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
Solution:1
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
Solution:2
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.
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