what's lost while link error "undefined reference" in g++

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What is an undefined reference/unresolved external symbol error and how do I fix it?

While g++ report "error: undefined reference to 'Log::flush()'", it means I lost a source code which define the member function flush of class Log like this:

class Log{   
   ... ...   
   int flush();   
   ... ... 
};

But if the g++ report "error: undefined reference to 'Log::outFile'", What is lost? There is no () so this is not a function, so why is there anything that needs defining?

did you provide `int Log::flush()` function definition and link it? — billz, Feb 01 '13 at 05:10
http://stackoverflow.com/questions/12573816/what-is-an-undefined-reference-unresolved-external-symbol-error-and-how-do-i-fix — chris, Feb 01 '13 at 05:12
I don't understand.. you practically answered your own question? the definition of outFile? — thang, Feb 01 '13 at 05:13
@thang: He has noticed that it is not a function, and does not understand why there is anything to define, because he has not read the chapter about static data members in his C++ book. — Lightness Races in Orbit, Feb 01 '13 at 05:14
@LightnessRacesinOrbit, Nice catch when the code sample is completely unrelated. Luckily, my link still applies :) — chris, Feb 01 '13 at 05:16
Not bad huh. This is quite a poor question, with an incomplete testcase and the premise left up to our imaginations. Fortunately I was a private investigator on TV in a previous life... — Lightness Races in Orbit, Feb 01 '13 at 05:16

score 4 · Answer 1 · answered Feb 01 '13 at 05:11

_{Well, isn't this a fun exercise in GCC diagnostic output jeopardy!}

But if the g++ report "error: undefined reference to 'Log::outFile'", What is lost?

More than likely, the definition of a static data member of Log called outFile.

Guessing:

class Log
{
   int flush();
   // ... more functions ...

   static std::string outFile;
   // ... more data ...
};

You'd need to write:

std::string Log::outFile;

in precisely one translation unit; put it in a .cpp file, probably the one in which you placed the definition for Log::flush().

what's lost while link error "undefined reference" in g++

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