Re: Assignement operator, templates, double associative array

From:

John Harrison <john_andronicus@hotmail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 20 Feb 2007 17:56:55 GMT

Message-ID:

<HDGCh.8886$fa.8528@newsfe1-win.ntli.net>

mscava@gmail.com wrote:

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?

remove_if tries to rearrange the sequence it is given. std::map's are
stored in a fixed order (i.e. they are ordered by a key) and they don't
like it when you try to rearrange them. That is what the error message
means.

Instead of trying fancy stuff in your CollectGarbage method you should
just write a loop through the entries of the map and delete any that are
garbage.

john