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Posted 28 Mar 2004


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To new is C++; To malloc is C; To mix them is sin

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29 Mar 20043 min read
Article explains the differences between malloc/free and new/delete in a C++ context


One of the most common questions that get asked during interviews for C++ programmers is to explain the differences between using malloc and using new. It's also a fairly common question in some of newsgroups and C++ forums. This article will try and explain as simply as possible how malloc and new are two entities that are essentially non-interchangeable, and there is nothing is this article that you wouldn't find in most decent C++ programming books; but the article tries to put all the information together in a single place using simple code snippets and is targeted at newbies who might be unfamiliar as to the differences.

Constructors and Destructors

When you new an object, space for the object is not only allocated but the object's constructor is called. And similarly when you

an object, the object's destructor is called before the memory is released. If you use malloc and free, the destructor and constructor do not get called respectively and obviously, this simply won't do in C++ except in certain very rare situations where you have classes without any specific destructor/constructors.

It's very easy to test this out by using the following test class.

class  Test
        cout  <<  "Test  :  ctor\r\n";
        cout  <<  "Test  :  dtor\r\n";
    void Hello()
        cout  <<  "Test  :  Hello  World\r\n";

Create, use and delete the object using new/delete as well as using malloc/free :-

int  _tmain(int  argc,  _TCHAR*  argv[])
  cout  <<  "1\r\n";
  Test*  t1  =  new  Test();
  delete  t1;

  cout  <<  "2\r\n";
  Test*  t2  =  (Test*)  malloc(sizeof  Test);

  return  0;

You'll see the following output:-

Test : ctor
Test : Hello World
Test : dtor
Test : Hello World

As obvious from the output, malloc/free did not result in either the destructor or the constructor being called.

Choosing constructor overloads

For non-array allocations, you can actually specify the specific overload of the constructor that you wish to use as in :- T t = new T(x, y, z); For array allocations using new, the default constructor will get used. If you attempt an array allocation on an object that does not have a default constructor, you get a compiler error :-

class Test2
  Test2(int y)


Test2* t2array = new Test2[10];

For example, if you attempt to compile the above snippet, you'll get an error C2512: 'Test2' : no appropriate default constructor available with the VC++ 7.1 compiler.

Type-casting forced by malloc

Because malloc returns a void* the caller has to do a type-cast to get it to compile.

Test* t1 = new Test();

is so much easier to code and is a lot more readable than

Test* t2 = (Test*) malloc(sizeof Test);

Native types

For native types new/delete and malloc/free work the same way except for the need to type-cast in the case of malloc/free. So it's just a matter of user preference.

//declaring native type

int* i1 = new int;
delete i1;

int* i2 = (int*) malloc(sizeof(int));

//declaring native type array

char** c1 = new char*[10];
delete[] c1;

char** c2 = (char**) malloc(sizeof(char)*10);

Safety tip

Always delete what you new, and free what you malloc, never mix new with free or malloc with delete.

The reason for this is that if you do that, then the behavior is technically undefined because there is no guarantee that new would internally use malloc, or that delete would internally use


Tip for scalar and vector new/delete

Thanks to Mike Dunn for referring me to Raymond Chen's blog on this issue. The basic point is that you should not mix scalar new with vector delete and vice versa.

Test* t = new Test[3]; 
delete t; // <-- This is very bad

The above code will result in a memory leak as only the first

object is deleted.

Test* t = new Test; 
delete[] t; // <-- This is even worse

Similarly, the above code snippet is just as bad and probably worse by a good deal. The vector delete will try to delete more objects depending on the random value it gets from its object-count location and will obviously result in heap corruption.

No realloc alternative for new/delete

The new/delete couple not have a realloc alternative that is available when you use the malloc/free pair. realloc is pretty handy if you want to resize the length of an array or a memory block dynamically, specially since the contents of the array/block remain same up to the shorter of the old and new sizes. To see this in action, see the following code snippet :-

char* p = (char*)malloc(sizeof(char)*12);
strcpy(p,"hello world");
cout << p << "\r\n";
p = (char*)realloc(p, sizeof(char)*24);
strcat(p," from Nish");
cout << p << "\r\n";

The output you get will be :-

hello world
hello world from Nish

As you can see from the output, the original contents were retained. Thanks to Minox for reminding me of the realloc issue.


Please feel free to send in further information on new/delete/malloc/free so that I can enhance the article.



