cpp:(.text+0x11): undefined reference to `thing::a'
collect2: error: ld returned 1 exit status

@πάνταῥεῖ Yeah, the code works fine when I use thing::a instead of just a — ankien
– ankien, Commented Jan 16, 2021 at 22:10
Sure, you must fully qualify your symbol names, unless you do using thing::a; statements for all the symbols defined in the .cpp file. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Jan 16, 2021 at 22:11
@πάνταῥεῖ So what rule makes it so I can't use using namespace thing? If I include iostream in the header, and using namespace std in the .cpp I can use it's functions/methods without the std namespace prefix just fine — ankien
– ankien, Commented Jan 16, 2021 at 22:17
Why not wrap the contents of thing.cpp in namespace thing { ... } like you did with thing.hpp? — Kevin
– Kevin, Commented Jan 16, 2021 at 22:26

Kevin · Accepted Answer · 2021-01-16 22:34:46Z

1

using namespace thing allows you to use identifiers from the thing namespace without prefixing them with thing::. It effectively pulls them into the namespace where the using directive is (or the global namespace).

It doesn't put further definitions into the namespace thing. So when you define int a;, it's just in the global namespace. You need to use int thing::a; or namespace thing { int a; } to define it in the namespace.

answered Jan 16, 2021 at 22:34

Kevin

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1 Comment

ankien Over a year ago

Great, I've changed the question title to match the answer I was looking for.

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