How to set static variable in another class?

c++, static-variables

Solution

Class declaration should be placed in the header (Coeffs.h)

#ifndef COEFFS_H
#define COEFFS_H
class Coeffs
{
    public:
    static double alpha5b;
};
#endif

but initialization of static member in a source file (.cpp,.cxx):

#include "Coeffs.h"

double Coeffs::alpha5b = 0.0;

Problem

I am trying to set a static variable in Coeffs.cpp: ``` #include "Coeffs.h" class Coeffs { public: double Coeffs::alpha5b = 0.0; }; ``` with the header file ``` #ifndef COEFFS_H #define GOEFFS_H class Coeffs { public: static double alpha5b; }; #endif ``` with the following code: ``` #include <iostream> #include <fstream> #include <string> #include "json/json.h" #include "Coeffs.h" using namespace std; int main() { cout << "start" << endl; string json; ifstream inputStream; inputStream.open("coeffTest.json"); inputStream >> json; Json::Value root; Json::Reader reader; bool parseSuccess = reader.parse(json, root); if(!parseSuccess) { cout << "failed" << endl; } else { Coeffs::alpha5b = 1.1; //Coeffs::alpha5b = root.get("alpha5b", "NULL").asDouble(); //double item1[] = root.get("delta21b", "NULL").asDouble(); //cout << "alpha5b is: " << Coeffs::alpha5b << endl; } cout << "done" << endl; } ``` but everytime I compile I get this: ``` pottsie@pottsie:~/Documents/CoeffsJSON$ g++ -o JsonToCoeffs JsonToCoeffs.cpp -ljson_linux-gcc-4.6_libmt /tmp/ccFxrr0k.o: In function `main': JsonToCoeffs.cpp:(.text+0x10b): undefined reference to `Coeffs::alpha5b' collect2: ld returned 1 exit status ``` Ive looked through some of the other similar questions and cant find anything that works. I've tried adding a constructor and making an object, but then I still get the same error. Anyone know what to do?

Original source