New subject: [Skunkworks] Size of an object in memory at runtime

28 Nov 2011

      I see.  You are right that sizeof won't work in this case since you need 
to get the size of the instance itself plus all the dynamically 
allocated objects (strings and pointers).  Sizeof can give the size of 
an object but of the object and all the objects it points too.  There is 
no simple way to do that in C++.  You would have to write code to take 
the sizeof the object itself and then add all the sizes of the objects 
that it points and the sizes of the objects that they point too etc...

But even if you could do that, it still wouldn't help with the problem 
you have.  The other objects that your object points too (strings, char 
pointers...) will not be in the same contiguous memory block as the 
object itself.  So even if you copy your object onto the clipboard and 
allocate enough space for the other objects, the other objects won't get 
copied onto the clipboard.  Here is an example of what I mean:

Say you have your drag and drop object DDObj that has a pointer to a 
Child object:

class Child
{
     int i;
}

class DDObj
{
    Child* c;
}

When you create the objects, each one gets created with a call to new:

DDObj dd = new DDObj();
dd.c = new Child();  // this might actually happen inside DDObj constructor

So instance dd might be at memory location 100 but since dd.c was 
created with a different call to new it could be at totally different 
memory location - 200 or 1000 or 10000 - anywhere.  So even though you 
know that the size of DDObj plus the size of c is say 16 bytes they 
aren't 16 bytes next to each other.  You probably have 8 bytes in one 
place and the other 8 bytes in another.  So if you try to copy 16 bytes 
onto the clipboard starting at the location of dd you will end picking 
some other values for dd.c.  Even if you did get lucky and the objects 
were lined up in memory when the other application at the other end of 
the clipboard got your object the pointer to dd.c would probably no 
longer be valid since the object would likely have been remapped to a 
different virtual address space.

To do this using the in memory representation of your objects you would 
have to either restrict the objects on the clipboard to simple value 
types and fixed size arrays (no std::string, no char*) or you would have 
to manage the memory yourself using some kind of variant of a mempool.

If I were you I would look for a library that you can use to 
marshal/serialize your objects.  Not sure if there is anything like for 
use with wxWidgets.  Using a JSON or XML representation of the objects 
is one way to serialize them although you may be able to find something 
slightly more efficient for an in memory representation.  In any case, 
it isn't something you want to code yourself as remapping the pointers 
is fairly tricky.

Josh

On 28/11/11 1:19 PM, skunkworks-request@lists.my.co.ke wrote:
...
Message: 3
Date: Mon, 28 Nov 2011 13:01:49 +0300
From: James Nzomo<kazikubwa@gmail.com>
To: Skunkworks Mailing List<skunkworks@lists.my.co.ke>
Subject: Re: [Skunkworks] Size of an object in memory at runtime
Message-ID:
  <CAChuKucuy6DM5ofjp-uoOdjG-=AQjZkFwnSNZE3sohOrUrHpkA@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
I'm using wxWidgets. It's clipboard mechanism extends the underlying
platform's clipboard mechanism.
The object is of a custom class.
This class contains various datatypes ranging from
ints,doubles,std::strings,char pointers to custom typedefs, structs and
other classes.
The objects i want to D&D are of different types. I will have a clipboard
data object for each type as opposed to having an abstract implementation
which is more complex.

Size of an object in memory at runtime

Josh Handley

James Nzomo

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