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c++iterator
roulette01
roulette01

Reputation: 2462

What does operator++ do on a forward iterator

From my understand, std::unordered_map and std::unordered_set use forward iterators.

I could do something like this

auto it = unordered_map_instance.begin(); 
it++;

But I can't do something like this

auto it = unordered_map_instance.begin(); 
it = it + 1;

I've always thought ++ is simply just incrementing by 1, but that doesn't appear to be so with forward iterators. Could someone explain what ++ does for forward iterators? I can't seem to google this operator. +

Upvotes: 1

Views: 118

Answers (1)

ChrisMM
ChrisMM

Reputation: 10020

If you refer to [iterator.requirements] from the C++ standard, iterator requires the operator++() (and operator++(int) for most types) to be overloaded. These are not to be confused with operator+(int), which would be the equivalent of your second example. The ++ operator is not the same as addition.

As to what the operator++() actually does, is it simply moves ahead by one in the list, i.e. to the next element. For example:

std::map<int,int> map_instance = { { 1,1 }, {2,2}, {3,3} };
auto it = map_instance.begin(); // it points to {1,1}
it++;  // now it points to {2,2}

Incrementing by more than one can be done with std::advance

Upvotes: 4

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