Item 8:  Understand the different meanings of new and delete.

It occasionally seems as if people went out of their way to make C++ terminology difficult to understand. Case in point: the difference between the new operator and operator new.

When you write code like this,

string *ps = new string("Memory Management");

the new you are using is the new operator. This operator is built into the language and, like sizeof, you can't change its meaning: it always does the same thing. What it does is twofold. First, it allocates enough memory to hold an object of the type requested. In the example above, it allocates enough memory to hold a string object. Second, it calls a constructor to initialize an object in the memory that was allocated. The new operator always does those two things; you can't change its behavior in any way.

What you can change is how the memory for an object is allocated. The new operator calls a function to perform the requisite memory allocation, and you can rewrite or overload that function to change its behavior. The name of the function the new operator calls to allocate memory is operator new. Honest.

The operator new function is usually declared like this:

void * operator new(size_t size);

The return type is void*, because this function returns a pointer to raw, uninitialized memory. (If you like, you can write a version of operator new that initializes the memory to some value before returning a pointer to it, but this is not commonly done.) The size_t parameter specifies how much memory to allocate. You can overload operator new by adding additional parameters, but the first parameter must always be of type size_t. (For information on writing operator new, consult Items E8-E10.)

You'll probably never want to call operator new directly, but on the off chance you do, you'll call it just like any other function:

void *rawMemory = operator new(sizeof(string));

Here operator new will return a pointer to a chunk of memory large enough to hold a string object.

Like malloc, operator new's only responsibility is to allocate memory. It knows nothing about constructors. All operator new understands is memory allocation. It is the job of the new operator to take the raw memory that operator new returns and transform it into an object. When your compilers see a statement like

string *ps = new string("Memory Management");

they must generate code that more or less corresponds to this (see Items E8 and E10, as well as the sidebar to my article on counting objects, for a more detailed treatment of this point):

void *memory =                              // get raw memory
  operator new(sizeof(string));             // for a string
                                            // object

call string::string("Memory Management")    // initialize the
on *memory;                                 // object in the
                                            // memory

string *ps =                                // make ps point to
  static_cast<string*>(memory);             // the new object

Notice that the second step above involves calling a constructor, something you, a mere programmer, are prohibited from doing. Your compilers are unconstrained by mortal limits, however, and they can do whatever they like. That's why you must use the new operator if you want to conjure up a heap-based object: you can't directly call the constructor necessary to initialize the object (including such crucial components as its vtbl — see Item 24).

Placement new

There are times when you really want to call a constructor directly. Invoking a constructor on an existing object makes no sense, because constructors initialize objects, and an object can only be initialized — given its first value — once. But occasionally you have some raw memory that's already been allocated, and you need to construct an object in the memory you have. A special version of operator new called placement new allows you to do it.

As an example of how placement new might be used, consider this:

class Widget {
public:
  Widget(int widgetSize);
  ...
};

Widget * constructWidgetInBuffer(void *buffer,
                                 int widgetSize)
{
  return new (buffer) Widget(widgetSize);
}

This function returns a pointer to a Widget object that's constructed within the buffer passed to the function. Such a function might be useful for applications using shared memory or memory-mapped I/O, because objects in such applications must be placed at specific addresses or in memory allocated by special routines. (For a different example of how placement new can be used, see Item 4.)

Inside constructWidgetInBuffer, the expression being returned is

new (buffer) Widget(widgetSize)

This looks a little strange at first, but it's just a use of the new operator in which an additional argument (buffer) is being specified for the implicit call that the new operator makes to operator new. The operator new thus called must, in addition to the mandatory size_t argument, accept a void* parameter that points to the memory the object being constructed is to occupy. That operator new is placement new, and it looks like this:

void * operator new(size_t, void *location)
{
  return location;
}

This is probably simpler than you expected, but this is all placement new needs to do. After all, the purpose of operator new is to find memory for an object and return a pointer to that memory. In the case of placement new, the caller already knows what the pointer to the memory should be, because the caller knows where the object is supposed to be placed. All placement new has to do, then, is return the pointer that's passed into it. (The unused (but mandatory) size_t parameter has no name to keep compilers from complaining about its not being used; see Item 6.) Placement new is part of the standard C++ library (see Item E49). To use placement new, all you have to do is #include <new> (or, if your compilers don't yet support the new-style header names (again, see Item E49), <new.h>).

If we step back from placement new for a moment, we'll see that the relationship between the new operator and operator new, though you want to create an object on the heap, use the new operator. It both allocates memory and calls a constructor for the object. If you only want to allocate memory, call operator new; no constructor will be called. If you want to customize the memory allocation that takes place when heap objects are created, write your own version of operator new and use the new operator; it will automatically invoke your custom version of operator new. If you want to construct an object in memory you've already got a pointer to, use placement new.

