static variable link error [duplicate]

I'm writing C++ code on a mac. Why do I get this error when compiling?:

Undefined symbols for architecture i386: "Log::theString", referenced from: Log::method(std::string) in libTest.a(Log.o) ld: symbol(s) not found for architecture i386 clang: error: linker command failed with exit code 1 (use -v to see invocation)

Not sure if my code is wrong or I have to add additional flags to Xcode. My current XCode configurations are the default ones for a 'static library' project.

My code:

Log.h------------

#include <iostream>
#include <string>

using namespace std;

class Log{
public:
    static void method(string arg);
private:
    static string theString ;
};

Log.cpp ----

#include "Log.h"
#include <ostream>

void Log::method(string arg){
    theString = "hola";
    cout   << theString << endl; 
}

I'm calling the 'method' from a test code, in this way: 'Log::method("asd"):'

thanks for your help.

You must define the statics in the cpp file.

Log.cpp

#include "Log.h"
#include <ostream>

string Log::theString;  // <---- define static here

void Log::method(string arg){
    theString = "hola";
    cout   << theString << endl; 
}

You should also remove using namespace std; from the header. Get into the habit while you still can. This will pollute the global namespace with std wherever you include the header.

You declared static string theString;, but haven't defined it.

Include

string Log::theString;

to your cpp file