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Thinking in C++
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Running out of storage

What happens when the operator new( ) cannot find a contiguous block of storage large enough to hold the desired object? A special function called the new-handler is called. Or rather, a pointer to a function is checked, and if the pointer is nonzero, then the function it points to is called.

The default behavior for the new-handler is to throw an exception, a subject covered in Volume 2. However, if you’re using heap allocation in your program, it’s wise to at least replace the new-handler with a message that says you’ve run out of memory and then aborts the program. That way, during debugging, you’ll have a clue about what happened. For the final program you’ll want to use more robust recovery.

You replace the new-handler by including new.h and then calling set_new_handler( ) with the address of the function you want installed:

//: C13:NewHandler.cpp
// Changing the new-handler
#include <iostream>
#include <cstdlib>
#include <new>
using namespace std;

int count = 0;

void out_of_memory() {
  cerr << "memory exhausted after " << count 
    << " allocations!" << endl;

int main() {
  while(1) {
    new int[1000]; // Exhausts memory
} ///:~

The new-handler function must take no arguments and have a void return value. The while loop will keep allocating int objects (and throwing away their return addresses) until the free store is exhausted. At the very next call to new, no storage can be allocated, so the new-handler will be called.

The behavior of the new-handler is tied to operator new( ), so if you overload operator new( ) (covered in the next section) the new-handler will not be called by default. If you still want the new-handler to be called you’ll have to write the code to do so inside your overloaded operator new( ).

Of course, you can write more sophisticated new-handlers, even one to try to reclaim memory (commonly known as a garbage collector). This is not a job for the novice programmer.

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire