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Problem with Returning Pointer to local variable

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Join Date: Jul 2006

Posts: 14

Reputation: ALAZHARY is an unknown quantity at this point

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ALAZHARY

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Problem with Returning Pointer to local variable

Nov 25th, 2008

Hello, everybody.
first, look at the following code:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <iostream>
 
using std::cout;
using std::endl;
using std::cin;
4
double* treble(double);
6
int main(void)
{
 double num = 5.0;
double* ptr = 0;
 ptr = treble(num);
 cout << endl
      << "Three times num = " << 3.0*num << endl;
 cout << "Result = " << *ptr;
 cout << endl;
 return 0;
}
18
double* treble(double data)
{
 double result = 0.0;
 result = 3.0*data;
 return &result;
}
#include <iostream>

using std::cout;
using std::endl;
using std::cin;
4
double* treble(double);
6
int main(void)
{
 double num = 5.0;
double* ptr = 0;
 ptr = treble(num);
 cout << endl
      << "Three times num = " << 3.0*num << endl;
 cout << "Result = " << *ptr;
 cout << endl;
 return 0;
}
18
double* treble(double data)
{
 double result = 0.0;
 result = 3.0*data;
 return &result;
}

the output from this program is something like:

Three times num = 15
Result = 4.10416e-230

I know that the returned pointer of the function treble is point to local variable so when I returned, the value which inside that address has been lost.
there is no problem till here,
but let us to change the lines 13,14 to become:

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 C++ Syntax (Toggle Plain Text) 
13        << "Three times num = " << 3.0*num << endl
14        << "Result = " << *ptr;
13        << "Three times num = " << 3.0*num << endl
14        << "Result = " << *ptr;

the output will become:

Three times num = 15
Result = 15

why?

I'd give my right arm to Improve my english

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Join Date: Jul 2006

Posts: 1,091

Reputation: MattEvans is a jewel in the rough

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MattEvans

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Re: Problem with Returning Pointer to local variable

Nov 25th, 2008

The behaviour when dereferencing an invalid pointer is undefined. So I can only speculate on the underlying reason: it probably has to do with how and where stack variables are allocated, e.g. sequential function calls within a given scope might allocate stack variables from the same starting place, so stuff that's stack-allocated in the first call to cout << is likely to overwrite anything that was stack allocated on the prior call to treble. When it's done all in one call to cout <<, the value is fetched back before calling any other function, and thus before it's likely for it to have been overwritten, and it appears to work ok.

You can occasionaly predict how the thing is going to behave, e.g., I was pretty sure that this would happen:

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double num = 5.0;
double* ptr = 0;
ptr = treble(num);
treble( 2.0 );
cout << endl << "Three times num = " << 3.0*num << endl << "Result = " << *ptr;
double num = 5.0;
double* ptr = 0;
ptr = treble(num);
treble( 2.0 );
cout << endl << "Three times num = " << 3.0*num << endl << "Result = " << *ptr;

Outputs:
Three times num = 15
Result = 6

On my machine, but I wouldn't bet on it being the same as yours, and I'd never rely on it working like that on mine.

A clearer example:

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#include <iostream>
 
int fn_a ( void )
{
  int a = 0;
  std::cerr << &a << std::endl;
}
 
int fn_b ( void )
{
  int b = 0;
  std::cerr << &b << std::endl;
}
 
int main ( void )
{
  fn_a ( );
  fn_b ( );
 
  return EXIT_SUCCESS;
}
#include <iostream>

int fn_a ( void )
{
  int a = 0;
  std::cerr << &a << std::endl;
}

int fn_b ( void )
{
  int b = 0;
  std::cerr << &b << std::endl;
}

int main ( void )
{
  fn_a ( );
  fn_b ( );

  return EXIT_SUCCESS;
}

Outputs:
0xbff36764
0xbff36764

E.g. both function calls allocated a temporary variable in the same place, on my machine, today. (emphasis: "don't ever rely on this")

The important question, why are you doing this anyway?

