How to define static method in a cpp file. (C )-CodePudding

I wan to have my class in a separate .h file, and the implementation in a .cpp file. The problem I run into is "Multiple definition of methodname". If I put the content of both files in a .hpp file it works, but I'm not sure if I should use that.

The .h File:

#pragma once

class Nincs;



class Sugar
{
    public:
        Sugar(){}

};

class Nincs : public Sugar
{
     private:
        static Nincs* ins;
        Nincs(): Sugar() {};
    public:
        static Nincs* instance();

};

the .cpp File:

#include "Sugar.h"


Nincs* Nincs::ins = nullptr;


Nincs* Nincs::instance() //the error is with this line
{
        if (ins == nullptr)
        {
            ins = new Nincs();
        }
        return ins;
};

The main .cpp file:

#include <iostream>

#include "Sugar.cpp"




using namespace std;

int main()
{

    Nincs* n = Nincs::instance();


    return 0;
}

Basically, I am struggling with making a simple singleton class in C .

CodePudding user response：

The problem is that you're including the source file(Sugar.cpp) instead of the header file(Sugar.h).

To solve this inside main.cpp, change #include "Sugar.cpp" to:

#include "Sugar.h"

Also it is recommended that we should never include source files.

Working demo