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Calling a constructor

Started by Newfound Ajarn Mar 8, 2004 at 6:49 PM 9 replies 1.2k views

Original Post

Mar 8, 2004 at 6:49 PM

Well, I've looked all over Google and Gamedev's search page and I still havent found a way to call a class constructor. I'm making amemory manager and I have it call new for an element, and when it's done it calls the destructor for that class. So, then I have the memory manager give out that memory the next time it needs a new class object. All is fine, but I cant call the class constructor. I'll give some examples of what I'm talking about.

template< typename T >
class MmoryManager
{
    T* New()
    {
        // Check to see if there is some "free" memory

        if( bFreeMemory )
        {
            // This is what I can't do

            //    pData is a pointer to the "free" deconstructed object

            pData->T(); // or pData->T::T();

        }
        else
        {
            return new T; // also keep track of this creation, but thats internal stuff

        }
    }

    void Delete( T *ppDel )
    {
        ppDel->~T(); // or ppDel->T::~T();

        // Some internal tracking of the memory for later use

    }
}

I have tried making a local copy of T and memcpy the data into the "free" object, but something seems to be going wrong with my code, and I can't figure out what that is. So, the main question: Is there a way to call the constructor for a class, or a alt for what I'm trying to do? [edited by - newfound ajarn on March 8, 2004 7:53:19 PM]

Oluseyi

Mar 8, 2004 at 6:52 PM

pData = new T();
..
delete pData;

Doc

Mar 8, 2004 at 6:58 PM

If you already have some memory pool that you wish the data to be constructed in there is a way to tell the new operator exactly where you want to put the object. Something like new (pData) T(); (Someone will correct me if I''m wrong.) I believe its then your resposibility to make sure the memory is indeed already allocated. I''m not too sure - someone help elaborate?

Newfound Ajarn

Mar 8, 2004 at 6:58 PM

That would work well (because that's the normal way of doing it), but I'm trying to make it so I only have to call new and delete once and reuse the memory I have by calling the constructor and destructor manually.

Doc: I would love to do it that way, but I don't know how to do that either. If someone could tell me how to do that it would solve all my problems.

[edited by - newfound ajarn on March 8, 2004 8:00:03 PM]

Oluseyi

Mar 8, 2004 at 7:00 PM

quote:
Original post by Newfound Ajarn
That would work well (because that''s the normal way of doing it), but I''m trying to make it so I only have to call new and delete once and reuse the memory I have by calling the constructor and destructor manually.

Google "placement new".

SiCrane

Mar 8, 2004 at 7:01 PM

It''s called "placement new". Example:

void * void_ptr = malloc(sizeof(T));
T * t_ptr = new (ptr) T();

To delete:

t_ptr->~T();
free(void_ptr);

Newfound Ajarn

Mar 8, 2004 at 7:08 PM

Thank you all! That''s what I was looking for.

iMalc

Mar 9, 2004 at 2:56 AM

Oh man, why didn''t someone tell me about this when I was writing virtually the same thing some time ago!

iMalc

Mar 9, 2004 at 2:58 AM

Oh btw does this work when the class has no constructor or destructor?

DrPizza

Mar 9, 2004 at 4:19 AM

One cannot have a class with no c-tor or d-tor. The c-tor or d-tor might be implicit, and they might be inaccessible, but they will always be there.

Unless you mean a class where the c-tor or d-tor are declared but not actually defined. No, that wouldn''t work, it should generate a linker error.

Newfound Ajarn

Mar 9, 2004 at 7:29 AM

My thing works with any type (int, float, cClassName), the typename T solves it (if there is not destructor, the compiler takes care of it). If you want to see my finally working version just ask here and I''ll provide a link (later today, because of stupid school).

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