Re: let baseclass know size of object

From:

Bart van Ingen Schenau <Bart.van.Ingen.Schenau@ict.nl>

Newsgroups:

comp.lang.c++

Date:

Fri, 14 Nov 2008 03:01:57 -0800 (PST)

Message-ID:

<b32da9ed-9d6e-44ab-be28-a3f4470fec1a@o40g2000prn.googlegroups.com>

On 13 nov, 16:50, ".rhavin grobert" <cl...@yahoo.de> wrote:

guess you have a bouquet of paddingless structs (and your compiler
already cares for that) that all have one in common: their first
memeber has to be their size. As fas as i know (am i right?) a
baseclass get's in it's ctor the _baseclass-size_ when doing a sizeof
(this), so you cant fill that member in ctor automatically. Lets say
you want the derived-class way, e.g.:

<snip>

is there a (perhaps templates) way to
say:

"if the struct is derived from 'base', one member of the ctors init-
list has to be a 'base(sizeof(this))' " ...?

If the restriction that it only works for dynamically-allocated
structures is not a problem, you could define a class-specific
operator new() to collect this information.
Something like this:

<pseudocode>
class Base {
private:
  static std::size_t lastSize;
public:
  static void* operator new(std::size_t size)
  {
    lastSize = size;
    return ::operator new(size);
  }
public:
  Base() : mSize(lastSize)
  {
    lastSize = 0; /* reset to avoid undetectable bogus results */
  }
private:
  unsigned int mSize;
};
</pseudocode>

Bart v Ingen Schenau