I just read the code for std::for_each:
template<class InputIterator, class Function>
Function for_each(InputIterator first, InputIterator last, Function f)
{
for ( ; first!=last; ++first ) f(*first);
return f;
}
and could not see any good reasons for this template function to return the input function. Does anyone have any examples on where this would be useful?
It's to allow you to accrue state in your function and then return it to your calling code. For instance, your function (as a functor class) could have a member int for counting the number of times it had been called.
Here is a page with some examples : https://web.archive.org/web/20171127171924/http://xenon.arcticus.com/c-morsels-std-for-each-functors-member-variables
If you pass in a function object, aka functor, and it has state, returning the function object allows you to access its state after iterating the sequence. Let's say you had a function object that computes three different variables from the sequence and holds them in member variables. Each time the functor is called, you update the counters. If for_each didn't return the object, how would you get the result?
Note... this is why you must always implement copy-construction, and assignment for function objects with state.
Returning the function basically makes std::for_each into a mediocre imitation of std::accumulate. It lets you accumulate something in the function/functor, and then retrieve that accumulated value when it's done. Almost any time you think this might be a useful thing to do, you should probably consider using std::accumulate instead.
its useful if you want to save (and later use) the functor state between calls for example, count the number of elements in a collections or indicate some failure to process an element by setting some internal variable.
No particular reason I guess. What you can do though is using the returned function in another foreach call, thus avoiding writing the function name twice and possibly making a mistake there.