"Undefined" reference to class member in C++

Question

While linking this code:

#include <map>
using std::map;
#include <string>
using std::string;
class C {
public:
    static void dump() {
        for (const auto& e : data) {
            string(e.first);
        }
    }
private:
    static map<string,map<string,string>> data;
};
int main() {
    C::dump();
}

... I get this error:

/tmp/cc4W2iNa.o: In function `C::dump()':
test.cpp:(.text._ZN1C4dumpEv[_ZN1C4dumpEv]+0x9): undefined reference to `C::data'
collect2: error: ld returned 1 exit status

... from g++ (GCC) 4.9.1. Am I doing anything wrong?

We really should find a less confusing duplicate for this question (or bang it into a more generic shape), but it's still the highest voted one to be found on topic. — πάντα ῥεῖ, Oct 06 '14 at 22:07
A more comprehensive duplicate here: http://stackoverflow.com/questions/7092765 — Mike Seymour, Oct 06 '14 at 22:17

score 8 · Accepted Answer · answered Oct 06 '14 at 21:56

8

You've declared C::data, but not defined it. Add a definition outside the class:

map<string,map<string,string>> C::data;

In a larger program, which more than one source file, this must go in just one source file to satisfy the One Definition Rule; while the class definition (including the declaration of data) might go in a header to be available wherever it's needed.

answered Oct 06 '14 at 21:56

Mike Seymour

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Some notes from your side about closing as a dupe, and finding a more appropriate one? – πάντα ῥεῖ Oct 06 '14 at 22:08
I have to define a non-function class member? Srsly?! – Alexander Aleksandrovič Klimov Oct 06 '14 at 22:12
@πάνταῥεῖ: This one, perhaps? http://stackoverflow.com/questions/7092765 – Mike Seymour Oct 06 '14 at 22:13
@AlexanderAlexandrowitschKlim: Yes, seriously. That's what the One Definition Rule requires. – Mike Seymour Oct 06 '14 at 22:14
@MikeSeymour Yeah, it's much more abstract/generic but lacks votes :P ... – πάντα ῥεῖ Oct 06 '14 at 22:18

"Undefined" reference to class member in C++

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