How to erase elements from boost::ptr_vector

Question

So I'm trying to get rid of my std::vector's by using boost::ptr_vector. Now I'm trying to remove an element from one, and have the removed element deleted as well. The most obvious thing to me was to do:

class A
{ int m; };

boost::ptr_vector<A> vec;
A* a = new A;
vec.push_back(a);
vec.erase(a);

But this won't even compile (see below for the full error message). I've tried the erase/remove idiom like I would on a std::vector but all the algorithms of boost::ptr_vector turn out to be slightly different from those in std::vector.

So my questions:

• How do I remove a pointer from a ptr_vector?
• Do I still need to manually delete() that element that I removed?

Compiler error:

1>------ Build started: Project: ptr_vector_test, Configuration: Debug Win32 ------
1>Compiling...
1>ptr_vector_test.cpp
1>c:\users\rvanhout\svn\trunk\thirdparty\boost\range\const_iterator.hpp(37) : error C2825: 'C': must be a class or namespace when followed by '::'
1>        c:\users\rvanhout\svn\trunk\thirdparty\boost\mpl\eval_if.hpp(63) : see reference to class template instantiation 'boost::range_const_iterator<C>' being compiled
1>        with
1>        [
1>            C=A *
1>        ]
1>        c:\users\rvanhout\svn\trunk\thirdparty\boost\range\iterator.hpp(63) : see reference to class template instantiation 'boost::mpl::eval_if_c<C,F1,F2>' being compiled
1>        with
1>        [
1>            C=true,
1>            F1=boost::range_const_iterator<A *>,
1>            F2=boost::range_mutable_iterator<A *const >
1>        ]
1>        c:\users\rvanhout\svn\trunk\thirdparty\boost\ptr_container\detail\reversible_ptr_container.hpp(506) : see reference to class template instantiation 'boost::range_iterator<C>' being compiled
1>        with
1>        [
1>            C=A *const 
1>        ]
1>        c:\tmp\ptr_vector_test\ptr_vector_test.cpp(21) : see reference to function template instantiation 'boost::void_ptr_iterator<VoidIter,T> boost::ptr_container_detail::reversible_ptr_container<Config,CloneAllocator>::erase<A*>(const Range &)' being compiled
1>        with
1>        [
1>            VoidIter=std::_Vector_iterator<void *,std::allocator<void *>>,
1>            T=A,
1>            Config=boost::ptr_container_detail::sequence_config<A,std::vector<void *,std::allocator<void *>>>,
1>            CloneAllocator=boost::heap_clone_allocator,
1>            Range=A *
1>        ]
1>c:\users\rvanhout\svn\trunk\thirdparty\boost\range\const_iterator.hpp(37) : error C2039: 'const_iterator' : is not a member of '`global namespace''
1>c:\users\rvanhout\svn\trunk\thirdparty\boost\range\const_iterator.hpp(37) : error C2146: syntax error : missing ';' before identifier 'type'
1>c:\users\rvanhout\svn\trunk\thirdparty\boost\range\const_iterator.hpp(37) : error C2208: 'boost::type' : no members defined using this type
1>c:\users\rvanhout\svn\trunk\thirdparty\boost\range\const_iterator.hpp(37) : fatal error C1903: unable to recover from previous error(s); stopping compilation
1>Build log was saved at "file://c:\tmp\ptr_vector_test\Debug\BuildLog.htm"
1>ptr_vector_test - 5 error(s), 0 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Answer 1

Curious thing: STL::vector<> is a Random Access Container, meaning that it uses Random Access Iterators.

So vec.erase(vec.begin()+N) will remove the element at index N.

Note that doing so breaks the whole iterator meme and you can no longer trivially switch between vectors and lists...

Answer 2

Well you can do that with a std::vector either.

In both cases erase takes an iterator as a parameter.
So before you can erase something from a vector (or a ptr_vector) you need to locate it.

Also note that the ptr_vector treats its content as if you have stored an object not a pointer. So any searching is done via the object.

So basically

 std::vector<A>       x;
 std::ptr_vector<A>   y;

 // These two object should behave in exactly the same way.
 // The ONLY difference is inserting values which for y are pointers.
 // Y take ownership of the pointer and all subsequent acesses to the
 // members of y look like they are objects

Example:

#include <boost/ptr_container/ptr_vector.hpp>
#include <vector>

class A
{ int m;
    public:
    A(int x):m(x)   {}
    bool operator==(A const& rhs)   {return m = rhs.m;}
};

int main()
{
    boost::ptr_vector<A>    x;
    x.push_back(new A(1));
    x.erase(std::find(x.begin(),x.end(),A(1)));


    std::vector<A>          y;
    y.push_back(A(2));
    y.erase(std::find(y.begin(),y.end(),A(2)));

    // To find an exact pointer don't modify the equality.
    // Use find_if and pass a predicate that tests for a pointer
    A* a = new A(3);
    boost:ptr_Vector<A>     z;
    z.push_back(a);
    z.erase(std::find_if(y.begin(),y.end(),CheckPointerValue(a));
}

struct CheckPointerValue
{
     CheckPointerValue(A* a):anA(a) {}
     bool operator()(A const& x)    { return &X == anA;}
     private:
        A* anA;
};

Answer 3

The semantics of ptr_vector are very similar to a regular vector. You have to find an element before you can erase it.

Answer 4

You need to use the member erase_if method with a suitable predicate. There's no need to delete the pointer, the container has ownership.

struct delete_a {
    bool operator()(boost::ptr_vector<A>::value_type inA) {
       return inA == a;
    }
}

vec.erase_if(delete_a());

(note this is just an example chosen for simplicity, for such situation in real code I suppose one would write a suitable bind/equal_to combo, or use lambda)

Or, as an alternative, call release on the correct iterator, if you still want to use the object.

Answer 5

I think you want to call .release() on the vector instead of erase. That removes the entry and deletes the memory.

See the section "New Functions" for details in the tutorial, or check the reference.

Alternatively, you need to get an iterator to an element in order to call erase(), I'm ot sure an A* counts in terms of a ptr_vector.

Answer 6

You can use erase_if template method.

vec.erase_if( predicate() );