gcnew requires Delete?

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4.00/5 (1 vote)

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This question always remains in mind as I got mixed answers from Google.

And also,
How would I delete if my variable is a return type?

For example, say I have a C++/Cli function which returns string and also has reference type string parameter. I have to call this function from C#

String^ GetStrings( [Out] String^ %outputstring)
{
    String^ result = gcnew(/*  C++ function that returns char*  */);
    outputstring = gcnew(/*  C++ function that returns char*  */);
    return result;
}

Will this cause a memory leak?
How would I go deleting these string object that I created with gcnew after I return these in C#?

Thanks in advance

Posted 9-Jun-11 8:19am

manumith

Updated 9-Jun-11 8:53am

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Sergey Alexandrovich Kryukov · Accepted Answer · 2011-06-09T09:05:00

No, never. This is .NET managed memory allocation, the object are destructed and memory reclaimed by the Garbage Collector (GC).

A memory leak is quite possible but not because you forgot to delete anything in a managed space. (However, a leak of unmanaged memory and resources is quite possible; clean-up is is usually placed in Dispose methods (but can be anywhere else); if you forget to dispose, there will be a leak.)

Managed memory is never reclaimed or disposed by the application. Here is how it works: when you Application Domain looses a reference to an object, the object is marked as a subject to destruction. However, it does not happen immediately; exact moment of time is determined by internal GC algorithm. By that reason, any code depending on the time and order of calling destructors is invalid. By the same reason, destructors are used quite rarely. CG mechanism is intellectual enough. For example, it handles cyclic associations: if object A references object B, object B references object C and object C references object A, all this structure is destructed anyway if currently running code cannot access all three objects.

—SA

CPallini · Accepted Answer · 2011-06-09T09:28:00

Solution 2

As SAKryukov correctly explained you must NOT explicitely delete what you allocated with gcnew. However the 'C++ function that returns char *' MIGHT require the deletion of the memory pointed by its return value. For instance, consider:

C++

char * str;
str = new char[3];
sprintf(str, "hi");
String ^ result = gcnew (p);
delete [] str;

in the above code the delete [] str; statement is required (otherwise... memory leaks!), while you MUST NOT delete result, of course.

Posted 9-Jun-11 9:28am

CPallini

Comments

Lutosław 9-Jun-11 16:01pm

It is worth to add that deleting 'str' will not spoil 'result', since the String's constructor copyies all characters from char*.

manumith 9-Jun-11 17:14pm

Sure! I understand. I realize I have given a bad example and it should be done as Pallini did. Infact I am doing like that in my code. I was worried about deleting the String^ result and outputstring. So, from all your replies I see that GC will take care of the deleting managed objects even though they are defined in C++/CLI space.

Thanks much every one!

Sergey Alexandrovich Kryukov 9-Jun-11 18:25pm

Good point, my 5. I failed to mention about what you demonstrated here -- mixed-mode C++/CLI + C++ (unmanaged) mode.
--SA