C++ - Iterators

Basic Usage

An iterator needs to be dereferenced *it or it->first so its internal data can be accessed.

Note, ContiguousIterator is introduced in C++20 to provide a contiguous memory guarantee

To advance an iterator, the safest bet is std::advance(it).

Iterator In std::map, std::set, std::tuple

If you have an std::unordered_map<Key, Value> (or std::map<Key, Value>), its iterator points to a key-value pair, represented as std::pair<const Key, Value>.

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auto it = my_map.find("some_key");
std::cout << (*it).first << " : " << (*it).second << std::endl; // see "some key", and value
std::cout << it->first << " : " << it->second << std::endl;

• std::set, std::unordered_set: Iterator points directly to the value (the key itself).

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std::unordered_set<int> s;
auto it = s.begin();
int value = *it;
int value2 = it.operator*();
int value3 = *it;         // same thing
int value4 = it->second;  // ❌ doesn't work! No .second — it's just a value

• std::vector<T> / std::deque<T> / std::list<T>:

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std::vector<int> v;
auto it = v.begin();
*it

• std::vector<std::pair<» or similar

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std::vector<std::pair<int, std::string>> vp;
auto it = vp.begin();
it->first    // valid!
it->second   // also valid!

• std::tuple in containers

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std::vector<std::tuple<int, std::string>> vt;
auto it = vt.begin();
std::get<0>(*it);  // get the first item
std::get<1>(*it);  // get the second item

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