Resource Management - Files

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08-14-2010
darren78

Resource Management - Files

Hi,

I'm just reading up on resource management and RAII (Resource Acquisition Is Initialisation) and it got me thinking regarding files.

If say I have a program that uses different file names. Would it be a good idea to create a file_handle class that simply takes a file name as an argument and the file is opened in the constructor and closed in the destructor?

I can then use objects of this type either as member objects of other classes or within functions, that way when the objects go out of scope the file is released. Would this be something that works in real world code?

Thanks.
08-14-2010
Elysia

Yes, but the class you're talking about already exists: ifstream and ofstream.
08-14-2010
darren78

aah okay. So ifstream and ofstream open the files and then close them when they go out of scope?

I have used them previously, I didn't realise they automatically closed the files though.

Thanks,
08-14-2010
laserlight

Yes.
08-14-2010
Elysia

Quote:

Originally Posted by darren78

aah okay. So ifstream and ofstream open the files and then close them when they go out of scope?

I have used them previously, I didn't realise they automatically closed the files though.

Thanks,

I don't know (or can't think) of any containers in the STL that does not clean up after itself. For files, that means closing them.
08-14-2010
darren78

Cheers LL. What sort of resources do I need to be looking at within constructors/destructors then? I know of memory allocated on the heap. As a beginner, are there any others that I should be aware of?

Thanks again.
08-14-2010
Elysia

STL pretty much takes care of everything.
For memory, there is unique_ptr and shared_ptr. For other resources, there is unique_ptr and shared_ptr with custom deleters.
08-14-2010
laserlight

Quote:

Originally Posted by darren78

What sort of resources do I need to be looking at within constructors/destructors then? I know of memory allocated on the heap.

Database and prepared statement handles are another example, e.g., I have written my own RAII wrappers for the SQLite C interface (and there are other C++ wrappers out there with the same idea).
08-14-2010
Elysia

You sure shared_ptr with custom deleter won't cut it?
08-14-2010
darren78

So when acquiring additional memory, if I use the smart pointers I'm not restricted to allocating and deallocating memory in constructors/destructors? I can just use the smart pointers inside a scope and when it goes out of scope the memory is released?
08-14-2010
Elysia

Yes, they are RAII and what you say is the purpose of RAII.
08-14-2010
darren78

Quote:

Originally Posted by laserlight

Database and prepared statement handles are another example, e.g., I have written my own RAII wrappers for the SQLite C interface (and there are other C++ wrappers out there with the same idea).

I did something with C# and SQL for a university project a few months ago and was a little unsure regarding database opening/closing.

Should I open the connection, do whatever needs to be done immediately and then close immediately, or is it acceptable to open the connection at the beginning of a programs execution and leave it open until the program closes?

The program I wrote was for an e-commerce website and when the customer browsed the products, the DB was queried and products displayed, information retrieved etc on stock levels, descriptions etc?

Should the DB be opened and closed with each query or is this too much overhead?

Thanks.
08-14-2010
laserlight

Quote:

Originally Posted by Elysia

You sure shared_ptr with custom deleter won't cut it?

That depends on whether RAII is the only thing that you are trying to cut :)

Quote:

Originally Posted by darren78

So when acquiring additional memory, if I use the smart pointers I'm not restricted to allocating and deallocating memory in constructors/destructors?

The idea is that if you use tools that provide you with RAII functionality, then you do not need to implement RAII yourself.

Quote:

Originally Posted by darren78

I can just use the smart pointers inside a scope and when it goes out of scope the memory is released?

Yes. This is the basic idea of RAII: the associate the lifetime of a resource with the lifetime of an object.

Quote:

Originally Posted by darren78

Should the DB be opened and closed with each query or is this too much overhead?

It depends. If you are waiting for user input, then there may be little point in maintaining the database connection, unless there is only one user. But if you are going to execute a series of queries, opening and closing the database connection for each of them is obviously silly.
08-14-2010
darren78

Quote:

Originally Posted by Elysia

Yes, they are RAII and what you say is the purpose of RAII.

Okay, cheers. The book i'm using only really touches briefly on auto_ptr and advocates the use of constructors/destructors for aquiring resources, etc.

It's good to know that I am not limited to that.
08-14-2010
darren78

Quote:

Originally Posted by laserlight

That depends on whether RAII is the only thing that you are trying to cut :)

The idea is that if you use tools that provide you with RAII functionality, then you do not need to implement RAII yourself.

Yes. This is the basic idea of RAII: the associate the lifetime of a resource with the lifetime of an object.

Thanks again. It's becoming a little clearer now!
08-14-2010
Elysia

Quote:

Originally Posted by laserlight

That depends on whether RAII is the only thing that you are trying to cut :)

Yeah, but you did say you wrote your own RAII wrappers.

Quote:

Originally Posted by darren78

Okay, cheers. The book i'm using only really touches briefly on auto_ptr and advocates the use of constructors/destructors for aquiring resources, etc.

Ah, unless you don't have access to C++0x, don't use auto_ptr. It's dangerous and deprecated. In C++0x and forward, you should use unique_ptr instead.
08-14-2010
laserlight

Quote:

Originally Posted by Elysia

Yeah, but you did say you wrote your own RAII wrappers.

Yes, and RAII is an important part. But the wrapper is another important part, i.e., a further level of abstraction.
08-14-2010
Elysia

True that, but was it worth time investigating in? It hardly seems like a thing a newbie would need.
08-14-2010
laserlight

Quote:

Originally Posted by Elysia

True that, but was it worth time investigating in? It hardly seems like a thing a newbie would need.

Custom deleters are hardly things that a newbie would need. Is it worth time investigating them? In my opinion, this depends on the path of learning that the newbie wishes to take. Seeing that darren78 started thinking about files, thinking about databases is along similiar lines.
08-14-2010
Elysia

But then it becomes a question of what is better - custom RAII wrappers or just custom deleters? Since custom deleters usually provide less work, they're more newbie friendly and integrates well with the STL.