Segmentation fault (core dumped) issue c++: dynamic memory -

 template<class t>    void simplelist<t>::in_node_l(void){        node* fin;        fin=new node;        (*last).next=fin;        last=fin;                size++; } 

i segmentation fault(core dumped) error when in_node_l member function called function push

 template<class t>    void stack<t>::push(t var){        simplelist<t>::in_node_l();        (*(this->last)).data=var;        position++;        return;   }  

stack derived class :

    class stack : public simplelist<t> 

node declared as

template<class t> struct simplelist<t>::node{   t data;   node* next;  }; 

if call in_node_l in main not errors.

the constructor class stack is:

template<class t>  stack<t>::stack(int s,string n) :simplelist<t>(s,n) {   position=1; } 

constructor simplelist given by

template<class t> simplelist<t>::simplelist(int s,string n){ int i; //name= input name node *prev,*current; size=s; name=n; if(s==1){     current=new node;     first=current;     last=current;     cout<<"simplelist constructur"<<endl; } else{     (i=1;i<s;i++){         current=new node;         if(i==1){             first=current;         }         else{             (*prev).next=current;             if(i==s-1){                 last=current;                 if(last=(*prev).next){                     cout<<"simplelist constructur2"<<endl;                 }                    }         }         prev=current;     } } 

}