Re: Better way of allocating a memory( auto_ptr vs new vs malloc)

James Kanze <>
Sun, 7 Dec 2008 01:48:12 -0800 (PST)
On Dec 7, 10:34 am, Erik Wikstr=F6m <> wrote:

On 2008-12-06 23:36, blargg wrote:

Ami wrote:
My god that is so hard to read. Look at the difference a
couple of find-and-replaces and some indention do:

And it lets us find some things to improve upon.

I have few qustions
Consider the allocation

  T p(T());

  T p(); will do just as well.

Not at local scope (which IIRC was the case). At local scope,
that declares a function.

T() in above would do default initialization right
then I can safely avoid the use of C style memset..
and then nevere have to de allocate the same.

As far as auto_ptr is concrned , do I again need to
initialize to T () as shown below

  auto_ptr<T>p = (new T()) ;

No need to use () around the new, auto_ptr<T>p = new T();
works just as fine.

It shouldn't. At least according to C++98. (I don't have a
copy of the C++03 standard here.) There's no assignment
operator of auto_ptr which takes a T*, and there's no implicit

another major concerned abt using auto_ptr is

after the memory allocation is done tht is lets say

  int main ()
      auto_ptr<T>p = (new T()) ;
      func(p.get()); //
      return ( EXIT_SUCESS);

  void func(T *output)
    // some code

do u see any problem with uisng an auto pointer like above
i.e passing as an argument in a function call .

No problem at all, but you might want to consider using a
shared pointer and pass that instead of using get(). Or pass a
reference to the auto ptr.

Or pass a reference to the object, using *p in the calling
function. (Unless the called function is capable of handling a
null pointer, this is the preferred solution.)

Is there any memory Leak ?

No, but there is always the risk that func() will somehow save
the pointer you pass in and someone else will later try to
access it.

I think the auto of auto ptr is the best way ..
Also in the code the malloc is done like

  T *p = (T *)malloc (sizeof(T)*(n+1));

I'm not sure how it is in C++, but in C you do not have to
cast the result of malloc.

In C++, you do.

so using new for above is like

  T *p = (T *)new char [sizeof(T)*(n+1)];

  T* p = new T[n+1];

Just a reminder, too. You can't use std::auto_ptr with array
new. In C++, the preferred solution for this would be:

    std::vector< char > obj( sizeof(T)*(n+1) ) ;

(Except that I suspect that what he wants is:

    std::vector< T > obj( n + 1 ) ;


James Kanze (GABI Software)
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