Functors

This is a discussion on Functors within the C++ Programming forums, part of the General Programming Boards category; I've read a couple of articles on these and can't understand why you would need them. Maybe I don't understand ...

  1. #1
    Supermassive black hole cboard_member's Avatar
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    Functors

    I've read a couple of articles on these and can't understand why you would need them. Maybe I don't understand exactly what they are.

    Could someone explain them to me in a "non-article" way?
    Good class architecture is not like a Swiss Army Knife; it should be more like a well balanced throwing knife.

    - Mike McShaffry

  2. #2
    Confused Magos's Avatar
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    A class overloading the () operator to make it look like a function is called when in fact the operator is called.
    MagosX.com

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  3. #3
    Supermassive black hole cboard_member's Avatar
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    Yeah I know what it is I just don't see why you would want to do that.
    Good class architecture is not like a Swiss Army Knife; it should be more like a well balanced throwing knife.

    - Mike McShaffry

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    lets say you want to pass the adress of a function in a class to another class...that will involve some really weird code and stuff. Instead you pass a functor. One thing i use them for when i need to.

  5. #5
    Guest Sebastiani's Avatar
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    a functor is just an object that behaves like a function. their useful when you need to store something that will later be applied to the data being passed to it.

    Code:
    int
    filter(int num)
    {
    	return num > 10 ? 10 : num < 5 ? 5 : num;
    }
    
    struct filter_object
    {
    	filter_object(int least, int most)
    	: minimum(least), maximum(most)
    	{
    	
    	}
    	
    	int
    	operator () (int num) const
    	{
    		return num > maximum ? maximum : num < minimum ? minimum : num;
    	}
    	
    	int minimum, maximum;
    };
    
    template <class Function>
    void
    apply_filter(int * ptr, const Function & fnc)
    {
    	*ptr = fnc(*ptr);
    }
    
    int
    main(void)
    {	
    	int x = 21;
    	apply_filter(&x, filter);
    	apply_filter(&x, filter_object(8, 20));
    	return 0;
    }
    Code:
    if( numeric_limits< byte >::digits != bits_per_byte )
        error( "program requires bits_per_byte-bit bytes" );
    24bbs.cpp

  6. #6
    Supermassive black hole cboard_member's Avatar
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    Ok thanks guys.
    Good class architecture is not like a Swiss Army Knife; it should be more like a well balanced throwing knife.

    - Mike McShaffry

  7. #7
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    You can think of a functor as a function that also has some variables to store data in. That way you can pass a function around with some data attached.

    Next up: functionoids.

  8. #8
    Supermassive black hole cboard_member's Avatar
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    Quote Originally Posted by 7stud
    You can think of a functor as a function that also has some variables to store data in. That way you can pass a function around with some data attached.

    Next up: functionoids.
    Ugh... I was going to ask about dators next, but go on.
    Good class architecture is not like a Swiss Army Knife; it should be more like a well balanced throwing knife.

    - Mike McShaffry

  9. #9
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    You can also use operator() to make your object emulate a multi dimensional array, kind of like operator[] only with multiple parameters.

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    Here's a small chunk of code I toyed about with a while ago, using functors to make life easier when using STL algorithms.

    Sometimes using STL algorithms, you wish to pass an extra parameter to the predicate - usually this is accomplished using std::bind1st or std::bind2nd. This is OK - except that you can't pass more than one extra parameter. Some people also don't like the way bind1st/bind2nd reads... so you can do this instead.

    Code:
    /* Functor to find a single character inside a string */
    
    class isCharInString
    {
       char ch;
    public:
       isCharInString( char c ): ch( c ) { }
           // Additional args passed through constructor
    
       bool operator()( const std::string& str ) const
       {
          return str.find( ch ) != std::string::npos;
       }
    };

    Example of using functor, with a vector of std::string:
    Code:
    typedef std::vector<std::string>::iterator vIterator;
    
    int main()
    {
        std::vector<std::string> foo;
         
        foo.push_back("qwertyuiop");
        foo.push_back("asdfghjkl");
        foo.push_back("zxcvbnm");
        
        vIterator iter = std::find_if(foo.begin(), foo.end(), 
                                      isCharInString('c') );
        if (iter != foo.end())
            std::cout << *iter;
    }
    The above should output the line
    Code:
    zxcvbnm
    Last edited by Bench82; 04-25-2006 at 09:51 AM.

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