c++ - Try to understand std::enable_shared_from_this<T> but cause a bad_weak_ptr using it -

i try understand behavior of std::enable_shared_from_this class, cant understand it. i've write simple program test different situations.

question

can explain me behavior of following code, because can't explain observed result.

thanks help.

code

#include <iostream> #include <memory>  struct c : std::enable_shared_from_this<c> { };  int main () {     {//test 1     std::shared_ptr<c> foo, bar;     foo = std::make_shared<c>();     bar = foo->shared_from_this(); //ok     std::cout<<"shared_ptr : ok"<<std::endl;     }      {//test 2     std::shared_ptr<c> foo = std::shared_ptr<c>(new c);     std::shared_ptr<c> bar;     bar = foo->shared_from_this(); //ok     std::cout<<"shared_ptr + new : ok"<<std::endl;     }      {//test 3     c* foo = new c;     std::shared_ptr<c> bar;     bar = foo->shared_from_this(); //throw std::bad_weak_ptr     std::cout<<"new : ok"<<std::endl;     }     {//test 4 (should make invalid free of that)     c foo;     std::shared_ptr<c> bar;     bar = foo.shared_from_this();//throw std::bad_weak_ptr     std::cout<<"local : ok"<<std::endl;    }    return 0; } 

and here output:

shared_ptr : ok shared_ptr + new : ok terminate called after throwing instance of 'std::bad_weak_ptr'    what():  bad_weak_ptr 

build informations

• system : windows 7 x64
• compiler : mingw 4.9.2 x64

the contract enable_shared_from_this provide shared_ptr when call shared_from_this() if object managed shared_ptr. cannot create shared_ptr if not being managed.

you figured out in last test case why bad idea allow shared_from_this() create shared pointer you...