Why does the STL provide a const overload for std::container::begin and std::container::end functions?

Question

Take any STL container in mind which uses iterators.

The following is valid:

iterator begin();
const_iterator begin() const;
const_iterator cbegin() const;

Now I don't understand why the second line exists. Could someone provide a nice example where iterator begin(); wouldn't work and it can't be replaced by const_iterator cbegin();... begging the need for const_iterator begin();?

Think of a container class member variable used in a `const` function. How would you access `begin()` without a `const_iterator`? — πάντα ῥεῖ, Nov 29 '15 at 13:38
http://stackoverflow.com/questions/12001410/what-is-the-reason-behind-cbegin-cend provides the answer to your question. — Tomasz Kaminski, Nov 29 '15 at 13:40
Possible duplicate of [What is the purpose of the const overloads of std::begin and end?](http://stackoverflow.com/q/33586029/1708801) — Shafik Yaghmour, Nov 29 '15 at 18:24

score 2 · Answer 1 · edited Jan 08 '17 at 00:13

There's probably no place in the language where you couldn't give something a different name; this is just a little act of kindness, a bit like size() and length() on std::string.

Remembering to use cbegin when working on a const container is a PITA, so the const_iterator begin() const is useful. At the same time, when I'm explicitly interested in obtaining a const iterator, without cbegin I'd have to do something convoluted like ((const std::vector<int> &)vec).begin(), which is certainly less readable/writable than vec.cbegin().

Now, you could argue that there are places where little acts of kindness like this are more sorely needed (and which scale better — supporting all the combinations of regular/reverse/const iterators quickly makes the interface explode), and I would certainly agree with you.

Incidentally, having the const_iterator begin avoids the need for other fiddling to make the range-based for loop work correctly on const objects.

score 2 · Accepted Answer · answered Nov 29 '15 at 14:31

First off, there is a simple historic reason: the original C++ standard didn't use cbegin() and only had the two begin() overloads. It was felt that sometimes it would be useful to obtain const_iterators to a non-const sequence and cbegin() and cend() were added (despite begin() and end() members actually being ill-advise: we'd be much better of if they were just non-member functions). That is, if the const overload of begin() had been removed when cbegin() was introduced, at lot of code would have been broken.

From a practical point of view it is also desirable to have both the non-const and the const overloads:

use of begin() and cbegin() in normal code could be done it would be quite annoying to actually do so when changing the constness of objects
more importantly, in generic (templatized) code where sequence could be either const or non-const it would be necessary to use some dance to to use the correct overload.

Why does the STL provide a const overload for std::container::begin and std::container::end functions?

2 Answers2