How do I allow unique ID with a class (C++)

Question

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I am trying to make a database, and i am using a static variable to keep track of the ID of each contact inside the database. When i try to use the variable though it changes any other contacts that also have an id.

This is the code i use with the static variable, holder is the new contact being added.
holder->id = idGen++;

What I have tried:

I have tried googling it but haven't found anything talking about my particular problem, and anything.

Posted 10-May-16 23:09pm

Member 12516566

Updated 11-May-16 3:54am

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Comments

Kornfeld Eliyahu Peter 11-May-16 5:12am

Why not to add a column to your database that stores the unique ID? A SID maybe...

Mohibur Rashid 11-May-16 5:54am

That's the property of static variable. Think different

Richard MacCutchan 11-May-16 7:18am

You need to show more of your code. It is impossible to guess why you are not getting the correct answer.

Member 12516566 11-May-16 7:21am

I cannot think of any code to add that would help clarify, unless you want me to just include the whole function I use it with.

Richard MacCutchan 11-May-16 7:49am

The only code you have shown is the single line in your question above. But it does not really give any clue as to why it does not work. We need more details in order to be able to help you. Have you used your debugger to identify where the code is going wrong?

Member 12516566 11-May-16 7:51am

It goes wrong after it increases the value of idGen. if i were to make two contacts in the database, the 1st one would get the id 1, then when it went to make the 2nd, both the 1st and 2nd get the value of 2.

Richard MacCutchan 11-May-16 7:55am

Well you are doing something wrong, but it is impossible for anyone here to guess what that may be.

Member 12516566 11-May-16 7:57am

Well i did ask what more code i should include to give people more to help. What i showed is the only location where i change the value of idGen

Richard MacCutchan 11-May-16 8:01am

We have no idea how your code is structured, or what instructions lead to this problem. You are the only person who can decide what needs to be shown for us to be able to help you.

Philippe Mori 11-May-16 12:14pm

You don't even show us the declaration of both variables so we don't know if they are static, class member, global or whatsover. We could try to make guess but it would probably not match what you use since if your code was correct it would be working.

But it seems you are too lazy to make minimal code to reproduce the problem. We don't need the whole code but the minimum code that compile and reproduce the problem.

Philippe Mori 11-May-16 12:11pm

If you cannot provide required information, you won't get much specific help. There are many ways to do things and no one will write whole code for you. It is up to you to show us enough code for us to figure the problem. By the way, it should also be easy to find the problem using a debugger.

3 solutions

Solution 3

Are you sure you need to synthesise a unique ID for each object?

In a database implemented in C++ each object is going to have two unique values already associated with it - the address of the object in memory or the offset of the object within the file the database is stored in. If your database is table based you've got a third unique identifier - the offset of the object from the start of the table. Any of these can be used to unambiguously identify an object and stored in other objects.

Posted 11-May-16 3:54am

Aescleal

Solution 1

Your static variable should provide the next ID value. Each contact should contain its assigned (by means of the static variable) ID.

Posted 11-May-16 0:02am

CPallini

Comments

Member 12516566 11-May-16 6:49am

That is what i do. holder->id is where the id of the contact is, which it is supposed to be equal to what idGen is. But when i make another contact, the id of the previous contact will be equal to the one that is being made.

CPallini 11-May-16 7:49am

If you use the static member in the Contact class then its value is shared among all the instances of such class. You need also a instance (non-static) member of the Contact class in order to store the actual ID of each class instance.

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Richard MacCutchan · Accepted Answer · 2016-05-11T00:30:00

Solution 2

You should use an auto-generated value within the database; see SQL AUTO INCREMENT a Field[^].

Posted 11-May-16 0:30am

Richard MacCutchan