Question

STL hash_multimap questions

Asked by: wayside

1) Is hash_multimap supported on the major Unix compilers? I would hope so, but in Visual Studio .Net 20003 it is in the stdext namespace instead of the std namespace, so I wasn't sure, and I don't have immediate access to a unix box. Is it in the std namespace on unix?

2) Is there any way to control the ordering of elements when inserting multiple elements with the same key? Basically, I'd like to get the elements out in the same order that I inserted them, for any given key. For Visual C++, they come out like this:

if I have 5 elements with the same key x, that are n, n+1, n+2, n+3, n+4, and I insert them in that order, I get them out in this order:

n+1, n+2, n+3, n+4, n

I could always sort them after I get them out of the multimap, or, if the retrieval order is the same on every platform I can account for that, although I expect that is implementation-specific. It would be much easier for me if I don't have to sort them though.

Thanks.

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Asked On
2006-12-15 at 10:58:22ID22094912
Tags

hash_multimap

,

stl

Topics

C++ Programming Language

,

Microsoft Visual C++

Participating Experts
2
Points
250
Comments
2

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Answers

 

by: Fippy_DarkpawPosted on 2006-12-15 at 11:43:43ID: 18148445

1) yes. Here's some GCC/Unix STL samples:

http://bobcat.webappcabaret.net/javachina/faq/stl_01.htm

2) AFAIK no. But if you really need them sorted it would probably be easier to have an array of linked lists.

- make your own hash function which yields an index in the array of linked lists

- when you hash an object place it in the linked list at that index

- there is a built in sort() function in stl linked lists, although it would probably be more effiecient to insert new objects in their correct place

 

by: AxterPosted on 2006-12-15 at 12:33:56ID: 18148827

Hi wayside,
> but in Visual Studio .Net 20003 it is in the stdext namespace instead
> of the std namespace

FYI:
That is the correct way to add objects that are not part of the official C++ std standard.
Compilers and Libraries that add non-standard object to std namespace are breaking the rules.  The std namespace is suppose to be only for the official C++ standard objects.
This is why the C++ commitee has decided to change the hash_map table name when it finally gets added to the standard.  They didn't want to break existing libraries and code using these bad libraries.

This is one of the rare cases, in which VC++ is actually following the standard, and compilers/libraries like GNU, are not.

David Maisonave (Axter)
Cheers!

20120131-EE-VQP-002

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