Undefined Reference to class static member in static member

问题

I am creating a linked list with self referential class in C++ and I want to have a static pointer of the type Item (Item is the class name) named "startPointer" so that when i call my static member function "free" , it can free up the memory by using Item::startPointer but i am getting an error(shown after code). Pls Help,

class Item
{
    public:
    std::string name;
    int row,column;
    int fileType;
    Item *ptr;
    static Item *startPointer;
    void setNextPointer(Item* ptr)
    {
        ptr=ptr;
    }
    Item *getNextPointer()
    {
        return ptr;
    }
    static void free()
        {
        Item *p,*temp;
        p=startPointer;
        while(p!=NULL)
        {
            temp=p;
            p=p->getNextPointer();
            delete temp;
        }
    }

};

---

cube.o:cube.cpp:(.text$_ZN4Item4freeEv[Item::free()]+0x8): undefined reference to `Item::startPointer'
collect2: ld returned 1 exit status

mingw32-make.exe: *** [cube.exe] Error 1

Execution terminated

回答1:

You have to define your static member like this:

Item* Item::startPointer = nullptr; // or = NULL; if your cpp version is below c++11

Write such a line in a single compilation unit (cpp file), otherwise the member is just declared.

来源：https://stackoverflow.com/questions/21366174/undefined-reference-to-class-static-member-in-static-member