c++ - Why do I get a segmentation fault when I am trying to insert into a tree* -

#include<iostream> #include<set> #include<stdlib.h> using namespace std; typedef set<short> pos; typedef struct tree {         pos first; }tree; class check { public:         pos last;         void check_set(); }; void check::check_set() {         tree *root,p;         root=(tree*)malloc(sizeof(tree));         root->first.insert(2);//segmentation fault here why???         p.first.insert(3);//no segmentation fault } int main() { check obj; obj.check_set(); obj.last.insert(1);//no error here return 0; } 

use new instead of malloc.

malloc allocates memory, doesn't initialize in way , doesn't construct objects living in memory. new on other had constructs c++ objects. valid tree object (with initialized first member), use this:

root = new tree(); 

later, when want release object, use delete:

delete root;