Memory leak in Double pointer

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4.00/5 (1 vote)

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Hello
I am using a structure like

C++

struct abc
{
	int		x;		
	int		y;
};

struct abcArray
{
	int iCount;
	abc **test;
};

Further Use this like(memory allocation):

C++

abcArray xyz;
xyz.test = new abc*;
for(int i = 0 ; i < 10 ; i++)
   xyz.test[i] = new xyz;

For deallocation:

C++

for(int i = 0 ; i < 10 ; i++)
{
   delete[] xyz.test[i];
   xyz.test[i] = NULL;
}

When I check for memory leaks,it says leak at

C++

xyz.test = new abc*;

Can anybody suggest me how I should deallocate to avoid this leak?

Posted 30-Jun-11 22:53pm

NipunG

Updated 30-Jun-11 22:57pm

Wild-Programmer

v2

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Richard MacCutchan 2-Jul-11 4:11am

From AndreFratelli:
Since you are already using structs, you could also go old school and implement a linked list yourself. STL is probably a better option, though.

3 solutions

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Richard MacCutchan · Answer 1 · 2011-06-30T23:14:00

Solution 1

abcArray xyz;
xyz.test = new abc*;
for(int i = 0 ; i < 10 ; i++)
   xyz.test[i] = new xyz;

You have allocated a pointer to a single abc structure and then proceeded to access the following 9 cells which do not belong to you. You should allocate like:

xyz.test = new abc*[10];

Posted 30-Jun-11 23:14pm

Richard MacCutchan

Comments

NipunG 1-Jul-11 5:25am

But I am not sure how many cells need to be allocated...It is to be allocated as and when required.I used 10 just for reference.Randomly I have to allocate cell one by one.
If I do new abc*[10]; way, the I always need to create a temporary array to add new set of data.

Richard MacCutchan 1-Jul-11 5:48am

If you don't know how many cells to allocate then you cannot do it with an array, as C++ arrays are fixed. Take a look at the STL collection classes and use one that allows this feature; the vector class would probably do it.

NipunG 1-Jul-11 10:03am

I found out the reason for memory leak and that is not related to this piece of code which I have posted.But I want to understand what is the problem with this piece of code.

Richard MacCutchan 1-Jul-11 12:10pm

I have explained what is wrong with your code; you allocate a single pointer and then use it as if you had allocated a 10 element array of pointers. You must either allocate the number you need at the start, or use a STL class that can grow dynamically.

AndreFratelli 2-Jul-11 1:45am

~~Since you are already using structs, you could also go old school and implement a linked list yourself. STL is probably a better option, though.~~

Moved this comment so OP gets it.

AndreFratelli 2-Jul-11 15:34pm

Sorry, what? =\

Richard MacCutchan 3-Jul-11 3:03am

You replied to me, rather than the OP.

AndreFratelli 3-Jul-11 13:20pm

Oh, Ok. But why strike?

(did I just double post? I'm still figuring out some details on this interface =P)

Richard MacCutchan 3-Jul-11 15:27pm

I moved your comment to where the OP would see it. So by striking out the original I show that it has been moved, but you (and others) can see the original so they know what I'm referring to.

AndreFratelli 4-Jul-11 11:36am

Oh.. =\ I get it.

Albert Holguin · Answer 2 · 2011-07-01T03:47:00

Solution 2

What Richard said is correct... but there's a variety of solutions available:
CArray (MFC)[^]
vector (STL)[^]
Linked list (there's several implementations out there)[^]

Posted 1-Jul-11 3:47am

Albert Holguin

Stefan_Lang · Answer 3 · 2011-07-04T02:06:00

First, you allocate xyz.test[i] using new, but then you deallocate it using delete[]. The two don't go together! Use delete on singular allocated objects and delete[] only on allocated arrays!

Also, you allocate xyz.test, but fail to deallocate it - this is causing the leak.

You said:
But I am not sure how many cells need to be allocated...It is to be allocated as and when required.I used 10 just for reference.Randomly I have to allocate cell one by one.

This means you can not store your structs in a simple C array. Instead you need a linked list. You should follow the last link posted by Albert Holguin if you don't want to implement your own.

If you are programming in C only (no C++ constructs, no templates), then check out this article about an Implementation of a single linked list in C. It has some very good explanations.