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c++stliteratorconstants
Konrad
Konrad

Reputation: 40947

What is the difference between const_iterator and non-const iterator in the C++ STL?

What is the difference between a const_iterator and an iterator and where would you use one over the other?

Upvotes: 177

Views: 126169

Answers (7)

Stack Overflow is garbage
Stack Overflow is garbage

Reputation: 247909

They should pretty much be self-explanatory. If iterator points to an element of type T, then const_iterator points to an element of type const T.

It's basically equivalent to the pointer types:

T* //   A non-const iterator to a non-const element. ≡ vector<T>::iterator
T* const // A const iterator to a non-const element. ≡ const vector<T>::iterator
const T* // A non-const iterator to a const element. ≡ vector<T>::const_iterator

A const iterator always points to the same element, so the iterator itself is const. But the element it points to does not have to be const, so the element it points to can be changed.

A const_iterator is an iterator that points to a const element, so while the iterator itself can be updated (incremented or decremented, for example), the element it points to cannot be changed.

Upvotes: 52

Ciro Santilli OurBigBook.com
Ciro Santilli OurBigBook.com

Reputation: 382552

Minimal runnable examples

Non-const iterators allow you to modify what they point to:

std::vector<int> v{0};
std::vector<int>::iterator it = v.begin();
*it = 1;
assert(v[0] == 1);

Const iterators don't:

const std::vector<int> v{0};
std::vector<int>::const_iterator cit = v.begin();
// Compile time error: cannot modify container with const_iterator.
//*cit = 1;

As shown above, v.begin() is const overloaded, and returns either iterator or const_iterator depending on the const-ness of the container variable:

A common case where const_iterator pops up is when this is used inside a const method:

class C {
    public:
        std::vector<int> v;
        void f() const {
            std::vector<int>::const_iterator it = this->v.begin();
        }
        void g(std::vector<int>::const_iterator& it) {}
};

const makes this const, which makes this->v const.

You can usually forget about it with auto, but if you starting passing those iterators around, you will need to think about them for the method signatures.

Much like const and non-const, you can convert easily from non-const to const, but not the other way around:

std::vector<int> v{0};
std::vector<int>::iterator it = v.begin();

// non-const to const.
std::vector<int>::const_iterator cit = it;

// Compile time error: cannot modify container with const_iterator.
//*cit = 1;

// Compile time error: no conversion from const to no-const.
//it = ci1;

Which one to use: analogous to const int vs int: prefer const iterators whenever you can use them (when you don't need to modify the container with them), to better document your intention of reading without modifying.

Upvotes: 10

Mr Coder
Mr Coder

Reputation: 8186

ok Let me explain it with very simple example first without using constant iterator consider we have collection of random integers collection "randomData"

    for(vector<int>::iterator i = randomData.begin() ; i != randomData.end() ; ++i)*i = 0;
for(vector<int>::const_iterator i = randomData.begin() ; i!= randomData.end() ; ++i)cout << *i;

As can be seen for writing/editing data inside collection normal iterator is used but for reading purpose constant iterator has been used . If you try using constant iterator in first for loop you will get error . As a thumb rule use constant iterator to read data inside collection .

Upvotes: 0

Magnus Andermo
Magnus Andermo

Reputation: 373

Unfortunaty, a lot of the methods for the STL containers takes iterators instead of const_iterators as parameters. So if you have a const_iterator, you can't say "insert an element before the element that this iterator points to" (saying such a thing is not conceptually a const violation, in my opinion). If you want do that anyway, you have to convert it to a non-const iterator using std::advance() or boost::next(). Eg. boost::next(container.begin(), std::distance(container.begin(), the_const_iterator_we_want_to_unconst)). If container is a std::list, then the running time for that call will be O(n).

So the universal rule to add const wherever it is "logical" to do so, is less universal when it comes to STL containers.

However, boost containers take const_iterators (eg. boost::unordered_map::erase()). So when you use boost containers you can be "const agressive". By the way, do anyone know if or when the STL containers will be fixed?

Upvotes: 8

Trey Jackson
Trey Jackson

Reputation: 74430

(as others have said) const_iterator doesn't allow you modify the elements to which it points, this is useful inside of const class methods. It also allows you to express your intent.

Upvotes: 0

Naveen
Naveen

Reputation: 73433

Use const_iterator whenever you can, use iterator when you have no other choice.

Upvotes: 7

Dominic Rodger
Dominic Rodger

Reputation: 99751

const_iterators don't allow you to change the values that they point to, regular iterators do.

As with all things in C++, always prefer const, unless there's a good reason to use regular iterators (i.e. you want to use the fact that they're not const to change the pointed-to value).

Upvotes: 162

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