  • Mar 29, 2004 - First published
  • Mar 30, 2004 - Updated
    • Added stuff on scalar/vector mixing with new/delete
    • Added stuff on realloc


This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here


About the Author

Nish Nishant
United States United States
Nish Nishant is a Principal Software Architect based out of Columbus, Ohio. He has over 17 years of software industry experience in various roles including Lead Software Architect, Principal Software Engineer, and Product Manager. Nish was a Microsoft Visual C++ MVP between 2002 and 2015.

Nish is an industry acknowledged expert in the Microsoft technology stack. He authored C++/CLI in Action for Manning Publications in 2005, and had previously co-authored Extending MFC Applications with the .NET Framework for Addison Wesley in 2003. In addition, he has over 140 published technology articles on and another 250+ blog articles on his WordPress blog. Nish is vastly experienced in team management, mentoring teams, and directing all stages of software development.

Contact Nish : If you are interested in hiring Nish as a consultant, you can reach him via his google email id voidnish.

Company Website :

Comments and Discussions

GeneralMy vote of 5 Pin
Manikandan1028-Jun-14 4:13
professionalManikandan1028-Jun-14 4:13 
GeneralMy vote of 1 Pin
My2Cents16-May-09 1:08
MemberMy2Cents16-May-09 1:08 
Questionnew and default constructor? Pin
seskapill18-Dec-08 1:33
Memberseskapill18-Dec-08 1:33 
AnswerRe: new and default constructor? Pin
Arne Mertz17-Jun-13 22:04
MemberArne Mertz17-Jun-13 22:04 
Questionwhat!? Pin
pdxmusl9-Oct-08 18:06
Memberpdxmusl9-Oct-08 18:06 
AnswerRe: what!? Pin
ChrisMcB6-Nov-09 9:15
MemberChrisMcB6-Nov-09 9:15 
Questiondoubt Pin
vikasetrx9-Jul-07 0:22
Membervikasetrx9-Jul-07 0:22 
Generalone more prominent difference Pin
pasisathe15-May-07 16:59
Memberpasisathe15-May-07 16:59 
GeneralRe: one more prominent difference Pin
Slothy16-Oct-07 2:10
MemberSlothy16-Oct-07 2:10 
Generalone more difference Pin
pasisathe15-May-07 16:55
Memberpasisathe15-May-07 16:55 
QuestionBetter way? Pin
Amro Ibrahim24-Apr-07 0:09
MemberAmro Ibrahim24-Apr-07 0:09 
AnswerRe: Better way? Pin
Nish Nishant24-Apr-07 3:55
sitebuilderNish Nishant24-Apr-07 3:55 
GeneralRe: Better way? Pin
Amro Ibrahim24-Apr-07 4:28
MemberAmro Ibrahim24-Apr-07 4:28 
GeneralMSMQ Pin
Kirubanandam7-Jun-05 18:37
MemberKirubanandam7-Jun-05 18:37 
GeneralNew/Delete Scope Pin
godbert14-Jan-05 6:32
Membergodbert14-Jan-05 6:32 
GeneralRe: New/Delete Scope Pin
jan larsen25-May-05 1:04
Memberjan larsen25-May-05 1:04 
GeneralRe: New/Delete Scope Pin
Anand Vivek Srivastava17-Aug-06 9:48
MemberAnand Vivek Srivastava17-Aug-06 9:48 
GeneralStrdup - Duplicating/Freeing strings using New Pin
Member 62427827-Apr-04 22:14
MemberMember 62427827-Apr-04 22:14 
GeneralRe: Strdup - Duplicating/Freeing strings using New Pin
Anonymous27-Apr-04 22:36
MemberAnonymous27-Apr-04 22:36 
GeneralNon-default Heaps Pin
Mike Dimmick8-Apr-04 9:26
MemberMike Dimmick8-Apr-04 9:26 
QuestionIs a mistake? Pin
wycolor6-Apr-04 15:11
Memberwycolor6-Apr-04 15:11 
AnswerRe: Is a mistake? Pin
badecas24-Jun-09 3:19
Memberbadecas24-Jun-09 3:19 
Generaltell the right way of coding Pin
Karthik Murugan30-Mar-04 17:26
MemberKarthik Murugan30-Mar-04 17:26 
GeneralRe: tell the right way of coding Pin
Nish Nishant30-Mar-04 17:42
sitebuilderNish Nishant30-Mar-04 17:42 
GeneralRe: tell the right way of coding Pin
Christian Graus30-Mar-04 18:13
protectorChristian Graus30-Mar-04 18:13 

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