Advantages of std::for_each over for loop

Question

Are there any advantages of std::for_eachover for loop? To me, std::for_each only seems to hinder the readability of code. Why do then some coding standards recommend its use?

Answer 1

The container vendor can add optimisations that aren't available to the for loop.

Answer 2

You can have the iterator be a call to a function that is performed on each iteration through the loop.

See here: http://www.cplusplus.com/reference/algorithm/for_each/

Answer 3

for_each is more generic. You can use it to iterate over any type of container (by passing in the begin/end iterators). You can potentially swap out containers underneath a function which uses for_each without having to update the iteration code. You need to consider that there are other containers in the world than std::vector and plain old C arrays to see the advantages of for_each.

The major drawback of for_each is that it takes a functor, so the syntax is clunky. This is fixed in C++0x with the introduction of lambdas:

std::vector<int> container;
...
std::for_each(container.begin(), container.end(), [](int& i){
    i+= 10;
});

This will not look weird to you in 3 years.

Answer 4

1. It seems to hinder readability just because you're not used to it and/or not using the right tools around it to make it really easy. (see boost::range and boost::bind/boost::lambda for helpers. Many of these will go into C++0x and make for_each and related functions more useful.)

2. It allows you to write an algorithm on top of for_each that works with any iterator.

3. It reduces the chance of stupid typing bugs.

And (the soon-to-be available range-style for_each):

for_each(monsters, boost::mem_fn(&Monster::think));

is IMO more readable than:

for(Monsters::iterator i = monsters.begin(); i != monsters.end(); ++i) {
    i->think();
} 

Also this (or with lambdas, see others):

for_each(bananas, boost::bind(&Monkey::eat, my_monkey, _1));

Is more concise than:

for(Bananas::iterator i = banans.begin(); i != bananas.end(); ++i) {
    my_monkey->eat(*i);
} 

Especially if you have several functions to call in order... but maybe that's just me. ;)

update: I tend to write my own wrappers of stl-algos that work with ranges instead of pair of iterators. boost::range_ex, once released, will include that and maybe it will be there in C++0x too?

Answer 5

its very subjective, some will say that using for_each will make the code more readable, as it allows to treat different collections with the same conventions. for_each itslef is implemented as a loop

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

so its up to you to choose what is right for you.

Answer 6

Take a look:

http://stackoverflow.com/questions/135129/should-one-prefer-stl-algorithms-over-hand-rolled-loops

Answer 7

for is for loop that can iterate each element or every third etc. for_each is for iterating only each element. It is clear from its name. So it is more clear what you are intending to do in your code.

Answer 8

Like many of the algorithm functions, an initial reaction is to think it's more unreadable to use foreach than a loop. It's been a topic of many flame wars.

Once you get used to the idiom you may find it useful. One obvious advantage is that it forces the coder to separate the inner contents of the loop from the actual iteration functionality. (OK, I think it's an advantage. Other's say you're just chopping up the code with no real benifit).

One other advantage is that when I see foreach, I know that either every item will be processed or an exception will be thrown.

A for loop allows several options for terminating the loop. You can let the loop run its full course, or you can use the break keyword to explicitly jump out of the loop, or use the return keyword to exit the entire function mid-loop. In contrast, foreach does not allow these options, and this makes it more readable. You can just glance at the function name and you know the full nature of the iteration.

Here's an example of a confusing for loop:

for(std::vector<widget>::iterator i = v.begin(); i != v.end(); ++i)
{
   /////////////////////////////////////////////////////////////////////
   // Imagine a page of code here by programmers who don't refactor
   ///////////////////////////////////////////////////////////////////////
   if(widget->Cost < calculatedAmountSofar)
   {
        break;
   }
   ////////////////////////////////////////////////////////////////////////
   // And then some more code added by a stressed out juniour developer
   // *#&$*)#$&#(#)$#(*$&#(&*^$#(*$#)($*#(&$^#($*&#)$(#&*$&#*$#*)$(#*
   /////////////////////////////////////////////////////////////////////////
   for(std::vector<widgetPart>::iterator ip = widget.GetParts().begin(); ip != widget.GetParts().end(); ++ip)
   {
      if(ip->IsBroken())
      {
         return false;
      }
   }
}

Answer 9

The for_each loop is meant to hide the iterators (detail of how a loop is implemented) from the user code and define clear semantics on the operation: each element will be iterated exactly once.

