is back_insert_iterator<> safe to be passed by value?

Question

I have a code that looks something like:

struct Data { int value; };
class A {
public:
    typedef std::deque<boost::shared_ptr<Data> > TList;
    std::back_insert_iterator<TList> GetInserter()
    {
        return std::back_inserter(m_List);
    }
private:
    TList m_List;
};
class AA {
    boost::scoped_ptr<A> m_a;
public:
    AA() : m_a(new A()) {}
    std::back_insert_iterator<A::TList> GetDataInserter()
    {
        return m_a->GetInserter();
    }        
};
class B {
    template<class OutIt>
    CopyInterestingDataTo(OutIt outIt)
    {
        // loop and check conditions for interesting data
        // for every `it` in a Container<Data*>
        // create a copy and store it
        for( ... it = ..; .. ; ..) if (...) {
            *outIt = OutIt::container_type::value_type(new Data(**it));
            outIt++; // dummy
        }
    }
    void func()
    {
        AA aa;
        CopyInterestingDataTo(aa.GetDataInserter());
        // aa.m_a->m_List is empty!
    }
};

The problem is that A::m_List is always empty even after CopyInterestingDataTo() is called. However, if I debug and step into CopyInterestingDataTo(), the iterator does store the supposedly inserted data!

update: I found the culprit. I actually have something like:

class AA {
    boost::scoped_ptr<A> m_a;
    std::back_insert_iterator<A::TList> GetDataInserter()
    {
        //return m_a->GetInserter(); // wrong
        return m_A->GetInserter(); // this is the one I actually want
    }        
    // ..... somewhere at the end of the file
    boost::scoped_ptr<A> m_A;
};

Now, which answer should I mark as answer? Really sorry for those not chosen, but you guys definitely got some up-votes : )

Answer 1

The short answer is yes, back_insert_iterator is safe to pass by value. The long answer: From standard 24.4.2/3:

Insert iterators satisfy the requirements of output iterators.

And 24.1.2/1

A class or a built-in type X satisfies the requirements of an output iterator if X is an Assignable type (23.1) ...

And finally from Table 64 in 23.1:

expression t = u return-type T& post-condition t is equivalent to u

EDIT: At a glance your code looks OK to me, are you 100% certain that elements are actually being inserted? If you are I would single step through the code and check the address of the aa.m_a->m_List object and compare it to the one stored in outIt in CopyInterestingDataTo, if they're not the same something's fishy.

Answer 2

The following code, which compiles, prints "1", indicating one item added to the list:

#include <iostream>
#include <deque>
#include "boost/shared_ptr.hpp"
#include "boost/scoped_ptr.hpp"

struct Data { 
    int value; 
    Data( int n ) : value(n) {}
};

struct A {
    typedef std::deque<boost::shared_ptr<Data> > TList;
    std::back_insert_iterator<TList> GetInserter()
    {
        return std::back_inserter(m_List);
    }
    TList m_List;
};

struct AA {
    boost::scoped_ptr<A> m_a;
    AA() : m_a(new A()) {}
    std::back_insert_iterator<A::TList> GetDataInserter()
    {
        return m_a->GetInserter();
    }        
};

struct B {
    template<class OutIt>
    void CopyInterestingDataTo(OutIt outIt)
    {
        *outIt = typename OutIt::container_type::value_type(new Data(0));
        outIt++; // dummy
    }
    int func()
    {
        AA aa;
        CopyInterestingDataTo(aa.GetDataInserter());
        return aa.m_a->m_List.size();
    }
};

int main() {
    B b;
    int n = b.func();    
    std::cout <<  n << std::endl;
}