c++ - Are free operator->* overloads evil?

Question

I was perusing section 13.5 after refuting the notion that built-in operators do not participate in overload resolution, and noticed that there is no section on operator->*. It is just a generic binary operator.

Its brethren, operator->, operator*, and operator[], are all required to be non-static member functions. This precludes definition of a free function overload to an operator commonly used to obtain a reference from an object. But the uncommon operator->* is left out.

In particular, operator[] has many similarities. It is binary (they missed a golden opportunity to make it n-ary), and it accepts some kind of container on the left and some kind of locator on the right. Its special-rules section, 13.5.5, doesn't seem to have any actual effect except to outlaw free functions. (And that restriction even precludes support for commutativity!)

So, for example, this is perfectly legal:

#include <utility>
#include <iostream>
using namespace std;

template< class T >
T &
operator->*( pair<T,T> &l, bool r )
    { return r? l.second : l.first; }

template< class T >
 T & operator->*( bool l, pair<T,T> &r ) { return r->*l; }

int main() {
        pair<int, int> y( 5, 6 );
        y->*(0) = 7;
        y->*0->*y = 8; // evaluates to 7->*y = y.second
        cerr << y.first << " " << y.second << endl;
}

It's easy to find uses, but alternative syntax tends not to be that bad. For example, scaled indexes for vector:

v->*matrix_width[2][5] = x; // ->* not hopelessly out of place

my_indexer<2> m( v, dim ); // my_indexer being the type of (v->*width)
m[2][5] = x; // it is probably more practical to slice just once

Did the standards committee forget to prevent this, was it considered too ugly to bother, or are there real-world use cases?

Answer 1

The best example I am aware of is Boost.Phoenix, which overloads this operator to implement lazy member access.

For those unfamiliar with Phoenix, it is a supremely nifty library for building actors (or function objects) that look like normal expressions:

( arg1 % 2 == 1 )     // this expression evaluates to an actor
                 (3); // returns true since 3 % 2 == 1

// these actors can also be passed to standard algorithms:
std::find_if(c.begin(), c.end(), arg1 % 2 == 1);
// returns iterator to the first odd element of c

It achieves the above by overloading operator% and operator==. - applied to the actor arg1 these operators return another actor. The range of expressions which can be built in this manner is extreme:

// print each element in c, noting its value relative to 5:
std::for_each(c.begin(), c.end(),
  if_(arg1 > 5)
  [
    cout << arg1 << " > 5
"
  ]
  .else_
  [
    if_(arg1 == 5)
    [
      cout << arg1 << " == 5
"
    ]
    .else_
    [
      cout << arg1 << " < 5
"
    ]
  ]
);

After you have been using Phoenix for a short while (not that you ever go back) you will try something like this:

typedef std::vector<MyObj> container;
container c;
//...
container::iterator inv = std::find_if(c.begin(), c.end(), arg1.ValidStateBit);
std::cout << "A MyObj was invalid: " << inv->Id() << std::endl;

Which will fail, because of course Phoenix's actors do not have a member ValidStateBit. Phoenix gets around this by overloading operator->*:

(arg1 ->* &MyObj::ValidStateBit)              // evaluates to an actor
                                (validMyObj); // returns true 

// used in your algorithm:
container::iterator inv = std::find_if(c.begin(), c.end(), 
      (arg1 ->* &MyObj::ValidStateBit)    );

operator->*'s arguments are:

• LHS: an actor returning MyObj *
• RHS: address of a member

It returns an actor which evaluates the LHS and looks for the specified member in it. (NB: You really, really want to make sure that arg1 returns MyObj * - you have not seen a massive template error until you get something wrong in Phoenix. This little program generated 76,738 characters of pain (Boost 1.54, gcc 4.6):

#include <boost/phoenix.hpp>
using boost::phoenix::placeholders::arg1;

struct C { int m; };
struct D { int n; };

int main() {
  ( arg1  ->*  &D::n ) (new C);
  return 0;
}