Singleton implementation static member error

Question

When implementing the Singleton standard the compiler gives the following error:

include graphdata. h|21|error: 'constexpr' needed for in-class initialization of Static data Member 'graphdata* graphdata::instance' of non-integral type [-fpermissive]|

The code I made is this::

class graphdata
{
    public:
        static graphdata& getinstance(){
            if(!instance)
                instance = new graphdata();
            return *instance;
        }
        void dfsR();
        graphdata(graphdata const&) = delete;
        void operator = (graphdata const&) = delete;
    protected:

    private:
        graphdata();
        static graphdata* instance = 0;
};

Answer 1

This is exactly what is written, if you want to initialize a value in the class declaration you need to be a constant expression (constexpr). If it cannot be constant it must initialize in the static constructor or its normal code before using the variable.