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From: David Byrden <[e-mail]>
Newsgroups: comp.std.c++
Subject: Re: Function object operator() is too anonymous.
Date: 28 Mar 1996 17:35:41 PST
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[e-mail] (Igor Boukanov) wrote:

>   I noticed that that it impossible to write Function object class
>(DWP 20.3) that can be used for example as Arithmetic operation object
>and Predicate object in the same time, because both require operator()
>that can not be overloaded!

>So why do not have instead of one anonymous
>operator() a set of function obect methods like
>unary_operation, binary_operation, predicate, comparison
>and each function from algorithm.h will call correspondent method.
>For example, find_if will call predicate method of function object and
>transform will call unary_operation or binary_operation
>instead of operator().
>
>In this case one can write:
>
>class F {
>   ...
>   bool predicate(int); // instead of bool operator()(int);
>   int unary_operation(int); // instead of int operator()(int);
>   int binary_operation(int, int); // instead of int operator()(int, int);
>   bool comparison(int, int); // instead of bool operator()(int, int);
>};
>
>and somewhere use F:
>
>vector<int> v1, v2;
>F f;
>
>...
>std::find_if(v1.begin(), v1.end(), f);
> // f.predicate(int) will  be called
>std::transform(v1.begin(), v1.end(), v2.begin(), v1.begin(), f);
> // f.binary_operation(int, int) will be called
>std::equal(v1.begin(), v1.end(), v2.begin(), f);
> // f.comparison(int, int) will be called
>


  Igor; perhaps you are not quite clear about how functors are used.

  They are interchangeable with ordinary function addresses; this would no
longer be possible if you were to replace operator() with a member
function called, for example, unary_operation, because function addresses
do not have it. For example;

  bool ordinary_function( int ) ;

  std::find_if(v1.begin(), v1.end(),  ordinary_function ) ;
  // works if the algorigthm calls operator()()
  // fails if the algorithm calls unary_operation()



   Also, there is no need to use a single functor class F in these two
cases, as you did;

 F f ;
 std::transform(v1.begin(), v1.end(), v2.begin(), v1.begin(), f );
 std::equal(v1.begin(), v1.end(), v2.begin(), f );

you would normally use different functors, depending on what you want the
algorithm to do.

The library provides many interchangeable functors, and you can build a
collection of your own ones. There is no requirement to use only one
functor in all cases.



     The purpose of a functor is to be a "function with attached data", as
this example makes clear;

   find_if( v.begin(),  v.end(),  IsEqualTo(6)  ) ;

The functor is effectively a function which I have customised by attaching
the number 6 to it. So, I am not obliged to write a whole set of ordinary
functions like these;

   bool IsEqualTo6( int ) ;
   bool IsEqualTo7( int ) ;
   bool IsEqualTo8( int ) ;
   ..................


                            David
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