On Functors

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wheaties Permalink
May 17, 2010, 15:15

You should put down accumulate instead of for_each in your C++ example as for_each is not required to pass the previous functor's state to the next call. That is, you could wind up with even == 0 and odd == 1 instead of 25 and 30.

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Roman Permalink
May 17, 2010, 18:09

How would it lose the state? The doc clearly states that for_each(first, last, f) applies f to the result of dereferencing every iterator in the range [first, last).

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May 18, 2010, 18:15

The standard just requires a copy of f to be returned. This means that, for instance, the following implementation is perfectly legal:

template< typename InputIterator, typename Function >
Function
for_each( InputIterator first, InputIterator last, Function f )
{
    if ( first != last ) {
        f( *first ) ;
        for_each( first + 1, last, f ) ;
    }
    return f ;
}

This is a typical gotcha for C++ beginners.

(There are also other small oversights in the code, but this is the main one.)

PS: Also, I think that standard-wise the only occurrence of the term "functor" I've seen is in the draft specification for unique_ptr (but I'd have to check if it's still there; if so it ought probably be replaced by "function object"). But of course it's a term in common usage, sometimes referring to classes, sometimes to objects (instances).

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May 19, 2010, 00:04

Oops, I forgot to stick to input iterators, sorry. It should of course be:

template< typename InputIterator, typename Function >
Function
for_each( InputIterator first, InputIterator last, Function f )
{
    if ( first != last ) {
        f( *first ) ;
        ++ first ;
        for_each( first, last, f ) ;
    }
    return f ;
}
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