c++ template NULL undeclared identifier -

i having problems in c++ templates , null. trying make bst in c++ using templates. have class element, treeelement , binarysearchtree.

a binarysearchtree has treeelements nodes. each treeelement has linkedlist of elements. each element has long indentify , check whether it's <, > or ==.

now error: null undeclared identifier. don't know how it's possible. here's code of treeelement error:

template <class t> class treeelement { private:     treeelement* lefttree;     treeelement* righttree;     treeelement* parent;     element<t>* value; public:     //here error!!     treeelement(element<t>* element){value = element; lefttree = null; righttree = null; parent = null;};     ~treeelement() {         element<t>* nextelement;         if (value != null) {             while (value->getnextelement() != null) {                 nextelement = value;                 value = nextelement->getnextelement();                 delete nextelement;             }             delete value;         }     };     treeelement* getlefttree(){return this->lefttree;};     treeelement* getrighttree(){return this->righttree;};     treeelement* getparent(){return this->parent;};     element<t>* getvalue(){return this->value;};      void setlefttree(treeelement* lefttree){this->lefttree = lefttree;};     void setrighttree(treeelement* righttree){this->righttree = righttree;};     void setparent(treeelement* parent){this->parent = parent;};     void setvalue(element<t>* value){this->value = value;}; }; 

three options:

• include header <cstddef> defines null
• use null pointer constant, 0 or (since c++11) nullptr rather dodgy macro
• don't compare null, use pointer in boolean context same effect (although applies 1 of uses of null here)