How to erase "this" from boost::ptr_vector

Question

I have a boost::ptr_vector containing pointers to class "holdable".

boost::ptr_vector<holdable> items;

I add new items to this vector from within the holdable class like this:

currentplanet->items.push_back(this);

where currentplanet is a pointer to an object of the class containing the ptr_vector. This is all fine.

What I'm confused about is how to remove an entry from the ptr_vector from a function within its own class. I'm trying:

currentplanet->items.erase(std::find(currentplanet->items.begin(),
                                     currentplanet->items.end(),
                                     this));

as in accordance with the answer to a similar question here: How to erase elements from boost::ptr_vector, but i've obviously gone wrong somewhere, probably with regard to the use of "this".

On trying to compile, i receive an error from stl_algo.h saying

stl_algo.h|174|error: no match for 'operator==' in '__first.boost::void_ptr_iterator<VoidIter, T>::operator*
  [with VoidIter = __gnu_cxx::__normal_iterator<void**, std::vector<void*, std::allocator<void*> > >, 
  T = holdable]() == __val'|

I'm sure it's something obvious, but i'm probably getting confused by ptr_vector's indirection... thanks for any answers in advance!

Answer 1

I understood that std::find requires a value (not a pointer) for the third param, so you pass a ["begin", "end") range in which to seek, and the value you seek

Is this what you meant ?

currentplanet->items.erase(std::find(currentplanet->items.begin(),
                                     currentplanet->items.end(),
                                     *this));

note the *this

Answer 2

Alf is correct, but for a reason that's somewhat peculiar to boost::ptr_vector (and the other related boost::reversible_ptr_container containers) that I think shoould be called out. Usually an iterator to a container's element will dereference to a reference to the the container's value_type.

boost:ptr_vector<T>::value_type is a typedef for T*; however dereferencing a boost::ptr_vector<T>::iterator doesn't result in a T* reference.

From the Boost docs (http://www.boost.org/doc/libs/1_52_0/libs/ptr_container/doc/reversible_ptr_container.html):

Also notice that

typedef ... iterator

allows one to iterate over T& objects, not T*.

Dereferencing a boost::ptr_vector<T>::iterator results in a T&. That's why you were getting confused by ptr_vector's indirection, and that's why the last argument to std::find() needs to be a holdable object rather than a holdable*.

Finally, note that Xeo's comment about ptr_vector taking ownership of the pointers to should be understood - it might make sense in very limited situations for an object to want to delete itself, but you should do so with the full understanding that after that erase() call, you can't do anything more with the object. If that's not the behavior you need, instead of erase() you might want to consider using release(), which will release the container's ownership of the pointer along with removing it from the container (so the object will not be destroyed).

Answer 3

As other wrote - it is true that ptr_vector takes ownership of your object, but if you insist to be able for object to be removed from ptr_vector by itself - use find_if, not find:

Holdable* ptr = this;
currentplanet->items.erase(std::find_if(currentplanet->items.begin(),
                                        currentplanet->items.end(),
                                        [ptr](const Holdable& other) 
                                        {return ptr == &other;} ));

Note that find used with *this might find other object which is equal to *this...

---

For compilers still not supporting lambda expression (this thing starting from [ptr]), use your own predicate:

struct CompareToPtr { 
    CompareToPtr(Holdable* ptr) : ptr(ptr) {}
    bool operator () (const Holdable& other) const { return &other == ptr; }
    Holdable* ptr;
};
currentplanet->items.erase(std::find_if(currentplanet->items.begin(),
                                        currentplanet->items.end(),
                                        CompareToPtr(this)));