(For additional insights into variants of new and delete, see Item E7 and my article on counting objects.)

Deletion and Memory Deallocation

To avoid resource leaks, every dynamic allocation must be matched by an equal and opposite deallocation. The function operator delete is to the built-in delete operator as operator new is to the new operator. When you say something like this,

string *ps;
...
delete ps;                          // use the delete operator

your compilers must generate code both to destruct the object ps points to and to deallocate the memory occupied by that object.

The memory deallocation is performed by the operator delete function, which is usually declared like this:

void operator delete(void *memoryToBeDeallocated);

Hence,

delete ps;

causes compilers to generate code that approximately corresponds to this:

ps->~string();                      // call the object's dtor

operator delete(ps);                // deallocate the memory
                                    // the object occupied

One implication of this is that if you want to deal only with raw, uninitialized memory, you should bypass the new and delete operators entirely. Instead, you should call operator new to get the memory and operator delete to return it to the system:

void *buffer =                      // allocate enough
  operator new(50*sizeof(char));    // memory to hold 50
                                    // chars; call no ctors

...

operator delete(buffer);            // deallocate the memory;
                                    // call no dtors

This is the C++ equivalent of calling malloc and free.

If you use placement new to create an object in some memory, you should avoid using the delete operator on that memory. That's because the delete operator calls operator delete to deallocate the memory, but the memory containing the object wasn't allocated by operator new in the first place; placement new just returned the pointer that was passed to it. Who knows where that pointer came from? Instead, you should undo the effect of the constructor by explicitly calling the object's destructor:

// functions for allocating and deallocating memory in
// shared memory
void * mallocShared(size_t size);
void freeShared(void *memory);

void *sharedMemory = mallocShared(sizeof(Widget));

Widget *pw =                                   // as above,
  constructWidgetInBuffer(sharedMemory, 10);   // placement
                                               // new is used

...

delete pw;            // undefined! sharedMemory came from
                      // mallocShared, not operator new

pw->~Widget();        // fine, destructs the Widget pointed to
                      // by pw, but doesn't deallocate the
                      // memory containing the Widget

freeShared(pw);       // fine, deallocates the memory pointed
                      // to by pw, but calls no destructor

As this example demonstrates, if the raw memory passed to placement new was itself dynamically allocated (through some unconventional means), you must still deallocate that memory if you wish to avoid a memory leak. (See the sidebar to my article on counting objects for information on "placement delete".)

Arrays

So far so good, but there's farther to go. Everything we've examined so far concerns itself with only one object at a time. What about array allocation? What happens here?

string *ps = new string[10];          // allocate an array of
                                      // objects

The new being used is still the new operator, but because an array is being created, the new operator behaves slightly differently from the case of single-object creation. For one thing, memory is no longer allocated by operator new. Instead, it's allocated by the array-allocation equivalent, a function called operator new[] (often referred to as "array new.") Like operator new, operator new[] can be overloaded. This allows you to seize control of memory allocation for arrays in the same way you can control memory allocation for single objects (but see Item E8 for some caveats on this).

(operator new[] is a relatively recent addition to C++, so your compilers may not support it yet. If they don't, the global version of operator new will be used to allocate memory for every array, regardless of the type of objects in the array. Customizing array-memory allocation under such compilers is daunting, because it requires that you rewrite the global operator new. This is not a task to be undertaken lightly. By default, the global operator new handles all dynamic memory allocation in a program, so any change in its behavior has a dramatic and pervasive effect. Furthermore, there is only one global operator new with the "normal" signature (i.e., taking the single size_t parameter — see Item E9), so if you decide to claim it as your own, you instantly render your software incompatible with any library that makes the same decision. (See also Item 27.) As a result of these considerations, custom memory management for arrays is not usually a reasonable design decision for compilers lacking support for operator new[].)

The second way in which the new operator behaves differently for arrays than for objects is in the number of constructor calls it makes. For arrays, a constructor must be called for each object in the array:

string *ps =               // call operator new[] to allocate
  new string[10];          // memory for 10 string objects,
                           // then call the default string
                           // ctor for each array element

Similarly, when the delete operator is used on an array, it calls a destructor for each array element and then calls operator delete[] to deallocate the memory:

delete [] ps;              // call the string dtor for each
                           // array element, then call
                           // operator delete[] to
                           // deallocate the array's memory

Just as you can replace or overload operator delete, you can replace or overload operator delete[]. There are some restrictions on how they can be overloaded, however; consult a good C++ text for details. (For ideas on good C++ texts, see the recommendations beginning on page 285.)

So there you have it. The new and delete operators are built-in and beyond your control, but the memory allocation and deallocation functions they call are not. When you think about customizing the behavior of the new and delete operators, remember that you can't really do it. You can modify how they do what they do, but what they do is fixed by the language.