ei9.htm

一个非常适合初学者入门的有关c++的文档

 Effective C++, 2E | Item 9: Avoid hiding the "normal" form of new. Back to Item 8: Adhere to convention when writing operator new and operator delete.
Continue to Item 10: Write operator delete if you write operator new.
Item 9: Avoid hiding the "normal" form of new.
A declaration of a name in an inner scope hides the same name in outer scopes, so for a function f at both global and class scope, the member function will hide the global function: 
void f();                             // global function

class X {
public:
  void f();                           // member function
};

X x;


f();                                  // calls global f


x.f();                                // calls X::f


This is unsurprising and normally causes no confusion, because global and member functions are usually invoked using different syntactic forms. However, if you add to this class an operator new taking additional parameters, the result is likely to be an eye-opener: class X {
public:
  void f();

  // operator new allowing specification of a
  // new-handling function
  static void * operator new(size_t size, new_handler p);
};


void specialErrorHandler();          // definition is elsewhere

X *px1 =
  new (specialErrorHandler) X;       // calls X::operator new


X *px2 = new X;                      // error!


By declaring a function called "operator new" inside the class, you inadvertently block access to the "normal" form of new. Why this is so is discussed in Item 50. Here we're more interested in figuring out how to avoid the problem.
One solution is to write a class-specific operator new that supports the "normal" invocation form. If it does the same thing as the global version, that can be efficiently and elegantly encapsulated as an inline function: class X {
public:
  void f();

  static void * operator new(size_t size, new_handler p);

  static void * operator new(size_t size)
  { return ::operator new(size); }
};

X *px1 =
  new (specialErrorHandler) X;      // calls X::operator
                                    // new(size_t, new_handler)


X* px2 = new X;                     // calls X::operator
                                    // new(size_t)


An alternative is to provide a default parameter value (see Item 24) for each additional parameter you add to operator new: class X {
public:
  void f();

  static
    void * operator new(size_t size,                // note default
                        new_handler p = 0);         // value for p
};


X *px1 = new (specialErrorHandler) X;               // fine

X* px2 = new X;                                     // also fine


Either way, if you later decide to customize the behavior of the "normal" form of new, all you need to do is rewrite the function; callers will get the customized behavior automatically when they relink. Back to Item 8: Adhere to convention when writing operator new and operator delete.
Continue to Item 10: Write operator delete if you write operator new.