Whats the point of .begin() and .end()?

Question

In C++ library arrays, what are some cases where it's useful to have the .begin() and .end() member functions?

On cplusplus.com, the example use is to iterate through an array:

for ( auto it = myarray.begin(); it != myarray.end(); ++it )

But

for (int i = 0; i < myarray.size(); i++)

can be used for that.

without it you wouldn't be able to use it in a ranged based for loop. — NathanOliver, Aug 20 '15 at 18:41
If you use plain arrays, stick to plain indexes or pointers. But as soon as you use the containers from C++ standard library, you should use the iterators to use a common syntax through all containers. — Serge Ballesta, Aug 20 '15 at 18:47
Not to mention `end()` is often used as a "not found" value by many standard algorithmic functions. — Banex, Aug 20 '15 at 21:30

score 26 · Accepted Answer · answered Aug 20 '15 at 18:43

begin() and end() return iterators. Iterators provide uniform syntax to access different types of containers. At the first glance they might look like an overkill for traversing a simple array, but consider that you could write the same code to traverse a list, or a map.

This uniform access to various containers will allow you to write algorithms that work on all of them without knowing their internal structure. A for loop from begin to end is just a first piece in a much larger mosaic. Just look up the list of standard algorithms to appreciate the power of this simple abstraction.

score 10 · Answer 2 · answered Aug 20 '15 at 18:40

10

The whole point of standard containers is the ability to change them and use the same syntax. If you had a linked list, the first syntax still works.

Also it is equivalent to a pointer. i is an index so myarray[i] is slightly slower than it.

answered Aug 20 '15 at 18:40

Robert Jacobs

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... and, more importantly, having the same syntax for iteration over the container means you can write a template that will work for any of them. – Jules Aug 20 '15 at 22:08
Most who care point to negligible difference. http://stackoverflow.com/questions/2524233/speed-accessing-a-stdvector-by-iterator-vs-by-operator-index – Robert Jacobs Aug 21 '15 at 13:11

Jarod42 · Answer 3 · 2015-08-21T07:23:48.150

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In addition to be generic with other containers, begin, end is useful for for range

for (const auto& e : myarray)

edited Aug 21 '15 at 07:23

answered Aug 20 '15 at 18:47

Jarod42

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score 0 · Answer 4 · answered Aug 21 '15 at 08:58

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Now try iterating through a linked list. The only efficient way is to iterate from one item to the next, until you reach the end.

answered Aug 21 '15 at 08:58

Graham

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Whats the point of .begin() and .end()?

4 Answers4