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Placement new and placement delete

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The C++ equivalent of malloc is operator new, and the C++ equivalent of free is operator delete. Unlike malloc and free, however, operator new and operator delete are overloadable functions, and as such they may take different numbers and types of parameters. This has always been the case for operator new, but until relatively recently, it wasn't valid to overload operator delete.

The "normal" signature for operator new is:

void * operator new(size_t) throw (std::bad_alloc);

(To simplify things from now on, I'll omit exception specifications. They're not germane to the points I want to make.) Overloaded versions of operator new are limited to adding additional parameters, so an overloaded version of operator new might look like:

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

This particular version of operator new — the one taking an extra void* argument specifying what pointer the function should return — is so commonly useful that it's in the Standard C++ library (declared in the header <new>) and it has a name, "placement new." The name indicates its purpose: to allow programmers to specify where in memory an object should be created (where it should be placed). Over time, the term placement new has come to be applied to any version of operator new taking additional arguments. (This terminology is actually enshrined in the grammar for C++ in the forthcoming International Standard.) Hence, when C++ programmers talk about the placement new function, they mean the function above, the one taking the extra void* parameter specifying where an object should be placed. When they talk about a placement new function, however, they mean any version of operator new taking more than the mandatory size_t argument. That includes the function above, but it also includes a plethora of operator new functions that take more or different parameter types. In other words, when the topic is memory allocation functions, "placement new" means "a version of operator new taking extra arguments." The term can mean still other things in other contexts, but we don't need to go down that road here, so we won't. For details, consult the suggested reading at the end of the article. By analogy with placement new, the term "placement delete" means "a version of operator delete taking extra arguments." The "normal" signature for operator delete is:

void operator delete(void*);

So any version of operator delete taking one or more arguments beyond the mandatory void* parameter is a placement delete function. Let us now revisit an issue discussed in the main article. What happens when an exception is thrown during construction of a heap object? Consider again this simple example:

class ABCD { ... };
ABCD *p = new ABCD;

Suppose the attempt to create the ABCD object yields an exception. The main article pointed out that if that exception came from the ABCD constructor, operator delete would automatically be called to deallocate the memory allocated by operator new. But what if operator new is overloaded, and what if different versions of operator new (quite reasonably) allocate memory in different ways? How can operator delete know how to correctly deallocate the memory? Furthermore, what if the ABCD object being created used placement new, as in:

void *objectBuffer = getPointerToStaticBuffer();
ABCD *p = new (objectBuffer) ABCD; // create an ABCD object in a static buffer

(The) placement new didn't actually allocate any memory. It just returned the pointer to the static buffer that was passed to it in the first place. So there's no need for any deallocation.

Clearly, the actions to be taken in operator delete to undo the actions of its corresponding operator new depend on the version of operator new that was invoked to allocate the memory.

To make it possible for programmers to indicate how the actions of particular versions of operator new can be undone, the C++ Standards committee extended C++ to allow operator delete to be overloaded, too. When an exception is thrown from the constructor for a heap object, then, the game is played a special way. The version of operator delete that's called is the one whose extra parameter types correspond to those of the version of operator new that was invoked.

If there's no placement delete whose extra parameters correspond to the extra parameters of the placement new that was called, no operator delete is invoked at all. So the effects of operator new are not undone. For functions like (the) placement version of operator new, this is fine, because they don't really allocate memory. In general, however, if you create a custom placement version of operator new, you should also create the corresponding custom placement version of operator delete to go with it.

Alas, most compilers don't yet support placement delete. With code generated from such compilers, you almost always suffer a memory leak if an exception is thrown during construction of a heap object, because no attempt is made to deallocate the memory that was allocated before the constructor was invoked. o

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