The problem with readability in the current standard is that it requires a functor as the last argument instead of a block of code, so in many cases you must write specific functor type for it. That turns into less readable code as functor objects cannot be defined in-place (local classes defined within a function cannot be used as template arguments) and the implementation of the loop must be moved away from the actual loop.

struct myfunctor {
   void operator()( int arg1 ) { code }
};
void apply( std::vector<int> const & v ) {
   // code
   std::for_each( v.begin(), v.end(), myfunctor() );
   // more code
}

Note that if you want to perform an specific operation on each object, you can use std::mem_fn, or boost::bind (std::bind in the next standard), or boost::lambda (lambdas in the next standard) to make it simpler:

void function( int value );
void apply( std::vector<X> const & v ) {
   // code
   std::for_each( v.begin(), v.end(), boost::bind( function, _1 ) );
   // code
}

Which is not less readable and more compact than the hand rolled version if you do have function/method to call in place. The implementation could provide other implementations of the for_each loop (think parallel processing).

The upcoming standard takes care of some of the shortcomings in different ways, it will allow for locally defined classes as arguments to templates:

void apply( std::vector<int> const & v ) {
   // code
   struct myfunctor {
      void operator()( int ) { code }
   };
   std::for_each( v.begin(), v.end(), myfunctor() );
   // code
}

Improving the locality of code: when you browse you see what it is doing right there. As a matter of fact, you don't even need to use the class syntax to define the functor, but use a lambda right there:

void apply( std::vector<int> const & v ) {
   // code
   std::for_each( v.begin(), v.end(), 
      []( int ) { // code } );
   // code
}

Even if for the case of for_each there will be an specific construct that will make it more natural:

void apply( std::vector<int> const & v ) {
   // code
   for ( int i : v ) {
      // code
   }
   // code
}

I tend to mix the for_each construct with hand rolled loops. When only a call to an existing function or method is what I need (for_each( v.begin(), v.end(), boost::bind( &Type::update, _1 ) )) I go for the for_each construct that takes away from the code a lot of boiler plate iterator stuff. When I need something more complex and I cannot implement a functor just a couple of lines above the actual use, I roll my own loop (keeps the operation in place). In non-critical sections of code I might go with BOOST_FOREACH (a co-worker got me into it)

Answer 10

Personally, any time I'd need to go out of my way to use std::for_each (write special-purpose functors / complicated boost::lambdas), I find BOOST_FOREACH and C++0x's range-based for clearer:

BOOST_FOREACH(Monster* m, monsters) {
     if (m->has_plan()) 
         m->act();
}

vs

std::for_each(monsters.begin(), monsters.end(), if_then(bind(&Monster::has_plan, _1), bind(&Monster::act, _1)));

Answer 11

Aside from readability and performance, one aspect commonly overlooked is consistency. There are many ways to implement a for (or while) loop over iterators, from:

for (C::iterator iter = c.begin(); iter != c.end(); iter++) {
    do_something(*iter);
}

to:

C::iterator iter = c.begin();
C::iterator end = c.end();
while (iter != end) {
    do_something(*iter);
    ++iter;
}

with many examples in between at varying levels of efficiency and bug potential.

Using for_each, however, enforces consistency by abstracting away the loop:

for_each(c.begin(), c.end(), do_something);

The only thing you have to worry about now is: do you implement the loop body as function, a functor, or a lambda using Boost or C++0x features? Personally, I'd rather worry about that than how to implement or read a random for/while loop.

Answer 12

Mostly you'll have to iterate over the whole collection. Therefore I suggest you write your own for_each() variant, taking only 2 parameters. This will allow you to rewrite Terry Mahaffey's example as:

for_each(container, [](int& i) {
    i += 10;
});

I think this is indeed more readable than a for loop. However, this requires the C++0x compiler extensions.

Answer 13

I used to dislike std::for_each and thought that without lambda, it was done utterly wrong. However I did change my mind some time ago, and now I actually love it. And I think it even improves readability, and makes it easier to test your code in a TDD way.

The std::for_each algorithm can be read as do something with all elements in range, which can improve readability. Say the action that you want to perform is 20 lines long, and the function where the action is performed is also about 20 lines long. That would make a function 40 lines long with a conventional for loop, and only about 20 with std::for_each, thus likely easier to comprehend.

Functors for std::for_each are more likely to be more generic, and thus reusable, e.g:

struct DeleteElement
{
    template <typename T>
    void operator()(const T *ptr)
    {
        delete ptr;
    }
};

And in the code you'd only have a one-liner like std::for_each(v.begin(), v.end(), DeleteElement()) which is slightly better IMO than an explicit loop.

All of those functors are normally easier to get under unit tests than an explicit for loop in the middle of a long function, and that alone is already a big win for me.

std::for_each is also generally more reliable, as you're less likely to make a mistake with range.

And lastly, compiler might produce slightly better code for std::for_each than for certain types of hand-crafted for loop, as it (for_each) always looks the same for compiler, and compiler writers can put all of their knowledge, to make it as good as they can.