 Help with Code Tags 
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 double treble(double data)
 {
  double result = 0.0;
   result = 3.0*data;
  return result;
}
 double treble(double data)
 {
  double result = 0.0;
   result = 3.0*data;
  return result;
}

Is much more normal and safe.

Plato forgot the nullahedron..

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Join Date: Nov 2008

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Reputation: minas1 is an unknown quantity at this point

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minas1

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Junior Poster in Training

Re: Problem with Returning Pointer to local variable

Nov 25th, 2008

result is a local variable, which is destroyed when the functions ends. So a garbage value is destroyed. The best solution is to do what is said by the poster above.

If you need to work with pointers you can do this:

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 C++ Syntax (Toggle Plain Text) 
double* treble(double data)
{
	double result = 0.0;
	result = 3.0*data;
	return new result;
}
double* treble(double data)
{
	double result = 0.0;
	result = 3.0*data;
	return new result;
}

now result isn't destroyed and a pointer to it is returned. However, this memory must be freed manually.

delete ptr;

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ArkM

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Postaholic

Re: Problem with Returning Pointer to local variable

Nov 25th, 2008

•

Originally Posted by minas1

result is a local variable, which is destroyed when the functions ends. So a garbage value is destroyed. The best solution is to do what is said by the poster above.

If you need to work with pointers you can do this:
Help with Code Tags

C++ Syntax (Toggle Plain Text)
double* treble(double data)
{
	double result = 0.0;
	result = 3.0*data;
	return new result;
}
double* treble(double data) { double result = 0.0; result = 3.0*data; return new result; }
now result isn't destroyed and a pointer to it is returned. However, this memory must be freed manually.

delete ptr;

No errors means even number of errors

Incorrect example and wrong explanation (result always "destroyed": it's local automatic variable). Right "solution":
[code=c++]
double* treble(double data)
{
return new double(3.0*data);
}
Never, ever do that. It's totally senseless and a very dangerous programming style.

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Join Date: Jul 2006

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ALAZHARY

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Re: Problem with Returning Pointer to local variable

Nov 26th, 2008

thank you minas1, but I think you need to read carefully what is the problem?

I'd give my right arm to Improve my english

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niek_e

Online

Roasting Maven

Re: Problem with Returning Pointer to local variable

Nov 26th, 2008

•

Originally Posted by ALAZHARY

thank you minas1, but I think you need to read carefully what is the problem?

Actually: YOU need to read the replies carefully. MattEvans already told you what the problem is:

•

Originally Posted by MattEvans

The behaviour when dereferencing an invalid pointer is undefined.

So just use the code posted by him and your problem will go away:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
 double treble(double data)
 {
  double result = 0.0;
   result = 3.0*data;
  return result;
}
 double treble(double data)
 {
  double result = 0.0;
   result = 3.0*data;
  return result;
}

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Join Date: Jul 2006

Posts: 14

Reputation: ALAZHARY is an unknown quantity at this point

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ALAZHARY

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Newbie Poster

Re: Problem with Returning Pointer to local variable

Nov 26th, 2008

I understood MattEvans well, but he speculate, and minas1 didn't understand my question

I'd give my right arm to Improve my english

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Join Date: Jul 2006

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MattEvans

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Re: Problem with Returning Pointer to local variable

Nov 26th, 2008

That "the behaviour when dereferencing an invalid pointer is undefined" is fact.

Other than that, the best you'll get (in terms of an explanation for why you get this output) is speculation or some implementation-specific reason. On my implementation (GCC) reasonable evidence (see second code I posted) suggests that local variables in any function get allocated from the same starting address (assuming the calls come from the same level).

(EDIT: and that would be sufficient explanation for your output, providing std::ostream::operator<< allocates at least one local variable)

There's no requirement for "reclaimed" memory to be zeroed or otherwise restored (by reclaimed, I mean memory under deleted pointers or under auto variables that go out of scope). The most efficient implementation strategy is to just leave the contents of such memory untouched, hence, before reclaimed memory is overwritten, it will tend to contain its old value. (You shouldn't rely on this, either)

Last edited by MattEvans; Nov 26th, 2008 at 7:46 am.

Plato forgot the nullahedron..