为什么重载运算符 - >（）有用？

Question

I saw the following example in book of the C++ Programming Language

class Ptr {
     X* operator->( );
};

 voide f(Ptr p)
 {
   p->m=7;
   X* q1 = p->;
   X* q2 = p.operator->();
 }

The book claims that 1)Objects of class Ptr can be used to access members of class X in a very similar manner to the way pointers are used. 2)The transformation of the object p into the pointer p.operator->() does not depend on the member m pointed to. That is the sense in which operator->( ) is a unary postfix operator.

For the first point, I do not understand why we need to this design, or in which scenarios should use this kind of design. For the second point, I am confused about the message that the author want to deliver.

Thanks.

Answer 1

This operator is overloaded when implementing objects that behave like (pretend to be) pointers.

A good example would be boost::shared_ptr , which is a classic reference counting pointer that automatically deletes the pointed-to object when all pointers are destroyed. Countless other similar "smart pointer" objects have been implemented by people for all sorts of reasons.

(and, as pointed out the stl iterators also use this to behave like pointers, allowing syntax like it->method(); or it->data; )

Answer 2

1. This design is extremely useful when we want to create a class that behaves like a pointer; this is the case of smart pointers (objects that have a pointer-like interface but that provide additional "services", eg automatic deallocation on destruction, ownership transfer, reference counting, ...).

   Notice that often this kind of object will also overload the * (dereference) operator to mimic pointers more closely.

2. The author wants to say that when you use the -> operator on Ptr , it's not relevant (as far as operator-> is concerned) what you put after it: in any case, the operator-> method will be called, that will return a pointer to the object that will be considered for the rest of the expression.

---

To make this more clear: quoting directly from the standard:

13.5.6 Class member access

operator-> shall be a non-static member function taking no parameters. It implements class member access using ->

 postfix-expression -> id-expression 

An expression x->m is interpreted as (x.operator->())->m for a class object x of type T if T::operator->() exists and if the operator is selected as the best match function by the overload resolution mechanism (13.3).

In other words:

• if you call operator->() directly (second and third example in the OP code), you will get the pointer returned by the operator-> method, just like what happens with any method;
• if you put stuff after the -> operator (eg x->m , as in the first example in the OP code), the overloaded operator-> will be called, and the returned pointer will be used as if the ->m was being used over it.

Answer 3

iterators ... a fundamental part of the C++ standard library .

std::list<std::string> list_of_string
std::list<std::string>::iterator i;
for(i = list_of_string.begin(); i != list_of_string.end(); ++i)
{
  //WHERE -> IS OVERLOADED...
  printf("%s\n", i->c_str()); //print to the screen, printf() is C and requires a C-string.
}

normally, i should be thought of as a std::list<> object, not a std::string . Normally c_str() would be inaccessible too, without the -> overloaded.

Answer 4

it allows for creating smart pointers. (little objects that behave just like normal pointers)

The main reason for this is automatic memory management. But can also be used for things like proxys