Replacing shared_ptr elements in std::vector

Question

I have a vector of shared_ptrs as below.

std::vector<std::shared_ptr<SharedThing>> things;

Now let's say I push a number of shared_ptrs onto the vector and each element now has a reference count of 1.

When I need to replace one of those elements with a new shared_ptr I want the old shared_ptr to go out of scope. Will regular element assignment achieve this or will it just copy the shared_ptr contents. For example:

things.at(0) = new_shared_ptr;

Will this decrement the reference count of things.at(0) and increment the count of new_shared_ptr?

Answer 1

Yes, basically you are right.
To be more accurate, the reference count of previous shared_ptr at(0) will be decremented. And then you assign it with a new shared_ptr, which may have the count 1. Looks like the reference count at(0) is the same, but it changed and changed back.

You can verify it by std::shared_ptr::use_cout()

For more details, we can debug into the STL, when things.at(0) = new_shared_ptr;

include/c++/4.8.3/bits/shared_ptr_base.h:556

  __shared_count&
  operator=(const __shared_count& __r) noexcept
  {    
_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
if (__tmp != _M_pi)
  {    
    if (__tmp != 0)
      __tmp->_M_add_ref_copy();
    if (_M_pi != 0)
      _M_pi->_M_release();
    _M_pi = __tmp;
  }    
return *this;
  }    

The new one _M_add_ref_copy(), then previous one _M_release(), which will decrease the _M_use_count by 1.