Re: new and delete

From:

"Victor Bazarov" <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Thu, 22 Jun 2006 14:15:59 -0400

Message-ID:

<e7emp2$1ll$1@news.datemas.de>

junw2000@gmail.com wrote:

Below is a small code about memory allocation and deallocation.

#include <cstdlib>
#include <iostream>
using namespace std;

class X {
public:
void* operator new(size_t sz) throw (const char*) {
   void* p = malloc(sz); //LINE1
   if (p == 0) throw "malloc() failed";
   return p;
}

void operator delete(void* p) {
   free(p);
}

};

class Y {
int filler[100];
public:

// two arguments
void operator delete(void* p, size_t sz) throw (const char*) {
   cout << "Freeing " << sz << " byte(s)" << endl; //LINE2
   free(p);
};

};

int main() {
X* ptr = new X; //LINE3

// call X::operator delete(void*)
delete ptr;

Y* yptr = new Y;

// call Y::operator delete(void*, size_t)
// with size of Y as second argument
delete yptr; //LINE4
}

My questions are:
When LINE3 is executed, LINE1 is executed. How is the variable sz
assigned sizeof(X)?

Compiler magic.

The output of the code is:
Freeing 400 byte(s)
When LINE4 is executed, LINE2 is executed. How does sz get the value
400?

Compiler magic.

The 'operator new' and 'operator delete' are called by the executing
environment when you use the "new expression". There are intervening
mechanisms that know how much to pass to *your* 'new'.

Put a breakpoint in your 'operator new' and examine the call stack
under the debugger, when the execution is suspended. YOu will see
something between the 'main' and 'operator new', most likely. What
it is, isn't defined in the language. But it will be there.

You might find "Inside the C++ Object Model" by Lippman worth a look.

V
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