Same applies to other std algorithms like find_if, transform etc.

Answer 14

The nice thing with C++0X, is that this tiresome debate will be settled.

I mean, no one in their right mind who want to iterate over a whole collection will still use this :

for(auto it = collection.begin(); it != collection.end() ; ++it)
{
   foo(*it);
}

Or this :

for_each(collection.begin(), collection.end(), [](Element& e)
{
   foo(e);
});

when the range-based for-loop syntax will be available:

for(Element& e : collection)
{
   foo(e);
}

This kind of syntax is available in Java or C# for some time now, and there is actually way more foreach loop than classical for loop in every recent Java or C# code I saw.

Answer 15

I find for_each to be bad for readability. The concept is a good one but c++ makes it very hard to write readable, at least for me. c++0x lamda expressions will help. I really like the idea of lamdas. However on first glance I think the syntax is very ugly and I'm not 100% sure I'll ever get used to it. Maybe in 5 years I'll have got used to it and not give it a second thought, but maybe not. Time will tell :)

I prefer to use

vector<thing>::iterator istart = container.begin();
vector<thing>::iterator iend = container.end();
for(vector<thing>::iterator i = istart; i != iend; ++i) {
  // Do stuff
}

I find an explicit for loop clearer to read and explicity using named variables for the start and end iterators reduces the clutter in the for loop.

Of course cases vary, this is just what I usually find best.

Answer 16

You're mostly correct: most of the time, std::for_each is a net loss. I'd go so far as to compare for_each to goto. goto provides the most versatile flow-control possible -- you can use it to implement virtually any other control structure you can imagine. That very versatility, however, means that seeing a goto in isolation tells you virtually nothing about what's it's intended to do in this situation. As a result, almost nobody in their right mind uses goto except as a last resort.

Among the standard algorithms, for_each is much the same way -- it can be used to implement virtually anything, which means that seeing for_each tells you virtually nothing about what it's being used for in this situation. Unfortunately, people's attitude toward for_each is about where their attitude toward goto was in (say) 1970 or so -- a few people had caught onto the fact that it should be used only as a last resort, but many still consider it the primary algorithm, and rarely if ever use any other. The vast majority of the time, even a quick glance would reveal that one of the alternatives was drastically superior.

Just for example, I'm pretty sure I've lost track of how many times I've seen people writing code to print out the contents of a collection using for_each. Based on posts I've seen, this may well be the single most common use of for_each. They end up with something like:

class XXX { 
// ...
public:
     std::ostream &print(std::ostream &os) { return os << "my data\n"; }
};

And their post is asking about what combination of bind1st, mem_fun, etc. they need to make something like:

std::vector<XXX> coll;

std::for_each(coll.begin(), coll.end(), XXX::print);

work, and print out the elements of coll. If it really did work exactly as I've written it there, it would be mediocre, but it doesn't -- and by the time you've gotten it to work, it's difficult to find those few bits of code related to what's going on among the pieces that hold it together.

Fortunately, there is a much better way. Add a normal stream inserter overload for XXX:

std::ostream &operator<<(std::ostream *os, XXX const &x) { 
   return x.print(os);
}

and use std::copy:

std::copy(coll.begin(), coll.end(), std::ostream_iterator<XXX>(std::cout, "\n"));

That does work -- and takes virtually no work at all to figure out that it prints the contents of coll to std::cout.

Answer 17

The advantage of writing functional for beeing more readable, might not show up when for(...) and for_each(...).

If you utilize all algorithms in functional.h, instead of using for-loops, the code gets a lot more readable;

iterator longest_tree = std::max_element(forest.begin(), forest.end(), ...);
iterator first_leaf_tree = std::find_if(forest.begin(), forest.end(), ...);
std::transform(forest.begin(), forest.end(), firewood.begin(), ...);
std::for_each(forest.begin(), forest.end(), make_plywood);

is much more readable than;

Forest::iterator longest_tree = it.begin();
for (Forest::const_iterator it = forest.begin(); it != forest.end(); ++it{
   if (*it > *longest_tree) {
     longest_tree = it;
   }
}

Forest::iterator leaf_tree = it.begin();
for (Forest::const_iterator it = forest.begin(); it != forest.end(); ++it{
   if (it->type() == LEAF_TREE) {
     leaf_tree  = it;
     break;
   }
}

for (Forest::const_iterator it = forest.begin(), jt = firewood.begin(); 
     it != forest.end(); 
     it++, jt++) {
          *jt = boost::transformtowood(*it);
    }

for (Forest::const_iterator it = forest.begin(); it != forest.end(); ++it{
    std::makeplywood(*it);
}

And that is what I think is so nice, generalize the for-loops to one line functions =)