Should one prefer STL algorithms over hand-rolled loops?

Question

I seem to be seeing more 'for' loops over iterators in questions & answers here than I do for_each(), transform(), and the like. Scott Meyers suggests that stl algorithms are preferred, or at least he did in 2001. Of course, using them often means moving the loop body into a function or function object. Some may feel this is an unacceptable complication, while others may feel it better breaks down the problem.

So... should STL algorithms be preferred over hand-rolled loops?

Answer 1

It depends on:

• Whether high-performance is required
• The readability of the loop
• Whether the algorithm is complex

If the loop isn't the bottleneck, and the algorithm is simple (like for_each), then for the current C++ standard, I'd prefer a hand-rolled loop for readability. (Locality of logic is key.)

However, now that C++0x/C++11 is supported by some major compilers, I'd say use STL algorithms because they now allow lambda expressions — and thus the locality of the logic.

Answer 2

I’m going to go against the grain here and advocate that using STL algorithms with functors makes code much easier to understand and maintain, but you have to do it right. You have to pay more attention to readability and clearity. Particularly, you have to get the naming right. But when you do, you can end up with cleaner, clearer code, and paradigm shift into more powerful coding techniques.

Let’s take an example. Here we have a group of children, and we want to set their “Foo Count” to some value. The standard for-loop, iterator approach is:

for (vector<Child>::iterator iter = children.begin();     iter != children.end();     ++iter) {     iter->setFooCount(n); } 

Which, yeah, it’s pretty clear, and definitely not bad code. You can figure it out with just a little bit of looking at it. But look at what we can do with an appropriate functor:

for_each(children.begin(), children.end(), SetFooCount(n)); 

Wow, that says exactly what we need. You don’t have to figure it out; you immediately know that it’s setting the “Foo Count” of every child. (It would be even clearer if we didn’t need the .begin() / .end() nonsense, but you can’t have everything, and they didn’t consult me when making the STL.)

Granted, you do need to define this magical functor, SetFooCount, but its definition is pretty boilerplate:

class SetFooCount { public:     SetFooCount(int n) : fooCount(n) {}      void operator () (Child& child)     {         child.setFooCount(fooCount);     }  private:     int fooCount; }; 

In total it’s more code, and you have to look at another place to find out exactly what SetFooCount is doing. But because we named it well, 99% of the time we don’t have to look at the code for SetFooCount. We assume it does what it says, and we only have to look at the for_each line.

What I really like is that using the algorithms leads to a paradigm shift. Instead of thinking of a list as a collection of objects, and doing things to every element of the list, you think of the list as a first class entity, and you operate directly on the list itself. The for-loop iterates through the list, calling a member function on each element to set the Foo Count. Instead, I am doing one command, which sets the Foo Count of every element in the list. It’s subtle, but when you look at the forest instead of the trees, you gain more power.

So with a little thought and careful naming, we can use the STL algorithms to make cleaner, clearer code, and start thinking on a less granular level.