CODEWITHSUNDEEP
c++11iteratorkeyunordered-map
actionjezus6
actionjezus6

Reputation: 47

std::unordered_map iterator returns erased keys, why and how to skip them

I am very new to C++ programming and have stumbled across a behaviour that confuses me and makes my coding harder. I have searched for answer a bit and could not find anything - I have also scrolled through C++ reference pages and that did not help either (please don't crucify me if the answer is in there - the page isn't role model for explaining things). Maybe I am missing something really obvious.

Could someone explain, the following behaviour of std::unordered_map ?

std::unordered_map<std::string, std::string> test_map;
test_map["test_key_1"] = "test_value_1";
test_map["test_key_2"] = "test_value_2";

std::cout <<  "'test_key_1' value: " << test_map["test_key_1"] << std::endl; // This returns "test_value_1"
std::cout << "test_map size before erase: " << test_map.size() << std::endl; // This returns 2

test_map.erase("test_key_1");

std::cout << "test_map size after erase: " << test_map.size() << std::endl; // This returns 1
std::cout << "'test_key_1' value after erase: " << test_map["test_key_1"] << std::endl; // This returns empty string
std::cout << "'non_existing_key' value: " << test_map["non_existing_key"] << std::endl; // This returns empty string

test_map.rehash(test_map.size()); // I am doing this because vague hints from internet, code behaves
                                  // same way without it.

for (std::unordered_map<std::string, std::string>::iterator it = test_map.begin();
     it != test_map.end(); ++it)
{
    std::cout << "Key: " << it->first << std::endl;
}
// Above loop return both 'test_key_1' and 'test_key_2'.
// WHY!?

Why iterator is returning items that were already erased ? How can I make iterator return only items that are present in map ? I will be grateful for any help, as I am really lost.

Upvotes: 0

Views: 474

Answers (1)

dewaffled
dewaffled

Reputation: 2963

You are using operator[] to access previously erased elements which

Returns a reference to the value that is mapped to a key equivalent to key, performing an insertion if such key does not already exist.

If you need just to search for given key, use find method that returns map.end() if element was not found.

Upvotes: 3

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