static vs non-static variables in namespace

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I have a namespace foo which contains an integer bar, declared so...

foo.h:

namespace foo {
    int bar;
}

Now if I include foo.h in only one file, this works just fine. But a problem arises when I include foo.h from two or more files: I get a linker error. I figured out that if I declare bar as static, I can include foo.h in more than one file. This seems strange to me, because I wasn't aware that one could declare a static variable inside of a namespace. (what does that even mean?)

Why does this work? And more importantly, why doesn't it work without static? What does static mean when used in a namespace?

static namespaces c++
4 Answers

Also note that const int at namespace (global) scope in C++ has static implicitly added by default: Define constant variables in C++ header

To better understand what is going on, do a readelf on the intermediate ELF object files of the compilation, and you will see clearly is symbols are defined twice or not. Here is a detailed example: What does "static" mean in C?

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