CODEWITHSUNDEEP
c++iterator
dtgee
dtgee

Reputation: 1272

Why does an iterator operator + return a copy?

If you had an iterator vector<int>::iterator i = vector.begin(), i++ moves the actual iterator down. But why does something like

i = i + 3

give you a new iterator three doors down?

Upvotes: 4

Views: 179

Answers (4)

Sarfaraz Nawaz
Sarfaraz Nawaz

Reputation: 361302

It is because x + n should not change the value of x irrespective of what x is, whether it is int or iterator. The idea is same.

However if you don't want to write this:

it = it + 3;

then you have an alternative, you could write this:

std::advance(it, 3);

Note that in case of some standard containers which do not support random access iterator1, you cannot write it = it + 3, but you can still write std::advance(it,3). For example:

std::list<int>::iterator it = lst.begin();

it  = it + 3; //COMPILATION ERROR. `it` is not random access iterator

std::advance(it,3); //okay

So in such cases, std::advance(it,3) is the only way (or else you've to write such functionality yourself).

1. Note that std::vector<T>::iterator is random access iterator, that is why you can write it+3.

Upvotes: 1

Benjamin Lindley
Benjamin Lindley

Reputation: 103693

When you use operator+, you don't expect either of the operands to be modified. So that means a new object must be created. Just like if you did this:

int a = 5;
int b = a + 3;

You would still expect a to be equal to 5.

Upvotes: 4

Robᵩ
Robᵩ

Reputation: 168616

So that you can write:

j = i + 3;

If operator+ didn't create a new copy, what would it do? Modify i?

Upvotes: 1

Joseph Mansfield
Joseph Mansfield

Reputation: 110648

To mimic the natural behaviour that one would expect from +. The same way that in:

int x = 0;
int y = x + 3;

The second line doesn't change x, it just evaluates to the value of 3. However, x++ would modify x.

If you want to advance a generic iterator, you should use std::advance(i, 3) (it will do i += 3 on a Random Access Iterator and i++ three times on any other).

Upvotes: 6

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