STL map and memory management

[paulix]

Hi,
I've a map of maps like the following:

map <int, map <int, map <string, int> > > mymap;

that, inside a big loop, use a lot of memory so that the entire computer memory is used. I would like to free the memory sometimes inside the loop (when I'm sure that some keys are not used anymore) but using clear() the map is empty but the objects are still in memory.

thanks in advance!

Pierpaolo

[weaknessforcats]

You may need to swap your empty map with a new one that has no elements and then copy back to the original.

STL containers do not always release memory when elements are removed.

Check out: Scott Meyers book Effective STL.

[paulix]

Please could you be more precise?
My code is something like:

map <int, map <int, map <string, int> > > mymap;
for(int l=0; l<nline; l++) {
....................
mymap[..][..][..]++;

if(some conditions){
for(map...iterator h mymap.begin(); h != mymap.end(); h++){
............
some print
}
}
}

after the print I should have to free the memory (for some keys) but I'm not able to obtain that using clear(). Not only....I noticed (using the linux command 'top') that when I'm inside the second loop (the one that use the iterator to have access to the map elements) the memory used it is doubled...as if the map and all its elements are copied into a second map in memory!

[weaknessforcats]

You are not in control of the container. You cannot tell the container to delete memory just because you deleted an element. You will have to create an empty container and swap it withe the full one.

OR:

Do not use objects in the map. Use a reference counted handle instread and have a map of handles. Handles are objects of an envelope class and somes times called smart, or managed pointers.

With a handle, then the handle id deleted, the object is manages id deleted also if it is the last handle. Otherwise, the reference count is just reduced by1.

This may require some rresearch. Check out Design Patterns ISBN 0-201-63361-2. You are interested in the Bridge pattern.

A reference counted handle will be available as a template in STL as shared_ptr in C++ 2003, which isn't quite out yet.

If you decide to go this way, let me know and I will post a template I have.

[paulix]

I've never used something like that, but if it works....why not? But let me explain better....what I'm reading are logfiles and each row of data contains the hour, so the first key in the map of maps is exactly the hour [0,..,23]. So I could also use an array of 24 map, i.e. instead of:

map <int, map <int, map <string, int> > > mymap;

I could use:
map <int, map <string, int> > mymap[24];

and because the logfiles are sorted text file as soon as the data concerning the first hour are finished I print the cumulative data and so I could delete the first element of the array (mymap[0]) and so on. I've tried but even in this way the memory increase with the same speed....
So of course if you could show me some pratical way to mantain the memory control usind the std map would be nice!

Many thanks!

[weaknessforcats]

I just posted an article on Handle classes. Have a look and tell me what you think.