[boost] Suggestion: "dereferencing" function object

From: Vincent Poinot (vincent.poinot_at_[hidden])
Date: 2007-06-20 14:16:43

Hello,

I recently developped some small utility classes and I wonder if this could
be interesting for other people.

The motivation is the following: when manipulating containers of pointers,
standard algorithms won't work out of the box, e.g. if I want to sort a
vector of pointers to a class X, I have to pass to sort() a new comparison
function object that will dereference its arguments so that the proper
operator< may be called on the values.

So I wrote a piece of code that creates this new "dereferencing" function
object automatically. Here is what it looks like, assuming a class X with a
properly defined operator<(const X&, const X&)

int main()
{
    vector<X*> v ;

    // Populate v here

    // Testing dereferencing of binary function object
    sort(v.begin(), v.end(), deref<X*>(less<X>())) ;
}

As you can see, the available comparison function objects operates on
objects X, the deref() helper function creates a new function object that
will call the original less<>() after dereferencing its arguments, allowing
sort() to work as expected.

Two other examples (assuming there is also an operator+ and an operator==
for X objects):

X sum = accumulate(v.begin(), v.end(), X(), deref<X*>(plus<X>())) ;
remove_if(v.begin(), v.end(), 
          deref<X*>(bind2nd(equal_to<X>(), X(something)))) ;

I am well aware that lambda functions can solve this problem much more
elegantly, but the reason why I wrote this is that I cannot use Boost at
work :-( And now that it is written, well, maybe somebody else can find
this useful...

(This stuff also works with smart pointers.)

Vincent Poinot.