Help storing an intrusive_ptr of a template class in a std::map

Question

I have a small template class of type Locker contained within a boost::intrusive_ptr that I want to store inside a std::map:

template <typename T>
bool LockerManager<T>::
  AddData(const std::string& id, T* pData)
{

  boost::intrusive_ptr<Locker<T> > lPtr(Locker<T>(pData)); // Line 359 - compiles

  mMap.insert(make_pair(id, lPtr));  // Line 361 - gives error
}

Locker is just a container class; its constructor looks like:

template <typename T>
Locker<T>::
  Locker(T* pData)
  : IntrusivePtrCountable(),
    mpData(pData),
    mThreadId(0),
    mDataRefCount(0)
{}

In my test of this class, I am trying to do the following:

class Clayton
{
public:
  static int count;

  Clayton()
  { mNumber = count++;}

  void GetNumber()
  { cerr<<"My number is: "<<mNumber<<endl; }

private:
  int mNumber;
};

int Clayton::count = 0;

class ClaytonManager
{
public:
  bool AddData(const std::string& id, Clayton* pData)
  { return mManager.AddData(id, pData); }

private:
  LockerManager<Clayton> mManager;
};

I get the following compile error:

Compiling LockerManagerTest.cpp                             : /usr/local/lib/gcc/i686-pc-linux-gnu/3.4.6/../../../../include/c++/3.4.6/bits/stl_pair.h: In constructor `std::pair<_T1, _T2>::pair(const std::pair<_U1, _U2>&) [with _U1 = std::basic_string<char, std::char_traits<char>, std::allocator<char> >, _U2 = boost::intrusive_ptr<Locker<Clayton> > (*)(Locker<Clayton>), _T1 = const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, _T2 = boost::intrusive_ptr<Locker<Clayton> >]':
../Utilities/include/LockerManager.h:361:   instantiated from `bool LockerManager<T>::AddData(const std::string&, T*) [with T = Clayton]'
src/LockerManagerTest.cpp:35:   instantiated from here
/usr/local/lib/gcc/i686-pc-linux-gnu/3.4.6/../../../../include/c++/3.4.6/bits/stl_pair.h:90: error: no matching function for call to `boost::intrusive_ptr<Locker<Clayton> >::intrusive_ptr(boost::intrusive_ptr<Locker<Clayton> > (* const&)(Locker<Clayton>))'
/usr/local/boost-1.36.0/include/boost-1_36/boost/intrusive_ptr.hpp:94: note: candidates are: boost::intrusive_ptr<T>::intrusive_ptr(const boost::intrusive_ptr<T>&) [with T = Locker<Clayton>]
/usr/local/boost-1.36.0/include/boost-1_36/boost/intrusive_ptr.hpp:70: note:                 boost::intrusive_ptr<T>::intrusive_ptr(T*, bool) [with T = Locker<Clayton>]
/usr/local/boost-1.36.0/include/boost-1_36/boost/intrusive_ptr.hpp:66: note:                 boost::intrusive_ptr<T>::intrusive_ptr() [with T = Locker<Clayton>]
Command exited with non-zero status 1
0:05.40

Please help

Answer 1

The compiler is doing its best to be helpful here. You need to define a method with the following signature:

boost::intrusive_ptr<Locker<Clayton> >::intrusive_ptr(
    boost::intrusive_ptr<Locker<Clayton> > (* const&)(Locker<Clayton>)
):

You need to define a copy constructor for types that you store in a std::map, that's what this error message is trying to convey. You should also implement the < operator so that the following is true:

- ( x < x ) == false
- if ( x < y ), then !( y < x )
- if ( x == y ), then ( x < y ) == ( y < x ) == false
- if ( x < y ) and ( y < z ), then ( x < z )

For many of the built-in types C++ will provide these for you, which is why some people don't know what you have to do to store custom objects in std::map and other STL containers.

Answer 2

Actually, intrusive_ptr already has a < operator and a copy constructor defined, so that wasn't the problem.

There were two main things that we were missing. First, we needed to use value_type, instead of make_pair, to avoid implicit type conversion in the insert statement. Second, we missed the fact that the intrusive_ptr constructor takes a pointer to the type that it is templated around.

So, the final, working method looks like this:

// ---------------------------------------------------------------------------
// LockerManager::AddData
// ---------------------------------------------------------------------------
template <typename T>
bool LockerManager<T>::
  AddData(const std::string& id, T* pData)
{
  Lock<MutualExclusion> lLock(mMutex);

  if ((pData == NULL) || (mMap.find(id) != mMap.end()))
    return false;

  mMap.insert(typename std::map<std::string, boost::intrusive_ptr<Locker<T> > >::value_type(id, new Locker<T>(pData)));

  return true;
} // LockerManager::AddData