How can I destroy an object, but not free the memory?

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Hello people,

I am writing a memory pool application and I have the below problem:

When I start, the application is allocating (by using malloc) a big amount of memory. Then, a list of char pointers point to different places of the allocated memory. When I call allocate, one of these pointers is returned.

The problem is, when I call deallocate, I only want to explicitly call the destructor of the object and then make the memory block available again. In other words, I do not want to free the memory.

So my deallocate looks like that:

C++

void Pool:: Deallocate(void* pObj)
{
.. Call object destructor here ..

m_ListOfSlices.push_back((char*)pObj);
m_AllocLeft += 1;
}

How can I call the destructor of the object that pObj points to?

Keep in mind that I do not really want a template solution.

There must be a way, since operator delete, is actually doing the same thing! I tried to debug the operator delete, but I had no luck.

Please help!

Revised:::
Thanks very much for your answers,

To answer answer 1, I am afraid you can actually do that. You can actually do pMem->~A(); and then call delete pMem;

To question answer2, I have read the parashift tutorials long ago, the problem is that they do not apply to my situation at all ;) . I am trying to do a different thing. Thanks very much for the reply though :)

Posted 10-Feb-10 7:34am

Andreas Tsiapis

Updated 11-Feb-10 6:54am

Sean Ewington

v3

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3 solutions

Solution 3

Why don't you simply provide a clear() method in your object that takes care of cleaning your object ?

By the way, your design of the pool doesn't look very C++ to me: first you are calling malloc (which is pure C) and then you are manipulating void pointers and casting them to char pointers... It really seems to be a mess.
In general, when you want to have a pool, it is mainly used to store objects of the same type. So in that case there's no need for all these ugly stuff you are doing.

What are your requirements in fact ? What are you trying to do precisely ?

Posted 10-Feb-10 20:06pm

Cedric Moonen

Solution 2

You may call it, but you shouldn't. See here [^] for details.
:)

Posted 10-Feb-10 8:37am

CPallini

v2

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Emilio Garavaglia · Accepted Answer · 2010-02-11T01:33:00

Lets go for steps:

Allocate without contruct: just use malloc, or cast the return of a new char array.
```
A* pA;
pA = (A*)new char[sizeof(A)];
```
Construct what is rawly allocate: use placement new:
```
new(pA)A(...);
```
Destroy without deallocate: explicitly call the destructor:
```
pA->~A();
```
Deallocate what has already been destroyed: use free or free the char array as you allocated it
```
delete[] (char*)pA; pA = 0;
```
Allocate and construct: just use new
```
pA = new A(...);
```
Destroy and deallocate: just use delete
```
delete pA; pA = 0;
```

This is what the STL implementation does inside std::allocator.