Assignment operator, templates, double associative container

I'm building DataManager<T>. A class where shared data will be stored.
One of things useful to implement is garbage collection. But it still
gives me this error:

stl_algo.h:1076: error: non-static const member `const std::string
DataMapPair::first', can't use default assignment operator

Here's my code:

typedef std::map< std::string, std::pair< bool, CountedPtr<T> > >
DataMap;
typedef std::pair< std::string, std::pair< bool, CountedPtr<T> > >
DataMapPair;

DataMap dataMap_;

template <typename T> bool DataManager<T> ::
IsGarbage( const DataMapPair& dmPair )
{
  return ( dmPair.second.first && dmPair.second.second.IsUnique() );

}

template <typename T> void DataManager<T> ::
CollectGarbage()
{
  dataMap_.erase( remove_if( dataMap_.begin(), dataMap_.end(),
IsGarbage ), dataMap_.end() );

}

Can anyone tell how should I oveload that operator=? Or should I do
something else?

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