c++ lambda why - Advantages of std::for_each over for loop

10 Answers

Here are some reasons:

  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.

  4. It also opens your mind to the rest of the STL-algorithms, like find_if, sort, replace, etc and these won't look so strange anymore. This can be a huge win.

Update 1:

Most importantly, it helps you go beyond for_each vs. for-loops like that's all there is, and look at the other STL-alogs, like find / sort / partition / copy_replace_if, parallell execution .. or whatever.

A lot of processing can be written very concisely using "the rest" of for_each's siblings, but if all you do is to write a for-loop with various internal logic, then you'll never learn how to use those, and you'll end up inventing the wheel over and over.

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

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

Or with C++x11 lambdas:

for_each(monsters, [](Monster& m) { m.think(); });

is IMO more readable than:

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

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 = bananas.begin(); i != bananas.end(); ++i) {

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

Update 2: I've written my own one-liner 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?

use vs ranged

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

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()) 


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

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)
   // 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)
         return false;

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;

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{

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

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)

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.

With C++11 and two simple templates, you can write

        for ( auto x: range(v1+4,v1+6) ) {
                cout<< x <<' ';

as a replacement for for_each or a loop. Why choose it boils down to brevity and safety, there's no chance of error in an expression that's not there.

For me, for_each was always better on the same grounds when the loop body is already a functor, and I'll take any advantage I can get.

You still use the three-expression for, but now when you see one you know there's something to understand there, it's not boilerplate. I hate boilerplate. I resent its existence. It's not real code, there's nothing to learn by reading it, it's just one more thing that needs checking. The mental effort can be measured by how easy it is to get rusty at checking it.

The templates are

template<typename iter>
struct range_ { 
                iter begin() {return __beg;}    iter end(){return __end;}
            range_(iter const&beg,iter const&end) : __beg(beg),__end(end) {}
            iter __beg, __end;

template<typename iter>
range_<iter> range(iter const &begin, iter const &end)
    { return range_<iter>(begin,end); }

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.

for_each allow us to implement Fork-Join pattern . Other than that it supports fluent-interface.

fork-join pattern

We can add implementation gpu::for_each to use cuda/gpu for heterogeneous-parallel computing by calling the lambda task in multiple workers.

// all summary is complete now
// go access the user-summary here.

And gpu::for_each may wait for the workers work on all the lambda-tasks to finish before executing the next statements.


It allow us to write human-readable code in concise manner.