C++ cannot initialize static data member

Question

I know this has been asked 10000 times, however, I'm still having issues getting this to compile. Notice the static member 'map'.

In the 'getMap()' function, I receive an undefined reference error referring to the map data member. I attempted to move that function to a cpp file and declare 'map' in that file. However, I then receive a conflicting definition error.

Can somebody explain to me what is going on? Thanks

Base.h

template<typename T> Base * createT() { return new T; }
typedef std::map<std::string, Base*(*)()> map_type;


class BaseFactory
{
    static Base* createInstance(std::string const& s)
    {
        map_type::iterator it = getMap()->find(s);
        if (it == getMap()->end())
            return 0;

        return it->second();
    }

protected:
    static map_type *getMap()
    {
        if (!map)
        {
            map = new map_type;
        }

        return map;
    }

private:
    static map_type * map;
     static Base* createInstance(std::string const* s);

public:
     BaseFactory();
     ~BaseFactory();

};

So in the 10001th time you missed the error message plus the actual line of code indicating the error — P0W, Oct 13 '13 at 19:55
You misunderstood the advice you were given. The answer is not to move the function to the cpp file. The solution is to *add new code* to the cpp file. This new code defines the static member. How can such a simple thing be so difficult? — john, Oct 13 '13 at 19:58
@john Thanks. I got this working. P.S. for everybody else, this question did not deserve the number of down votes received. — tier1, Oct 13 '13 at 20:00
@tkcsam put your mouse cursor, on the down arrow button, and see why it deserved that — P0W, Oct 13 '13 at 20:03

score 0 · Accepted Answer · edited May 23 '17 at 12:21

0

The following declaration within a definition of your class is not enough:

private:
    static map_type * map;

In one .cpp file, you must also add:

map_type* BaseFactory::map = NULL;

This article might help you too: Static data members (C++ only)
+ this question, which has almost 100k views by now: Initializing private static members

edited May 23 '17 at 12:21

Community

1
1

answered Oct 13 '13 at 19:54

LihO

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I didn't make my original point clear. I wanted to keep everything in the header file, but I moved your suggestion beneath the class definition and it worked. Thanks for your help. – tier1 Oct 13 '13 at 19:59
1

@tkcsam: I've edited my answer to make it more clear. – LihO Oct 13 '13 at 20:02

C++ cannot initialize static data member

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