Re: Overloading new[]

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maynard wrote: 
I'm attempting to overload the new[] operator.  This is my first
attempt at doing so, but I've found quite a few articles to help me
out.  Here is the source for my overloaded operator new[]:

class myClass
{
public:
     myClass();
     ~myClass();
     void *operator new[](size_t, long);
protected:
     double *v1;
     double *v2;
     long num_pts;
};

myClass::myClass()
{ //ctor... }

myClass::~myClass()
{ //dtor... }

void *myClass::operator new[](size_t sz, long numpts)
{


Have you seen what 'sz' is before you scale it down?

sz /= sizeof(myClass); 

I am not sure why but you actually lose a chunk of your array here.

myClass *x = ::new myClass[sz]; 

I don't think you're supposed to do that here.  You're supposed to
allocate a char array of size 'sz' (without dividing it) and then just
return it.


	for(size_t j = 0; j < sz; j++){
		x[j].v1 = new double[numpts];
		x[j].v2 = new double[numpts];
		for (long i=0;i<numpts;++i){
			x[j].v1[i] = 0.;
			x[j].v2[i] = 0.;
		}
		x[j].num_pts = numpts;
	}
	return x;
}

I pass the "message" as follows:

          int numObjs = 100;
          long totalPts = 100000;
          myClass* myObj = new(totalPts) myClass[numObjs];

When I step through this in the debugger (MS VC++ .NET), it appears
that the local "x" (in operator new[]) is properly allocated and
initialized.  However, when it returns, myObj isn't.  I also noticed in
the debugger that the memory location for "x" is 0x00321714, but myObj
(after new[] returns) is 0x00321718.

With that said, two questions...have I properly defined & implemented
my new[] operator?  Is there a problem in the debugger??


I don't think you properly implemented it.  Check out the following page
http://www.codeguru.com/cpp/tic/tic0139.shtml

The system calls "new operator" that figures out the size (your 'sz') and
then calls "operator new[]", after which it takes the returned pointer and apparently adjusts it. You're not supposed to fiddle with the array
before "new operator" gives it back to your 'myObj'.

I understand *why* you wanted to do what you did.  You just tried doing it
at the wrong time...

V
.