无法在函数返回的不可复制容器上进行迭代

Question

I'm not sure of the title, because I'm not sure the issue comes from the "copyablility" of my container. I tryied quite everything but I can't get rid of this error.

Here is a simplified version of my code (please do not challenge the class design, I really would like to keep the end-used syntax in the BOOST_FOREACH):

template <typename T>
class MyContainer
{
public:
    typedef typename std::vector<T>::iterator iterator;
    typedef typename std::vector<T>::const_iterator const_iterator;

    MyContainer(std::vector<T>& vec, boost::mutex& mutex) :
        m_vector(vec),
        m_lock(mutex)
    {
    }

    iterator begin() { return m_vector.begin(); }
    const_iterator begin() const { return m_vector.begin(); }
    iterator end() { return m_vector.end(); }
    const_iterator end() const { return m_vector.end(); }


private:
    std::vector<T>& m_vector;
    boost::lock_guard<boost::mutex> m_lock;
};

template <typename T>
struct GetContainer
{
    GetContainer(std::vector<T>& vec, boost::mutex& mutex) :
        m_vector(vec),
        m_mutex(mutex)
    {
    }

    MyContainer<T> Get()
    {
        return MyContainer<T>(m_vector, m_mutex);
    }

    std::vector<T>& m_vector;
    boost::mutex& m_mutex;
};



int main()
{
    std::vector<int> v;
    v.push_back(1);
    v.push_back(2);
    boost::mutex m;

    GetContainer<int> getter(v, m);

    BOOST_FOREACH(int i, getter.Get())
    {
        std::cout << i << std::endl;
    }

    return 0;
}

The compiler complains about not having a copy constructor for MyContainer::MyContainer(const MyContainer&). I also have : error: no matching function for call to 'MyContainer::MyContainer(boost::foreach_detail_::rvalue_probe >::value_type)'

I follow the extensibility tips: http://www.boost.org/doc/libs/1_58_0/doc/html/foreach/extensibility.html#foreach.extensibility.making__literal_boost_foreach__literal__work_with_non_copyable_sequence_types

But, making

MyContainer<T> : private boost::noncopyable

doesn't solve the issue. Nor defining the function

boost_foreach_is_noncopyable

or specializing the template struct

is_noncopyable

for MyContainer (in fact, how would I specialize this template for a template type ?)

Last "tip" : If I remove the mutex and the lock from everywhere (I just pass the vector to GetContainer and to MyContainer), it works. But it doesn't work if I make

MyContainer<T> : private boost::noncopyable

(I expected it should, so I'm not sure my problem is with BOOST_FOREACH, but maybe because I return a copy of MyContainer with my getter ?)

I thank you if you read me until here, and thanks in advance for help.

Answer 1

Seems to be a limitation of BOOST_FOREACH with move-only types. I didn't find a way around it¹ (except for the - ugly - obvious approach to put the lock_guard in a shared_ptr ).

You didn't specify a c++03 requirement, though, so you can make it work without BOOST_FOREACH by replacing lock_guard with unique_lock .

Here's my take on things in c++11 (note how generic it is):

Live On Coliru

#include <boost/thread.hpp>
#include <boost/range.hpp>

namespace detail {
    template <typename R, typename M>
    struct RangeLock {
        RangeLock(R&r, M& m) : _r(r), _l(m) {}
        RangeLock(RangeLock&&) = default;

        using iterator = typename boost::range_iterator<R>::type;
        iterator begin() { using std::begin; return begin(_r); }
        iterator end  () { using std::end;   return end  (_r); }

        using const_iterator = typename boost::range_iterator<R const>::type;
        const_iterator begin() const { using std::begin; return begin(_r); }
        const_iterator end  () const { using std::end;   return end  (_r); }

     private:
        R& _r;
        boost::unique_lock<M> _l;
    };
}

template <typename R, typename M>
    detail::RangeLock<R,M> make_range_lock(R& r, M& mx) { return {r,mx}; }
template <typename R, typename M>
    detail::RangeLock<R const,M> make_range_lock(R const& r, M& mx) { return {r,mx}; }

#include <vector>
#include <map>

int main() {

    boost::mutex mx;

    std::vector<int> const vec { 1, 2 };
    std::map<int, std::string> const map { { 1, "one" }, { 2, "two" } };

    for(int i : make_range_lock(vec, mx))
        std::cout << i << std::endl;

    for(auto& p : make_range_lock(map, mx))
        std::cout << p.second << std::endl;

    for(auto& p : make_range_lock(boost::make_iterator_range(map.equal_range(1)), mx))
        std::cout << p.second << std::endl;

}

Prints

1
2
one
two
one

---

¹ not even using all the approaches from Using BOOST_FOREACH with a constant intrusive list

Answer 2

I post my answer if it can help...

, I finally provide a copy constructor to be able to use the class with BOOST_FOREACH. So the issue is moved to another topic: make the class copied in a logic and suitable way.

In my case, I "share the lock and the vector", the user shouldn't use this copy itself if he doesn't want to do bugs, but in BOOST_FOREACH it's okay:

• I change the mutex to a recursive_mutex

• I change the lock to an unique_lock and:

   MyContainer(const MyContainer& other) : m_vector(other.vec), m_lock(*other.m_lock.mutex()) { } 

Thanks to Chris Glover on the boost mailling list, a C++11 solution:

You can't do what you are trying to do in C++03. To accomplish it, you need C++11 move semantics to be able to move the MyContainer out of the Get function. Even without using BOOST_FOREACH, the following code fails;

 GetContainer<int> getter(v, m); MyContainer<int> c = getter.Get(); // <-- Error. 

Here's an example with the necessary changes; I changed the scoped_lock to a unique_lock and added a move constructor.

 template <typename T> class MyContainer { public: [...] MyContainer(MyContainer&& other) : m_vector(other.m_vector) { m_lock = std::move(other.m_lock); other.m_vector = nullptr; }