When you create a new-expression, two things occur. First, storage is allocated using the operator new( ), then the constructor is called. In a delete-expression, the destructor is called, then storage is deallocated using the operator delete( ). The constructor and destructor calls are never under your control (otherwise you might accidentally subvert them), but you can change the storage allocation functions operator new( ) and operator delete( ).

The memory allocation system used by new and delete is designed for general-purpose use. In special situations, however, it doesn’t serve your needs. The most common reason to change the allocator is efficiency: You might be creating and destroying so many objects of a particular class that it has become a speed bottleneck. C++ allows you to overload new and delete to implement your own storage allocation scheme, so you can handle problems like this.

Another issue is heap fragmentation. By allocating objects of different sizes it’s possible to break up the heap so that you effectively run out of storage. That is, the storage might be available, but because of fragmentation no piece is big enough to satisfy your needs. By creating your own allocator for a particular class, you can ensure this never happens.

In embedded and real-time systems, a program may have to run for a very long time with restricted resources. Such a system may also require that memory allocation always take the same amount of time, and there’s no allowance for heap exhaustion or fragmentation. A custom memory allocator is the solution; otherwise, programmers will avoid using new and delete altogether in such cases and miss out on a valuable C++ asset.

When you overload operator new( ) and operator delete( ), it’s important to remember that you’re changing only the way raw storage is allocated. The compiler will simply call your new instead of the default version to allocate storage, then call the constructor for that storage. So, although the compiler allocates storage and calls the constructor when it sees new, all you can change when you overload new is the storage allocation portion. (delete has a similar limitation.)

When you overload operator new( ), you also replace the behavior when it runs out of memory, so you must decide what to do in your operator new( ): return zero, write a loop to call the new-handler and retry allocation, or (typically) throw a bad_alloc exception (discussed in Volume 2, available at www.BruceEckel.com).

Overloading new and delete is like overloading any other operator. However, you have a choice of overloading the global allocator or using a different allocator for a particular class.