Programming Forums > Application Development > C++

aloksave · Sep 17th, 2005, 10:29 AM

suppose we have allocated dynamic memory by using new operator...can we use free to deallocte this memory..
working out with a example i think it works..but they say its not a good practice to do this,can anyone tell why?

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main()
{	
	int *k= new int;
	int *j = (int*)malloc(sizeof(int));
	*j=22;
	*k=21;
  	cout<<"value at j = "<<*j<<"\n";
  	cout<<"value at k = "<<*k<<"\n";
  	free(k);
  	free(j);
  	cout<<"value at k = "<<*k<<"\n";
  	cout<<"value at j = "<<*j<<"\n";
}

coldDeath · Sep 17th, 2005, 10:45 AM

I believe you would need to use the delete operator.

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delete k
delete j

DaWei · Sep 17th, 2005, 10:55 AM

Use delete with new and free with malloc. In a majority of the implementations, new uses malloc, and delete will use free. There are additional things that new does over and above the malloc that are undone by delete, but not by free.

coldDeath · Sep 17th, 2005, 11:06 AM

Is this statement correct?
malloc() and free() are C.
new and delete are C++.

stevengs · Sep 17th, 2005, 11:07 AM

Bjarne Stroustrup - "The C++ Programming Language"

Quote:

From C, C++ inherited a functional interface to dynamic memory. It can be found in <cstdlib>.

void* malloc(size_t, s);
void* calloc(size_t n, size_t s);
void free(void* p)
void* realloc( void* p, size_t s);

These functions should be avoided in favor of new, delete, and standard containers. In particular, free() does not invoke destructors for the memory it frees. An implementation of new and delete may use these functions, but there is no guarantee that it does. For example, allocating an object using new and deleting it using free() is asking for trouble.

coldDeath · Sep 17th, 2005, 11:09 AM

Thanks stevengs, that has cleared things up for me, and probably the OP aswell.

DaWei · Sep 17th, 2005, 11:45 AM

Here is a little MS C++ compiled program:

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int main (void)
{
   char *myArray = new char (25);

   return 0;
}

Here is what that line invokes:

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/***
*new.cxx - defines C++ new routine
*
*       Copyright (c) 1990-1998, Microsoft Corporation.  All rights reserved.
*
*Purpose:
*       Defines C++ new routine.
*
*******************************************************************************/


#include <cruntime.h>
#include <malloc.h>
#include <new.h>
#include <stdlib.h>
#ifdef WINHEAP
#include <winheap.h>
#else  /* WINHEAP */
#include <heap.h>
#endif  /* WINHEAP */

void * operator new( unsigned int cb )
{
    void *res = _nh_malloc( cb, 1 );

    return res;
}

"nh_malloc" is the "new handler malloc." It checks to see whether the ordinary or the debug heap allocation method is in order and makes the appropriate further calls.

aloksave · Sep 17th, 2005, 12:21 PM

Quote:

Originally Posted by stevengs

Bjarne Stroustrup - "The C++ Programming Language"

Thanks a lot...very true,free doesnt call the destructors...just checked out with the ex below...

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#include <iostream.h>
#include <iostream.h>
#include <malloc.h>
class Base
{
       public:
          Base(){ cout<<"Constructor: Base"<<endl;}
        virtual ~Base(){ cout<<"Destructor : Base"<<endl;}
};
class Derived: public Base
{
     //Doing a lot of jobs by extending the functionality
       public:
           Derived(){ cout<<"Constructor: Derived"<<endl;}
           ~Derived(){ cout<<"Destructor : Derived"<<endl;}
};
void main()
{
     Base *Var = new Derived;
     //delete Var;
     free (Var);
}

stevengs · Sep 17th, 2005, 12:38 PM

My quote was merely supporting David's (DaWei) statement, but your quite welcome.

