c++ - Can't convert from one data type to the same? -

i'm trying implement own set template, , have issues trying breadth- first search using queue template works independently.

the weird part error in set template when trying compile. why can't convert 1 pointer different 1 same data type?

error c2440: '=' : cannot convert 'set<t>::node<t> *' 'set<t>::node<t> *'             [           t=std::string       ]       types pointed unrelated; conversion requires reinterpret_cast, c-style cast or function-style cast       c:\users\programming\set\set.h(96) : while compiling class template member function 'void set<t>::print(set<t>::node<t> *)'             [           t=std::string       ]       c:\users\programming\set\main.cpp(13) : see reference class template instantiation 'set<t>' being compiled             [           t=std::string       ] 

queue class template

template <typename t> class queue ... t* front() {     if (first != null)         return first->item;     else         return null; } 

set class template

template <typename t> class set ... queue<node<t> *> q; void print(node<t> *p) {     q.push(p);     while (p != null)     {         cout << p->item << "(" << p->height << ") ";         if (p->left != null)             q.push(p->left);         if (p->right != null)             q.push(p->right);         if (!q.size())         {             // error @ line             p = q.front();             q.pop();         }         else             p = null;     }     cout << endl; } 

your queue class instantiated node<t>* type ... attempting return pointer type t queue<t>::front method. if instantating queue<t> class t=node<t>*, need return type t front method, not t*. change front method signature following:

template <typename t> class queue ... t front() {     if (first != null)         return first->item;     else         return null; } 

now cause bunch of problems if t not pointer type ... therefore may want create specialization of queue<t>::front method cases t pointer type. instance:

//pointer-type specialization template<typename t> t queue<t*>::front() {     if (first != null)         return first->item;     else         return null; }  //non-specialized version t not pointer-type template<typename t> t* queue<t>::front() {     if (first != null)         return &(first->item);     else         return null; }