c++ - Pointers and reference issue -

i'm creating similar structure list. @ beginning of main declare null pointer. call insert() function couple of times, passing reference pointer, add new elements.

however, seems wrong. can't display list's element, std::cout breaks program, though compiler without warning.

#include <iostream>  struct node {     node *p, *left, *right;     int key; };  void insert(node *&root, const int key) {     node newelement = {};     newelement.key = key;      node *y = null;     std::cout << root->key; // line     while(root)     {         if(key == root->key) exit(exit_failure);         y = root;         root = (key < root->key) ? root->left : root->right;     }      newelement.p = y;      if(!y) root = &newelement;     else if(key < y->key) y->left = &newelement;     else y->right = &newelement; }  int main() {     node *root = null;     insert(root, 5);         std::cout << root->key; // works if delete cout in insert()     insert(root, 2);         std::cout << root->key; // program breaks before line     return 0; } 

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as can see, create new structure element in insert function , save inside root pointer. in first call, while loop isn't initiated works, , i'm able display root's element in main function.

but in second call, while loop works, , problem described.

there's wrong root->key syntax because doesn't work if place in first call.

what's wrong, , what's reason?

also, i've seen inserting new list's elements through pointers this:

node newelement = new node(); newelement->key = 5; root->next = newelement; 

is code equal to:

node newelement = {}; newelement.key = 5; root->next = &newelement; 

? bit cleaner, , there wouldn't need delete memory.

the problem because passing pointer local variable out of function. dereferencing such pointers undefined behavior. should allocate newelement new.

this code

node newelement = {}; 

creates local variable newelement. once function over, scope of newelement ends, , memory gets destroyed. however, passing pointer destroyed memory outside function. references memory become invalid function exits.

this code, on other hand

node *newelement = new node(); // don't forget asterisk 

allocates object on free store. such objects remain available until delete them explicitly. that's why can use them after function creating them has exited. of course since newelement pointer, need use -> access members.