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Local NEW or Automatic heap cleanup when leaving scope

, 29 Sep 2012
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Improve exception safety and reducin memory leaks
This is an old version of the currently published article.

Introduction    


Pointers always envied automatic cleanup that is in c++ provided only for big fat slow Static Object or their arrays stored by value.  

Manual cleanup is source of bugs. 

Automatic cleanup provided for static objects saves time lines of code and a lot and I mean really lot of bugs. Bugs by forgetting to match all allocations with deallocations. 
Bugs by memory being leaked when exception or error handler exits something somewhere prematurely and not all control paths contain correct number of release statements. 
Bugs by releasing objects twice or more and corrupting memory by confused condition statements.

Performance  

So we definitely want automatic cleanup that is provided for static objects. But pretty much any program serious about performance will store objects by pointers and not by value because price for reallocation insert or sort is horrendous. 

So can't we have both? Performance and efficiency of pointers and safety and simplicity of static objects by automatic cleanup when leaving scope?

Reinventing Wheel  

Turns out that I am not the first one thinking along those lines and Boost template library already provides us ptr_vector.  vector of shared_ptr or shared_array.
Templates are great when everything is working. But take this example of guy having trouble sorting simple array of pointers and geting responses like

"...The IS specifies that a predicate should not assume that the dereferenced iterator yields a non-const object. Arguments should therefore be taken by reference-to-const or by value.

The IS also specifies that algorithms that take function objects as arguments are permitted to copy those function objects. So in most cases, the overloaded function call operator would be a const member function." 

It it unbelivable what complexity we vere able introduce to "simplify our life"
So if you are like me preffering things small and simple so you know by simple look at source what is going on.
Or just don't plan or can't rewrite tousands of lines of code to use templates stl and boost
Let's try simpler approach.  

Scopes for dynamic heap objects ?  

So how automatic cleanup for static objects work? 
It works by adding pointer of every static object within scope to internal invisible static array and when leaving calling destructors for each of them. Can we make something similar for dynamic objects?
Why not. We can have array of void pointers and store pointers to various object types in one single array.
The only issue are calling proper object specific destructors. Unfortunately as far as I know c++ doesnt allow to get destructor adresses. So the only sollution I come up so far is to use virtual destructor in all stored objects so polymorphysm selects proper one for us. 

First Primitive implementation of Scope.h 

struct Scope { // This is Just simple linked list
       struct Obj { virtual ~Obj() {} };
       Scope*    prev;  Obj* ptr;
       Scope() : prev(0) ,   ptr(0) {}	
      ~Scope() { delete ptr; delete prev; } // deleting all childs in reverse order on exit
};

inline void * __cdecl operator new(unsigned int size,Scope& scope)
{
    void *ptr = (void *)malloc(size); // we add all new objects to this linked list
    if(scope.ptr) { Scope* prev=0; prev=(Scope*)calloc(sizeof(Scope),1); *prev=scope;
    scope.prev=prev; }
    scope.ptr=(Scope::Obj*)ptr; 
    return(ptr);
};

This very simple nothrow new overload just as proof of concept;
Also Scope is very primitive linked list kept simple on purpose. 
 
Beware: only objects having virtual destructor work right now for this scoped new. 

Example code 

#include <stdio.h>
#include <stdlib.h>
#include "Scope.h"   

struct A {  // only objects with virtual destructor are right now supported by Scope
             A(){ printf("\n  A %x",this); }
    virtual ~A(){ printf("\n ~A %x",this); }
};

Scope global; // this one deallocates all associated objects on program exit no matter 
              // what exception or error in program happens

void Test() {
    Scope local; // all objects associated with scope will be deallocated by leaving procedure
    for(int i=0;i<3;i++) {
        A* a=new(local) A();
    }
    A* b=new(global) A(); //associate with global scope
}

void main() {
    Test();
}                 

Output:

   A 689718   A 68b110   A 68b198   A 68b220 ~A 68b198 ~A 68b110 ~A 689718 ~A 68b220

Points of Interest 

The fact that only objects with virtual destructor can be autodeallocated is kinda sad. I hope that some way to get around this limit is found.
Also if you want to autodeallocate memory from malloc/calloc just create new type of Scope and call free instead of delete in its destructor. 

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Author

Ladislav Nevery
Software Developer (Senior)
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Comments and Discussions


Discussions posted for the Published version of this article. Posting a message here will take you to the publicly available article in order to continue your conversation in public.
 
QuestionDidn't know there is a new operator in C PinmemberStan Mihai18-Jul-13 4:41 
QuestionWhy not std::unique_ptr<T>? [modified] PinmemberC++ Pirate Programmer18-Oct-12 5:46 
AnswerRe: Why not std::unique_ptr? [modified] PinmemberLadislav Nevery18-Oct-12 16:02 
QuestionThanks PinmemberYannAchard18-Oct-12 1:29 
AnswerRe: Thanks PinmemberLadislav Nevery18-Oct-12 13:00 
GeneralMy vote of 2 PinmemberSeattleC++15-Oct-12 9:53 
GeneralRe: My vote of 2 PinmemberLadislav Nevery16-Oct-12 9:56 
GeneralRe: My vote of 2 PinmemberSeattleC++16-Oct-12 11:24 
GeneralRe: My vote of 2 [modified] PinmemberLadislav Nevery17-Oct-12 5:44 
GeneralRe: My vote of 2 PinmemberSeattleC++17-Oct-12 10:29 
GeneralRe: My vote of 2 [modified] PinmemberLadislav Nevery17-Oct-12 23:06 
QuestionWhat's about an exception handling? Pinmemberkonik1015-Oct-12 6:05 
AnswerRe: What's about an exception handling? [modified] PinmemberLadislav Nevery16-Oct-12 1:47 
GeneralRe: What's about an exception handling? [modified] Pinmemberkonik1023-Oct-12 21:24 
GeneralRe: What's about an exception handling? PinmemberLadislav Nevery24-Oct-12 22:58 
GeneralRe: What's about an exception handling? Pinmemberkonik1025-Oct-12 1:20 
GeneralRe: What's about an exception handling? PinmemberLadislav Nevery25-Oct-12 4:41 
GeneralRe: What's about an exception handling? [modified] Pinmemberkonik1025-Oct-12 5:43 
GeneralRe: What's about an exception handling? PinmemberLadislav Nevery25-Oct-12 12:50 
QuestionException / Thread safety? PinmemberPetro Protsyk8-Oct-12 1:20 
AnswerRe: Exception / Thread safety? PinmemberLadislav Nevery8-Oct-12 7:54 
GeneralMy vote of 2 PinmemberPatrick Niedzielski7-Oct-12 5:38 
GeneralRe: My vote of 2 [modified] PinmemberLadislav Nevery7-Oct-12 10:26 
GeneralRe: My vote of 2 PinmemberPatrick Niedzielski7-Oct-12 10:30 
GeneralMy vote of 5 Pinmembernv33-Oct-12 22:03 

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