c++ - Why value of local variable(its address is stored in global variable) is freed when function has completed its execution -

this code

#include<iostream> using namespace std; int *ptr; void hold(){   int a=12;   ptr=&a; };  int main(){   hold();   cout<<"value of a="<<*ptr; }; 

and value of a=12 must not happen because compiler has freed value @ address of why 12 comes?

freeing doesn't mean system modify value stored there (which involve unnecessary overhead). means memory space made available future storage.