C / C++ / MFC

3-Oct-11 21:05

Hello Friends

I am creating a map of key as string and data is of One class Object reference.
As i am inserting data into map as i am getting new string,so before loading new object I am deleting last object.but As i delete object,It is clearing from map also.

Here is the sample of code

C++

class Test
{
  int a;
  int b;
}

map<string,Test*> mapTest;

As I run a loop of my some condition
On adding every new string that is some file name i am reading file values into that class.

Test* obj = new Test();
for(.......)
{
 ...
 ...
 reading File int obj pointer
 And then inserting into map
 mapTest.insert(make_pair(string,objTest));
}

delete obj;
obj=NULL;
//As i delete obj then it gets cleared from map also.

Any Suggestions.

Thanks & Regards
Yogesh

Richard MacCutchan3-Oct-11 21:26

3-Oct-11 21:26

yogeshs wrote:
//As i delete obj then it gets cleared from map also.

Any Suggestions.

Don't delete it.

Unrequited desire is character building. OriginalGriff

002comp3-Oct-11 22:05

3-Oct-11 22:05

But If i dont delete then it is eating my memory.
As i my Class Size is too Long bcoz I am using for reading a file.

Regards
Yogesh

Richard MacCutchan3-Oct-11 22:24

3-Oct-11 22:24

yogeshs wrote:
But If i dont delete then it is eating my memory.

Well, how exactly do you expect to process the information in your objects if they are not in memory? If your application is memory constrained then you need to reduce the requirements by reading and processing your file one record at a time.

Unrequited desire is character building. OriginalGriff

002comp3-Oct-11 22:39

3-Oct-11 22:39

Hello,I agree with you that for memory constraint i need to create one object and processing should be done.I was doing this previously.
But my requirements are changed i need to keep in memry until application doesnt close.So that if user chooses same file I dont need to read it again.

This is the scenario.
Any Ideas.
Thanks & regards
yogesh

Richard MacCutchan4-Oct-11 1:11

4-Oct-11 1:11

Think about this logically: you need all the records from the file in memory at the same time, so the laws of physics state that you will consume record_count x record_size bytes of memory at minimum. The only alternative is what I suggested earlier, read in only the records you need at any one time. The overhead to do this is unlikely to be great enough to cause you any major problems.

Unrequited desire is character building. OriginalGriff

002comp4-Oct-11 2:22

4-Oct-11 2:22

Hello
I am totally agree with you Mr. macCutchan.

And we were doing like that but now Requirements are changed.
Regards
Yogesh

Richard MacCutchan4-Oct-11 2:25

4-Oct-11 2:25

yogeshs wrote:
but now Requirements are changed

Well you need to look at your design as there is no way you can have this data in memory without it using space in memory!

Unrequited desire is character building. OriginalGriff

Re: How to clear Object memory of Map [ how to delete newed object tht we store in map]

ThatsAlok3-Oct-11 23:53

ThatsAlok

3-Oct-11 23:53

yogeshs wrote:
Test* obj = new Test();
for(.......)
{
...
...
reading File int obj pointer
And then inserting into map
mapTest.insert(make_pair(string,objTest));
}

because map keep reference to memory allocated to the object, since you deleting the obj afterward the actual memory got deleted and reference in map just point to deleted memory!.

if memory is not constraint you can create copy of memory variable when inserting into map!,however at the time of clearing of map you have to manually delete memory associated with the variables.

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow
Never mind - my own stupidity is the source of every "problem" - Mixture

cheers,
Alok Gupta
VC Forum Q&A :- I/IV
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002comp4-Oct-11 0:10

Re: How to clear Object memory of Map [ how to delete newed object tht we store in map]

4-Oct-11 0:10

Thnks Alok,But my main constraint is memory here only.
Is there any other idea to erase memory after i use in map?

As you told map keep reference of object pointer.

Then how can i use map as a storage container of class objects.
Is there any other way to store class objects in some STL and then delete tht newed class object.
Bcoz otherwise if we dont delete then heap memory will get full or memory leaks.

Thanks In Advance.

Regards
Yogesh

modified 4-Oct-11 7:53am.

ThatsAlok4-Oct-11 21:07

ThatsAlok

4-Oct-11 21:07

yogeshs wrote:
As you told map keep reference of object pointer.

it's depend on the way you implement the collection class, in your case your container is of string and object pointer.

yogeshs wrote:
Then how can i use map as a storage container of class objects.

Is there any other way to store class objects in some STL and then delete tht newed class object.

see if you want anything readily available, it should be present in memory,see if you delete the newly created class-object using new, the object is lost till you have copy of same available for you. there is no way AFAIK to have something with having nothing.

Now, this a design issue not programming issue, so you can relax Smile | :)

, design can be created only if i have full requirement otherwise there always flaw in design which can lead to defects Smile | :)

cheers,
Alok Gupta
VC Forum Q&A :- I/IV
Support CRY- Child Relief and You

Stefan_Lang4-Oct-11 5:56

Stefan_Lang

4-Oct-11 5:56

First, when you delete an object using a pointer then other pointers, including the copy you stored in the map, will not be changed - instead they will then point to invalid memory. Trying to use them causes undefined behaviour. In short: don't do that!

Second, don't store pointers, store the objects instead. That way you'll save one pointer entry per entry, and the map will destroy the objects once you remove the entry from the map.

Third, from other comments I've gathered that you cannot actually afford to keep the whole map in memory. If that is the problem, then my best advice is: do not try to program that! Instead, use a database. The only alternative would be to selectively remove rarely used entries from your map and reread them later, when needed. But that would be inefficient, if the file you are reading is not itself indexed in some way. not to mention that such a paging mechanism isn't exactly easy to implement. A database however has these mechanisms, and rereading individual records back into memory later is fast.

ThatsAlok4-Oct-11 20:55

ThatsAlok

4-Oct-11 20:55

Stefan_Lang wrote:
Second, don't store pointers, store the objects instead. That way you'll save one pointer entry per entry, and the map will destroy the objects once you remove the entry from the map.

For discussion sake, if we kept object instead, still it will create copy of original object (see it call copy constructor,if not properly implemented, there can be problem of shallow copy and dangling pointers), however the only advantage i know would be you don't need to worry about destruction as OOPS by heart take responsibility of destroying of object!

Regards
Alok

cheers,
Alok Gupta
VC Forum Q&A :- I/IV
Support CRY- Child Relief and You