Why do C++ STL container begin and end functions return iterators by value rather than by constant reference?

Question

As I look at the standard for different STL objects and functions, one thing that doesn't make sense to me is why would the begin() and end() functions for container objects return an iterator by value rather than by constant reference? It seems to me that iterators could be held by the container object internally and adjusted whenever the container is mutated. This would mitigate the cost of creating unnecessary temporaries in for loops like this:

for (std::vector<int>::iterator it=my_vec.begin(); it!=my_vec.end(); ++it){
    //do things
}

Is this a valid concern? Is there something about using references to iterators that makes this a bad idea? Do most compiler implementations optimize this concern away anyway?

But then a container would be a container of objects and iterators. That sounds like a lot of overhead. You could be holding many, many iterators into the same container. — Lara Bailey
– Lara Bailey, Commented Feb 26, 2013 at 19:42
Indeed, there are reasonable algorithms that construct a second container of iterators, just as big as the initial container, to establish a second, independent order. — MSalters
– MSalters, Commented Feb 26, 2013 at 19:50
Try to follow that train of thought and see where it takes you... begin() returns a reference to an iterator, what happen if you copy it (so that you can increment it)? How does the container know how many copies were done? Where would it hold those iterators? — David Rodríguez - dribeas
– David Rodríguez - dribeas, Commented Feb 26, 2013 at 20:14
@DavidRodríguez-dribeas I don't think it would be the container's responsibility to keep track of copies of that iterator. But yes, it seems it would be silly to have begin() return a reference given that the user would be forced to manually copy the iterator to be able to iterate over the container. — c.hughes
– c.hughes, Commented Feb 26, 2013 at 20:20
Are your going to implement device drivers in C++? Always remember the golden rule of software-engineering: "Think about optimization only if the bottleneck was proven". I remember a project, where the usage of STL was prohibited because of performance. But, they used tons of images in GUI-themes and the software was slow because of silly GUI design. — Valentin H
– Valentin H, Commented Feb 26, 2013 at 23:11

Kerrek SB · Accepted Answer · 2013-02-26 19:42:07Z

5

Iterators are designed to be light-weight and copyable (and assignable). For example, for a vector an iterator might literally just be a pointer. Moreover, the whole point of iterators is to decouple algorithms from containers, and so the container shouldn't have to care at all what kind of iterators anyone else is currently holding

answered Feb 26, 2013 at 19:42

Kerrek SB

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Mark Ransom · Accepted Answer · 2013-02-26 19:57:13Z

5

If the begin and end methods returned a reference, the container would be forced to have each of those iterators as members. The standards people try to leave as much flexibility to an implementation as possible.

For example you can create a simple wrapper for an array that behaves as a standard container and doesn't consume any extra memory. If this wrapper were required to contain the iterators it wouldn't be so simple or small anymore.

answered Feb 26, 2013 at 19:57

Mark Ransom

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1 Comment

Manu343726 Over a year ago

"For example you can create a simple wrapper for an array that behaves as a standard container and doesn't consume any extra memory" Thank you C++11: std::array

Valentin H · Accepted Answer · 2013-02-26 19:58:10Z

0

Well, if you choose right iterator, STL would :-)

for (
    std::vector<int>::const_iterator it=my_vec.begin(), 
    end=my_vec.end(); 
    it!=end; 
    ++it)
{
    //do things
}

Iterator in STL has pointer-semantic. Const iterator has const pointer semantic.

answered Feb 26, 2013 at 19:58

Valentin H

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3 Comments

c.hughes Over a year ago

The end() function still returns a const_iterator by value in that situation. I was stating concern over the performance costs of creating and destroying a bunch of temporary iterator objects.

Valentin H Over a year ago

@c.hughes: In this version of loop end() is called only once. In your version - size-times of my_vec. Btw: on Win64, VS2008 sizeof(std::vector<T>::const_iterator) returns 12. It's not that much.

c.hughes Over a year ago

Oh, I see what you did there. Your comments made me only pay attention to the fact that you used const_iterator.

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