Overloading new and delete

^{Note: This only deals with the syntax of overloading new and delete, not with the implementation of such overloaded operators. I think that the semantics of overloading new and delete deserve their own FAQ, within the topic of operator overloading I can never do it justice.}

Basics

In C++, when you write a new expression like new T(arg) two things happen when this expression is evaluated: First operator new is invoked to obtain raw memory, and then the appropriate constructor of T is invoked to turn this raw memory into a valid object. Likewise, when you delete an object, first its destructor is called, and then the memory is returned to operator delete.
C++ allows you to tune both of these operations: memory management and the construction/destruction of the object at the allocated memory. The latter is done by writing constructors and destructors for a class. Fine-tuning memory management is done by writing your own operator new and operator delete.

The first of the basic rules of operator overloading – don’t do it – applies especially to overloading new and delete. Almost the only reasons to overload these operators are performance problems and memory constraints, and in many cases, other actions, like changes to the algorithms used, will provide a much higher cost/gain ratio than attempting to tweak memory management.

The C++ standard library comes with a set of predefined new and delete operators. The most important ones are these:

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

The first two allocate/deallocate memory for an object, the latter two for an array of objects. If you provide your own versions of these, they will not overload, but replace the ones from the standard library.
If you overload operator new, you should always also overload the matching operator delete, even if you never intend to call it. The reason is that, if a constructor throws during the evaluation of a new expression, the run-time system will return the memory to the operator delete matching the operator new that was called to allocate the memory to create the object in. If you do not provide a matching operator delete, the default one is called, which is almost always wrong.
If you overload new and delete, you should consider overloading the array variants, too.

Placement new

C++ allows new and delete operators to take additional arguments.
So-called placement new allows you to create an object at a certain address which is passed to:

class X { /* ... */ };
char buffer[ sizeof(X) ];
void f()
{ 
  X* p = new(buffer) X(/*...*/);
  // ... 
  p->~X(); // call destructor 
} 

The standard library comes with the appropriate overloads of the new and delete operators for this:

void* operator new(std::size_t,void* p) throw(std::bad_alloc); 
void  operator delete(void* p,void*) throw(); 
void* operator new[](std::size_t,void* p) throw(std::bad_alloc); 
void  operator delete[](void* p,void*) throw(); 

Note that, in the example code for placement new given above, operator delete is never called, unless the constructor of X throws an exception.

You can also overload new and delete with other arguments. As with the additional argument for placement new, these arguments are also listed within parentheses after the keyword new. Merely for historical reasons, such variants are often also called placement new, even if their arguments are not for placing an object at a specific address.

Class-specific new and delete

Most commonly you will want to fine-tune memory management because measurement has shown that instances of a specific class, or of a group of related classes, are created and destroyed often and that the default memory management of the run-time system, tuned for general performance, deals inefficiently in this specific case. To improve this, you can overload new and delete for a specific class:

class my_class { 
  public: 
    // ... 
    void* operator new();
    void  operator delete(void*,std::size_t);
    void* operator new[](size_t);
    void  operator delete[](void*,std::size_t);
    // ... 
}; 

Overloaded thus, new and delete behave like static member functions. For objects of my_class, the std::size_t argument will always be sizeof(my_class). However, these operators are also called for dynamically allocated objects of derived classes, in which case it might be greater than that.

Global new and delete

To overload the global new and delete, simply replace the pre-defined operators of the standard library with our own. However, this rarely ever needs to be done.