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Constructor call with malloc?

Started by skyfire360 Mar 16, 2005 at 6:30 PM 9 replies 7.8k views

Original Post

Mar 16, 2005 at 6:30 PM

I've been debugging my engine off and on all night when I made an odd observation: when you malloc something, the constructor is never called! For example:


//creates a new instance of Octtree and calls the default constructor
Octtree *tree = new Octtree() 

//Allocates memory for an Octtree but no constructor call
Octtree *tree = (Octtree *) malloc(sizeof(Octtree))

//Allocates memory and sets all pointers to null, but still doesn't call default constructor
Octtree *tree = (Octtree *) malloc(sizeof(Octtree))
ZeroMemory(tree, sizeof(Octtree))

Seeing as I'm a die-hard C programmer and shudder to think of anyone but me messing around with my memory allocation, is there any way to call the default/any constructor after mallocing? Or is it necessary to use the new keyword?

Evil Steve

Mar 16, 2005 at 6:33 PM

That's right. malloc() never calls the constructor, it just allocates raw chunks of memory. To call the constructor, you need to use new or malloc and placement new.

Agony

Mar 16, 2005 at 6:43 PM

Might I recommend merely using new, rather than a malloc()/placement new combination? Because new isn't managing memory any more significantly than malloc() is, unless you think that an automatic call to the constructor is an evil thing to perform without your consent. (Have fun dealing with uglier code, though; I hope I never have to mess with it. [wink])

Palidine

Mar 16, 2005 at 6:49 PM

new is just malloc with a constructor call. I believe that it also sets up the vtables and whatnot for all your polymorphic needs. I see no reason to not use it other than a desire for pain coupled with a fear of change. =)

-me

Oluseyi

Mar 16, 2005 at 6:52 PM

Quote:
Original post by skyfire360
Seeing as I'm a die-hard C programmer and...

Blah blah blah. Program in C if you're a C programmer. Otherwise, get with C++. Stop resisting the language.

smart_idiot

Mar 16, 2005 at 8:28 PM

I think this is a pretty bad idea, but here's an example of how you could do it.

#include <iostream>#include <string>#include <new>#include <cstdlib>int main() {  using namespace std;    // Get some memory with malloc.  void *memory = malloc(sizeof(string));    // If we didn't get the memory, then it's really time that you upgraded.  if(!memory) return -1;     // Create a string object in that memory.  string *string_pointer = new(memory) string("Hello World!");    // Use that string object for something.  cout << *string_pointer << endl;    // Call its destructor.  string_pointer->~string();    // Free the memory.  free(memory);      cin.get();  return 0; }

While we're on the subject, always release your memory with whatever you used to get it, i.e.:

malloc/realloc/calloc --> free new --> delete new [] --> delete [] operator new --> operator delete

If you match up the wrong ones your computer will implode and create a minature black hole that will swallow you! Don't fool around with these things!

Yohumbus

Mar 16, 2005 at 11:55 PM

Well seeing as you dont trust the compiler to allocate memory for you with its new operator, and you have to use new of some kind to construct the allocated object, Why dont you make your own new. The awsome thing about C++ is that(among other things) you can overload most operators. I would suggest reading up on the operator new and operator delete functions and their array equivelents because they give you all the controll you want. These operators can be defined at the global level and they can be static functions for a class if that class should be allocated differently.

JohnBolton

Mar 17, 2005 at 12:36 PM

Quote:
Original post by skyfire360
Seeing as I'm a die-hard C programmer...

Seeing as you are a die-hard C programmer, you shouldn't be using constructors in the first place.

Qw3r7yU10p!

Mar 17, 2005 at 1:09 PM

Usually if people are doing this (zeroing memory) I assume it is because they are just avoiding writing constructors which initialise values to zero.

This is just plain lazy and when you get into using virtual functions or member data which isn't builtin types, it will cause you big problems.

Let C++ do what it's supposed to do. Its type system brings all kinds of benefits. Don't try and subvert it.

Trust the compiler, but look at the generated code if you have any doubts.

If there's something special you need to do such as using a memory pool to speed things up then look at using a library such as boost or Loki to look after it for you. I simply added Loki's small object allocator (you derive from it) and it vastly improved the performance of my application.

Yohumbus

Mar 17, 2005 at 3:29 PM

AP, I dont meen for him to change the behavior of new and delete to something other than allocating memory. If he wants to ensure that malloc is used when new is used all he has to do is overload new and delete to allocate with malloc even though that's what the default behavoir should already be. There are no maintenence costs with overloading new and delete, that's why they exist and the best way to get with the language would be to use its more powerfull features (operator overloading and the like) to make his code easier to use.

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