Creating shared_ptr from raw pointer

Question

I have a pointer to an object. I would like to store it in two containers which both have the ownership. So I think I would be good to make it a shared_ptr of C++0x. How could I convert a raw pointer to a shared_pointer?

typedef unordered_map<string, shared_ptr<classA>>MAP1; MAP1 map1; classA* obj = new classA(); map1[ID] = how could I store obj in map1?? 

Thanks

Answer 1

You need to make sure you don't initialize both shared_ptr objects with the same raw pointer, or it will be deleted twice. A better (but still bad) way to do it:

classA* raw_ptr = new classA; shared_ptr<classA> my_ptr(raw_ptr);  // or shared_ptr<classA> my_ptr = raw_ptr;  // ...  shared_ptr<classA> other_ptr(my_ptr); // or shared_ptr<classA> other_ptr = my_ptr; // WRONG: shared_ptr<classA> other_ptr(raw_ptr); // ALSO WRONG: shared_ptr<classA> other_ptr = raw_ptr; 

WARNING: the above code shows bad practice! raw_ptr simply should not exist as a variable. If you directly initialize your smart pointers with the result of new, you reduce your risk of accidentally initializing other smart pointers incorrectly. What you should do is:

shared_ptr<classA> my_ptr(new classA);  shared_ptr<classA> other_ptr(my_ptr); 

What's nice is that the code is more concise as well.

EDIT

I should probably elaborate on how it would work with a map. If you had a raw pointer and two maps, you could do something similar to what I showed above.

unordered_map<string, shared_ptr<classA> > my_map; unordered_map<string, shared_ptr<classA> > that_guys_map;  shared_ptr<classA> my_ptr(new classA);  my_map.insert(make_pair("oi", my_ptr)); that_guys_map.insert(make_pair("oi", my_ptr)); // or my_map["oi"].reset(my_ptr); // or my_map["oi"] = my_ptr; // so many choices! 

Answer 2

You can use a variety of ways, but reset() would be good:

map1[ID].reset(obj); 

And to address the issue of having two maps refer to the same shared_ptr, we can have:

map2[ID] = map1[ID]; 

Note that the trick in general to avoid a double delete is to try to avoid raw pointers at all. Hence avoid:

classA* obj = new classA(); map1[ID].reset(obj); 

but instead put the new heap object straight into a shared_ptr.