What is the correct way to free memory in C

Question

I have a timer in C  which executes some code inside it s method  Inside the code I m using several temporary objects    If I have something like Foo o   new Foo    inside the method  does that mean that each time the timer ticks  I m creating a new object and a new reference to that object  If I have string foo   null and then I just put something temporal in foo  is it the same as above  Does the garbage collector ever delete the object and the reference or objects are continually created and stay in memory  If I just declare Foo o  and not point it to any instance  isn t that disposed when the method ends  If I want to ensure that everything is deleted  what is the best way of doing it    with the using statement inside the method by calling dispose method at the end by putting Foo o  outside the timer s method and just make the assignment o   new Foo   inside  so then the pointer to the object is deleted after the method ends  the garbage collector will delete the object

User · Answer

Objects are eligable for garbage collection once they ~~go out of scope~~ become unreachable (thanks ben!). The memory won't be freed unless the garbage collector believes you are running out of memory.

For managed resources, the garbage collector will know when this is, and you don't need to do anything.

For unmanaged resources (such as connections to databases or opened files) the garbage collector has no way of knowing how much memory they are consuming, and that is why you need to free them manually (using dispose, or much better still the using block)

If objects are not being freed, either you have plenty of memory left and there is no need, or you are maintaining a reference to them in your application, and therefore the garbage collector will not free them (in case you actually use this reference you maintained)

User · Answer

The  NET garbage collector takes care of all this for you   It is able to determine when objects are no longer referenced and will  eventually  free the memory that had been allocated to them

User · Answer

As Brian points out the GC can collect anything that is unreachable including objects that are still in scope and even while instance methods of those objects are still executing  consider the following code   class foo       static int liveFooInstances       public foo                 Interlocked Increment ref foo liveFooInstances              public void TestMethod                 Console WriteLine  entering method            while  Interlocked CompareExchange ref foo liveFooInstances  1  1     1                        Console WriteLine  running GC Collect                GC Collect                GC WaitForPendingFinalizers                      Console WriteLine  exiting method                foo                 Console WriteLine  in  foo            Interlocked Decrement ref foo liveFooInstances             class Program        static void Main string   args                foo aFoo   new foo            aFoo TestMethod              Console WriteLine aFoo ToString        if this line is uncommented TestMethod will never return           if run with a debug build  with the debugger attached  or with the specified line uncommented TestMethod will never return  But running without a debugger attached TestMethod will return

User · Answer

Let s answer your questions one by one    Yes  you make a new object whenever this statement is executed  however  it goes  out of scope  when you exit the method and it is eligible for garbage collection  Well this would be the same as  1  except that you ve used a string type   A string type is immutable and you get a new object every time you make an assignment  Yes the garbage collector collects the out of scope objects  unless you assign the object to a variable with a large scope such as class variable  Yes  The using statement only applies to objects that implement the IDisposable interface  If that is the case  by all means using is best for objects within a method s scope   Don t put Foo o at a larger scope unless you have a good reason to do so   It is best to limit the scope of any variable to the smallest scope that makes sense

User · Answer

1 If I have something like Foo o   new Foo    inside the method  does that   mean that each time the timer ticks    I m creating a new object and a new   reference to that object    Yes      2 If I have string foo   null and then I just put something temporal in foo    is it the same as above    If you are asking if the behavior is the same then yes      3 Does the garbage collector ever delete the object and the reference or   objects are continually created and   stay in memory    The memory used by those objects is most certainly collected after the references are deemed to be unused      4 If I just declare Foo o  and not point it to any instance  isn t that   disposed when the method ends    No  since no object was created then there is no object to collect  dispose is not the right word       5 If I want to ensure that everything is deleted  what is the best way of   doing it   If the object s class implements IDisposable then you certainly want to greedily call Dispose as soon as possible  The using keyword makes this easier because it calls Dispose automatically in an exception-safe way   Other than that there really is nothing else you need to do except to stop using the object  If the reference is a local variable then when it goes out of scope it will be eligible for collection 1 If it is a class level variable then you may need to assign null to it to make it eligible before the containing class is eligible     1This is technically incorrect  or at least a little misleading   An object can be eligible for collection long before it goes out of scope  The CLR is optimized to collect memory when it detects that a reference is no longer used  In extreme cases the CLR can collect an object even while one of its methods is still executing   Update   Here is an example that demonstrates that the GC will collect objects even though they may still be in-scope  You have to compile a Release build and run this outside of the debugger   static void Main string   args        Console WriteLine  Before allocation        var bo   new BigObject        Console WriteLine  After allocation        bo SomeMethod        Console ReadLine           The object is technically in-scope here which means it must still be rooted     private class BigObject       private byte   LotsOfMemory   new byte Int32 MaxValue   4        public BigObject                 Console WriteLine  BigObject                  BigObject                 Console WriteLine   BigObject                 public void SomeMethod                 Console WriteLine  Begin SomeMethod            GC Collect            GC WaitForPendingFinalizers            Console WriteLine  End SomeMethod              On my machine the finalizer is run while SomeMethod is still executing

User · Answer

Yes What do you mean by the same  It will be re-executed every time the method is run  Yes  the  Net garbage collector uses an algorithm that starts with any global in-scope variables  traverses them while following any reference it finds recursively  and deletes any object in memory deemed to be unreachable  see here for more detail on Garbage Collection Yes  the memory from all variables declared in a method is released when the method exits as they are all unreachable  In addition  any variables that are declared but never used will be optimized out by the compiler  so in reality your Foo variable will never ever take up memory  the using statement simply calls dispose on an IDisposable object when it exits  so this is equivalent to your second bullet point  Both will indicate that you are done with the object and tell the GC that you are ready to let go of it  Overwriting the only reference to the object will have a similar effect

User · Answer

Here s a quick overview    Once references are gone  your object will likely be garbage collected  You can only count on statistical collection that keeps your heap size normal provided all references to garbage are really gone  In other words  there is no guarantee a specific object will ever be garbage collected   It follows that your finalizer will also never be guaranteed to be called  Avoid finalizers   Two common sources of leaks   Event handlers and delegates are references  If you subscribe to an event of an object  you are referencing to it  If you have a delegate to an object s method  you are referencing it  Unmanaged resources  by definition  are not automatically collected  This is what the IDisposable pattern is for   Finally  if you want a reference that does not prevent the object from getting collected  look into WeakReference    One last thing  If you declare Foo foo  without assigning it you don t have to worry - nothing is leaked  If Foo is a reference type  nothing was created  If Foo is a value type  it is allocated on the stack and thus will automatically be cleaned up

User · Answer

The garbage collector will come around and clean up anything that no longer has references to it  Unless you have unmanaged resources inside Foo  calling Dispose or using a using statement on it won t really help you much   I m fairly sure this applies  since it was still in C   But  I took a game design course using XNA and we spent some time talking about the garbage collector for C   Garbage collecting is expensive  since you have to check if you have any references to the object you want to collect  So  the GC tries to put this off as long as possible  So  as long as you weren t running out of physical memory when your program went to 700MB  it might just be the GC being lazy and not worrying about it yet   But  if you just use Foo o outside the loop and create a o   new Foo   each time around  it should all work out fine

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