aloksave · Sep 17th, 2005, 12:41 PM

Thanks to both of you

Sep 17th, 2005, 10:29 AM	#1
aloksave Programmer Join Date: Sep 2005 Posts: 33 Rep Power: 0	memory allocation and deallocation suppose we have allocated dynamic memory by using new operator...can we use free to deallocte this memory.. working out with a example i think it works..but they say its not a good practice to do this,can anyone tell why? (Toggle Plain Text) main() { int k= new int; int j = (int)malloc(sizeof(int)); j=22; k=21; cout<<"value at j = "<<j<<"\n"; cout<<"value at k = "<<k<<"\n"; free(k); free(j); cout<<"value at k = "<<k<<"\n"; cout<<"value at j = "<<j<<"\n"; } main() { int k= new int; int j = (int)malloc(sizeof(int)); j=22; k=21; cout<<"value at j = "<<j<<"\n"; cout<<"value at k = "<<k<<"\n"; free(k); free(j); cout<<"value at k = "<<k<<"\n"; cout<<"value at j = "<<j<<"\n"; } Last edited by big_k105; Sep 17th, 2005 at 10:31 AM. Reason: Added Code Tags

Sep 17th, 2005, 10:45 AM	#2
coldDeath Expert Programmer Join Date: Aug 2005 Location: UK Posts: 862 Rep Power: 4	I believe you would need to use the delete operator. (Toggle Plain Text) delete k delete j delete k delete j __________________ Programming Tutorials - Linux Installation Guide - Learn Python - Learn C++ Got Slack? Join us at #programmingforums @ irc.freenode.net! My software never has bugs. It just develops random features.

Sep 17th, 2005, 10:55 AM	#3
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	Use delete with new and free with malloc. In a majority of the implementations, new uses malloc, and delete will use free. There are additional things that new does over and above the malloc that are undone by delete, but not by free. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Sep 17th, 2005, 11:06 AM	#4
coldDeath Expert Programmer Join Date: Aug 2005 Location: UK Posts: 862 Rep Power: 4	Is this statement correct? malloc() and free() are C. new and delete are C++. __________________ Programming Tutorials - Linux Installation Guide - Learn Python - Learn C++ Got Slack? Join us at #programmingforums @ irc.freenode.net! My software never has bugs. It just develops random features.

Sep 17th, 2005, 11:09 AM	#6
coldDeath Expert Programmer Join Date: Aug 2005 Location: UK Posts: 862 Rep Power: 4	Thanks stevengs, that has cleared things up for me, and probably the OP aswell. __________________ Programming Tutorials - Linux Installation Guide - Learn Python - Learn C++ Got Slack? Join us at #programmingforums @ irc.freenode.net! My software never has bugs. It just develops random features.

Sep 17th, 2005, 11:45 AM	#7
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	Here is a little MS C++ compiled program: (Toggle Plain Text) int main (void) { char myArray = new char (25); return 0; } int main (void) { char myArray = new char (25); return 0; } Here is what that line invokes: (Toggle Plain Text) /*** new.cxx - defines C++ new routine * Copyright (c) 1990-1998, Microsoft Corporation. All rights reserved. * Purpose: Defines C++ new routine. * ******************************************************************************/ #include <cruntime.h> #include <malloc.h> #include <new.h> #include <stdlib.h> #ifdef WINHEAP #include <winheap.h> #else / WINHEAP / #include <heap.h> #endif / WINHEAP / void operator new( unsigned int cb ) { void res = _nh_malloc( cb, 1 ); return res; } /** new.cxx - defines C++ new routine * Copyright (c) 1990-1998, Microsoft Corporation. All rights reserved. * Purpose: Defines C++ new routine. * ******************************************************************************/ #include <cruntime.h> #include <malloc.h> #include <new.h> #include <stdlib.h> #ifdef WINHEAP #include <winheap.h> #else / WINHEAP / #include <heap.h> #endif / WINHEAP / void operator new( unsigned int cb ) { void res = _nh_malloc( cb, 1 ); return res; } "nh_malloc" is the "new handler malloc." It checks to see whether the ordinary or the debug heap allocation method is in order and makes the appropriate further calls. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers*

Sep 17th, 2005, 12:38 PM	#9
stevengs Professional Programmer Join Date: May 2005 Location: Bad Nauheim, Germany Posts: 436 Rep Power: 4	My quote was merely supporting David's (DaWei) statement, but your quite welcome. __________________ -Steven "Is this a piece of your brain?" - Basil Fawlty

Sep 17th, 2005, 12:41 PM	#10
aloksave Programmer Join Date: Sep 2005 Posts: 33 Rep Power: 0	Thanks to both of you

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Programming Forums > Application Development > C++

memory allocation and deallocation