auto_ptr pointing to a dynamic array

Question

In my code, I am allocating an integer array using new. After that I am wrapping this pointer to an auto_ptr. I know that the auto_ptr call its destructor automatically. Since my auto_ptr is pointing to an array (allocated using new), Will the array get deleted along with auto_ptr or will it cause a memory leak. Here is my sample code.

std::auto_ptr<int> pointer;

void function()
{
  int *array = new int[2];
  array[0] = 10;
  array[1] = 20;

  pointer.reset((int*) array);
}

int _tmain(int argc, _TCHAR* argv[])
{

    function();
return 0;
}

Use std::vector.

James McNellis
– James McNellis

2013-07-17 05:38:31 +00:00
Commented Jul 17, 2013 at 5:38 — James McNellis
– James McNellis, Commented Jul 17, 2013 at 5:38

Jerry Coffin · Accepted Answer · 2013-07-17 05:42:00Z

9

The array will not be deleted correctly. auto_ptr uses delete contained_item;. For an array it would need to use delete [] contained_item; instead. The result is undefined behavior.

As James McNellis said, you really want std::vector here -- no new, no auto_ptr and no worries.

answered Jul 17, 2013 at 5:42

Jerry Coffin

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Comments

fatihk · Accepted Answer · 2013-07-17 05:51:30Z

7

You can not use std::auto_ptr to handle dynamic arrays, because it can not know how to differentiate between delete and delete[].

Moreover, auto_ptr is deprecated, in C++11 you can use std::unique_ptr with:

int *array = new int[2];
std::unique_ptr<int[]> pointer(array);

edited Jul 17, 2013 at 5:51

answered Jul 17, 2013 at 5:42

fatihk

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5 Comments

juanchopanza Over a year ago

You would need to instantiate it with a special deleter, otherwise it will just call delete.

fatihk Over a year ago

@juanchopanza, in msdn.microsoft.com/en-us/library/vstudio/ee410601.aspx it is stated that a partial specialization unique_ptr<Type[]>manages array objects allocated with new[], and has the default deleter default_delete<Type[]>, specialized to call delete[] _Ptr

Jagannath Over a year ago

@juanchopanza The template specialization will take care of it. No need of custom deleter.

juanchopanza Over a year ago

Perfect, that is something I didn't know!

ogni42 Over a year ago

I prefer std::unique_ptr<int[]> pointer(new int[2]); as it avoids copying the pointer. Note, that with C++14 there will be make_unique() as we have make_shared() already today. This will provide strong exception safety when allocation unique pointers

madnut · Accepted Answer · 2013-07-17 05:45:18Z

1

As others said auto_ptr is the wrong thing to use, std::vector is best. But you may also use boost::scoped_array. But note that you want to reset at time of creation otherwise you might as well use delete. pointer.reset(new int[2]);

or better yet like this boost::scoped_array arr(new int[2]);

Also there is no point to to create a static std::auto_ptr pointer; in global scope. This means it will get deleted only when the program exists, that happens anyway even if you leak memory.

answered Jul 17, 2013 at 5:45

madnut

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2 Comments

Potatoswatter Over a year ago

scoped_array does not appear to do anything that unique_ptr doesn't.

madnut Over a year ago

yep its just there in older vc's if you use boost.

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