STL map and memory management (clear() )

[Jan]

Hi there,
i've got an STL map with something like this ( map<string, Object*>
xyz; )
What happens when I call xyz.clear()?
Is only the map cleared or the map and the Objects, so that the memory
is free again?
Have I to delete every single Object?

Thanks for replying,
Jan

[Mark P]

Jan wrote:

Hi there,
i've got an STL map with something like this ( map<string, Object*>
xyz; )
What happens when I call xyz.clear()?
Is only the map cleared or the map and the Objects, so that the memory
is free again?
Have I to delete every single Object?

Thanks for replying,
Jan

clear() empties out the contents of the map. It doesn't do anything
with the Object*; in particular it doesn't delete them.

[Jan]

thanks :-)

[Axter]

Jan wrote:

Hi there,
i've got an STL map with something like this ( map<string, Object*>
xyz; )
What happens when I call xyz.clear()?
Is only the map cleared or the map and the Objects, so that the memory
is free again?
Have I to delete every single Object?

Thanks for replying,
Jan

None of the STL containers will delete a pointer type.
I recommend you use smart pointers instead of raw pointers with STL
containers.
You can not use auto_ptr with STL containers, but you can use other
common smart pointers like boost::shared_p tr and clone pointers like
copy_ptr and cow_ptr.

http://www.boost.org/libs/smart_ptr/shared_ptr.htm

http://code.axter.com/copy_ptr.h

http://code.axter.com/cow_ptr.h

[Mateusz Loskot]

Mark P wrote:

It doesn't do anything with the Object*; in particular it doesn't delete them.

Wrong!
Object* is a pointer or more descriptively - it is a pointer type to
objects of type of Object.
Every STL container destroys its elements while being destroyed itself.
So, when you call std::map::clear () then all contained pointers will be
destroyed, BUT NOT objects pointed by them. All those objects will stay
alive in memory.
In your case, container-of-pointers, no STL container will delete
instances of Object class for you because it stores pointers, not
objects. So, you have to destroy those instances by yourself: iterated
through std::map elements and call delete on contained pointer.

Cheers
--
Mateusz Loskot
http://mateusz.loskot.net

[Mateusz Loskot]

I'd like to complement my last post.
Currently, I'm reading amazing book "C++ Gotchas: Avoiding Common
Problems in Coding and Design" by Stephen C. Dewhurst.
There is "Gotcha #83: Failure to Distinguish Aggregation and
Acquaintance" chapter which, what at first sight may seem, does not
drift a-way from the subject.

It is really woth to read, especially when you are working with
containers-of-pointers.

At the end, author express' very neatly - in one sentence - what I
wanted to say to my previous post:

"For pointer elements, the standard* containers will clean up the
pointers but not the objects to which they refer."

* - means STL containers
Cheers
--
Mateusz Loskot
http://mateusz.loskot.net

[Stephen Howe]

> Wrong!

Object* is a pointer or more descriptively - it is a pointer type to
objects of type of Object.
Every STL container destroys its elements while being destroyed itself.
So, when you call std::map::clear () then all contained pointers will be
destroyed...

How is Mark P wrong?
A pointer has no destructor, so clear() has nothing to do for each value
element.

Stephen Howe

[Mateusz Loskot]

I said this sentence is wrong: "It doesn't do anything with the
Object*".
OK maybe not wrong, but not precise.

Your sentence "has nothing to do" is also not precise.
Why? Pointers are "cleaned up"*** after std::map::clear () is called.

*** - See my previous post where I quoted Stephen C. Dewhurst

If there "nothing is done" there also pointers would not be cleared,
pointers are data, they occupy space in memory, just as integers or
objects. So, they must be cleaned (we can say "destroyed" and it won't
be mistake).

Cheers
--
Mateusz Loskot
http://mateusz.loskot.net

[Michiel.Salters]

Jan wrote:

Hi there,
i've got an STL map with something like this ( map<string, Object*>
xyz; )
What happens when I call xyz.clear()?
Is only the map cleared or the map and the Objects, so that the memory
is free again?
Have I to delete every single Object?

Depends. If the Object* are non-owning (e.g. because xyx is just a way
of associating names with list<Object> entries), no. The list<Object>
will get them. However, if the Object* are owning, then yes of course.

Besides, how would the STL know whether to call delete or delete[]
or &Object::Del ete ? And what if it wasn't Object* but char*?

HTH,
Michiel Salters

[Stephen Howe]

> Your sentence "has nothing to do" is also not precise.

Why? Pointers are "cleaned up"*** after std::map::clear () is called.

If there "nothing is done" there also pointers would not be cleared,
pointers are data, they occupy space in memory, just as integers or
objects. So, they must be cleaned (we can say "destroyed" and it won't
be mistake).

Your not being precise.
What do you mean by pointers must be cleaned?
What does "cleaning" involve?

Stephen Howe