c++ - Segmentation Fault when accessing a struct -

i have header file define methods. trying implement doubly linked list store data of pairs. cont.h

template <class key, class t> class storage;  template < class key, class t, class compare = std::less<key> >  class cont{     private:         storage <key, t> data;     public:         typedef size_t size_type;         typedef key key_type;         typedef t mapped_type;          //default constructor         cont(){}          cont(key_type k, mapped_type m)         {             std::pair<key, t> x = std::make_pair(k,m);             data.begin_list(x);              //std::pair<key, t> = std::make_pair(k,m);             //data.push(a);         }    };  template <class key, class t> struct node {     node *prev, *next;     std::pair<key, t> node_data; };  template <class key, class t> class storage{     private:         node <key, t> *head;         node <key, t> *tail;         node <key, t> *curr;         int size;     public:         //default ctor         storage(){}          void begin_list(std::pair <key, t> h)         {             size=1;                     //bottom 2 lines induce segmentation fault             //head->node_data=h;             //head->prev=null;          }  }; 

main.cc this:

#include "cont.h" int main() {     cont<double, int> s(6.6, 3); } 

i don't understand why segfault. should dynamically allocating memory each node?

well, head has not been initialized time begin_list gets executed, dereferencing gives undefined behavior:

    void begin_list(std::pair <key, t> h)     {         size=1;          head->node_data=h; // <= dereferencing head without having initialized         head->prev=null; // <== same here     } 

here meant dynamically allocate object of type node in constructor of storage, assigning result of corresponding new expression head (and making tail , curr point same object.

however, please consider not using new, delete, , raw pointers memory management purposes. prefer using smart pointers express ownership.