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:

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.

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.

This is the drastic approach, when the global versions of new and delete are unsatisfactory for the whole system. If you overload the global versions, you make the defaults completely inaccessible – you can’t even call them from inside your redefinitions.

The overloaded new must take an argument of size_t (the Standard C standard type for sizes). This argument is generated and passed to you by the compiler and is the size of the object you’re responsible for allocating. You must return a pointer either to an object of that size (or bigger, if you have some reason to do so), or to zero if you can’t find the memory (in which case the constructor is not called!). However, if you can’t find the memory, you should probably do something more informative than just returning zero, like calling the new-handler or throwing an exception, to signal that there’s a problem.

The return value of operator new is a void*, not a pointer to any particular type. All you’ve done is produce memory, not a finished object – that doesn’t happen until the constructor is called, an act the compiler guarantees and which is out of your control.

The operator delete takes a void* to memory that was allocated by operator new. It’s a void* because operator delete only gets the pointer after the destructor is called, which removes the object-ness from the piece of storage. The return type is void.

Here’s a simple example showing how to overload the global new and delete:

Here you can see the general form for overloading new and delete. These use the Standard C library functions malloc( ) and free( ) for the allocators (which is probably what the default new and delete use as well!). However, they also print messages about what they are doing. Notice that printf( ) and puts( ) are used rather than iostreams. This is because when an iostream object is created (like the global cin, cout, and cerr), it calls new to allocate memory. With printf( ), you don’t get into a deadlock because it doesn’t call new to initialize itself.

In main( ), objects of built-in types are created to prove that the overloaded new and delete are also called in that case. Then a single object of type S is created, followed by an array of S. For the array, you’ll see from the number of bytes requested that extra memory is allocated to store information (inside the array) about the number of objects it holds. In all cases, the global overloaded versions of new and delete are used.

Although you don’t have to explicitly say static, when you overload new and delete for a class, you’re creating static member functions. As before, the syntax is the same as overloading any other operator. When the compiler sees you use new to create an object of your class, it chooses the member operator new over the global version. However, the global versions of new and delete are used for all other types of objects (unless they have their own new and delete).

In the following example, a primitive storage allocation system is created for the class Framis. A chunk of memory is set aside in the static data area at program start-up, and that memory is used to allocate space for objects of type Framis. To determine which blocks have been allocated, a simple array of bytes is used, one byte for each block:

The pool of memory for the Framis heap is created by allocating an array of bytes large enough to hold psize Framis objects. The allocation map is psize elements long, so there’s one bool for every block. All the values in the allocation map are initialized to false using the aggregate initialization trick of setting the first element so the compiler automatically initializes all the rest to their normal default value (which is false, in the case of bool).

The local operator new has the same syntax as the global one. All it does is search through the allocation map looking for a false value, then sets that location to true to indicate it’s been allocated and returns the address of the corresponding memory block. If it can’t find any memory, it issues a message to the trace file and throws a bad_alloc exception.

This is the first example of exceptions that you’ve seen in this book. Since detailed discussion of exceptions is delayed until Volume 2, this is a very simple use of them. In operator new there are two artifacts of exception handling. First, the function argument list is followed by throw(bad_alloc), which tells the compiler and the reader that this function may throw an exception of type bad_alloc. Second, if there’s no more memory the function actually does throw the exception in the statement throw bad_alloc. When an exception is thrown, the function stops executing and control is passed to an exception handler, which is expressed as a catch clause.

In main( ), you see the other part of the picture, which is the try-catch clause. The try block is surrounded by braces and contains all the code that may throw exceptions – in this case, any call to new that involves Framis objects. Immediately following the try block is one or more catch clauses, each one specifying the type of exception that they catch. In this case, catch(bad_alloc) says that that bad_alloc exceptions will be caught here. This particular catch clause is only executed when a bad_alloc exception is thrown, and execution continues after the end of the last catch clause in the group (there’s only one here, but there could be more).

In this example, it’s OK to use iostreams because the global operator new and delete are untouched.

The operator delete assumes the Framis address was created in the pool. This is a fair assumption, because the local operator new will be called whenever you create a single Framis object on the heap – but not an array of them: global new is used for arrays. So the user might accidentally have called operator delete without using the empty bracket syntax to indicate array destruction. This would cause a problem. Also, the user might be deleting a pointer to an object created on the stack. If you think these things could occur, you might want to add a line to make sure the address is within the pool and on a correct boundary (you may also begin to see the potential of overloaded new and delete for finding memory leaks).

operator delete calculates the block in the pool that this pointer represents, and then sets the allocation map’s flag for that block to false to indicate the block has been released.

In main( ), enough Framis objects are dynamically allocated to run out of memory; this checks the out-of-memory behavior. Then one of the objects is freed, and another one is created to show that the released memory is reused.

Because this allocation scheme is specific to Framis objects, it’s probably much faster than the general-purpose memory allocation scheme used for the default new and delete. However, you should note that it doesn’t automatically work if inheritance is used (inheritance is covered in Chapter 14).

If you overload operator new and delete for a class, those operators are called whenever you create an object of that class. However, if you create an array of those class objects, the global operator new is called to allocate enough storage for the array all at once, and the global operator delete is called to release that storage. You can control the allocation of arrays of objects by overloading the special array versions of operator new[ ] and operator delete[ ] for the class. Here’s an example that shows when the two different versions are called:

Here, the global versions of new and delete are called so the effect is the same as having no overloaded versions of new and delete except that trace information is added. Of course, you can use any memory allocation scheme you want in the overloaded new and delete.

You can see that the syntax of array new and delete is the same as for the individual object versions except for the addition of the brackets. In both cases you’re handed the size of the memory you must allocate. The size handed to the array version will be the size of the entire array. It’s worth keeping in mind that the only thing the overloaded operator new is required to do is hand back a pointer to a large enough memory block. Although you may perform initialization on that memory, normally that’s the job of the constructor that will automatically be called for your memory by the compiler.

The constructor and destructor simply print out characters so you can see when they’ve been called. Here’s what the trace file looks like for one compiler:

Creating an individual object requires 40 bytes, as you might expect. (This machine uses four bytes for an int.) The operator new is called, then the constructor (indicated by the *). In a complementary fashion, calling delete causes the destructor to be called, then the operator delete.

When an array of Widget objects is created, the array version of operator new is used, as promised. But notice that the size requested is four more bytes than expected. This extra four bytes is where the system keeps information about the array, in particular, the number of objects in the array. That way, when you say

the brackets tell the compiler it’s an array of objects, so the compiler generates code to look for the number of objects in the array and to call the destructor that many times. You can see that, even though the array operator new and operator delete are only called once for the entire array chunk, the default constructor and destructor are called for each object in the array.