static variable link error

Question

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

Undefined symbols for architecture i386: \"Log::theString\", referenced f

Answer 1

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.

Answer 2

In C++17, there is an easier solution using inline variables:

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

This is a definition, not just a declaration and similar to inline functions, multiple identical definitions in different translation units do not violate ODR. There is no longer any need to pick a favourite .cpp file for the definition.

Answer 3

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

Include

string Log::theString;

to your cpp file