Save Pointer to Object in Vector in a Loop

Question

Is it possible to save a pointer to an object in a vector inside a loop?

For example:

rpc::session* session=NULL;

//Find Sessions with same UserID, Remove if > 1
for(std::vector<rpc::session>::iterator it = session_manager::sessions.begin(); it != session_manager::sessions.end();) {
  if(it->userid == userid){
    if(session == NULL) {
      *session = *it;
      ++it;
    } else {
      it = session_manager::sessions.erase(it);
    }
  } else {
    ++it;
  }
}

The code gets compiled but as soon as it reaches *session = *it, it crashes with a Access violation..

Tony The Lion · Accepted Answer · 2013-04-02 12:52:24Z

2

Because you're not assigning the address of your session to your session variable, and seeing your vector is storing session objects and not pointers, you need to assign the address of your variable to your pointer.

session = &(*it);

This assigns the address of what is in *it to session

answered Apr 2, 2013 at 12:52

Tony The Lion

63.6k71 gold badges252 silver badges424 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Tony The Lion Over a year ago

@LuchianGrigore haaha, I misread the intent of the code. Fixed

Luchian Grigore · Accepted Answer · 2013-04-02 12:52:59Z

2

That's because you're attempting to dereference a NULL pointer. If you want to save a pointer to that object, you need to assign the pointer session itself - session = &(*it).

answered Apr 2, 2013 at 12:52

Luchian Grigore

260k68 gold badges465 silver badges634 bronze badges

Comments

ogni42 · Accepted Answer · 2013-04-02 13:07:24Z

This is because you try to copy the contents of a session object into a nullptr. You either have to allocate memory and copy into that memory or copy the pointer into another location.

Please be aware, that taking the address of the element in the vector is no good solution in your case as a vector::erase() will move the elements in the vector, making a previously stored address invalid.

In your case it might still work, as you will store only the last session found in the vector. If you want to store all sessions with the correct user id you need to do something like

std::vector<rpc::session> result;
std::copy_if(session_manager::sessions.begin(), 
             session_manager::sessions.end(), 
             result.begin(), 
             [&](rpc::session s) -> bool 
             {
                 return s.userid == userid;
             }
             );

filipe · Accepted Answer · 2013-04-02 12:54:59Z

1

You can get a pointer to an element in a vector, but what you're doing with *session = *it; is not getting the pointer to the element but getting its value and trying to copy it to another rpc::session object that was never allocated. If you want the pointer to the rpc::session object in the vector, you should do

session = &(*it);

answered Apr 2, 2013 at 12:54

filipe

3,3801 gold badge18 silver badges22 bronze badges

1 Comment

filipe Over a year ago

I need to learn to type faster... two similar answers got posted in the time I took to write mine.

Collectives™ on Stack Overflow

Save Pointer to Object in Vector in a Loop

4 Answers 4

1 Comment

Comments

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

1 Comment

Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related