Using _set_new_handler

In some circumstances, corrective action can be taken during memory allocation and the request can be fulfilled. To gain control when the global operator new function fails, use the _set_new_handler function (defined in NEW.H) as follows:

#include <stdio.h>
#include <new.h>

// Define a function to be called if new fails to allocate memory.
int MyNewHandler( size_t size )
    clog << "Allocation failed. Coalescing heap." << endl;

    // Call a fictitious function to recover some heap space.
    return CoalesceHeap();

void main()
    // Set the failure handler for new to be MyNewHandler.
    _set_new_handler( MyNewHandler );

    int *pi = new int[BIG_NUMBER];

In the preceding example, the first statement in main sets the new handler to MyNewHandler. The second statement tries to allocate a large block of memory using the new operator. When the allocation fails, control is transferred to MyNewHandler. The argument passed to MyNewHandler is the number of bytes requested. The value returned from MyNewHandler is a flag indicating whether allocation should be retried: a nonzero value indicates that allocation should be retried, and a zero value indicates that allocation has failed.

MyNewHandler prints a warning message and takes corrective action. If MyNewHandler returns a nonzero value, the new operator retries the allocation. When MyNewHandler returns a 0 the new operator stops trying and returns a zero value to the program.

The _set_new_handler function returns the address of the previous new handler. Therefore, if a new handler needs to be installed for a short time, the previous new handler can be reinstalled using code such as the following:

#include <new.h>


_PNH old_handler = _set_new_handler( MyNewHandler );

// Code that requires MyNewHandler.

// Reinstall previous new handler.
_set_new_handler( old_handler );

A call to _set_new_handler with an argument of 0 causes the new handler to be removed. There is no default new handler.

The new handler you specify can have any name, but it must be a function returning type int (nonzero indicates the new handler succeeded, and zero indicates that it failed).

If a user-defined operator new is provided, the new handler functions are not automatically called on failure.

The prototype for _set_new_handler and the type _PNH is defined in NEW.H:

_PNH _set_new_handler( _PNH );

The type _PNH is a pointer to a function that returns type int and takes a single argument of type size_t.