Is for(auto i : unordered_map) guaranteed to have the same order every time?

Question

When I iterate over a std::unordered_map with the range based for loop twice, is the order guaranteed to be equal?

std::unordered_map<std::string, std::string> map;  std::string query = "INSERT INTO table ("; bool first = true; for(auto i : map) {     if(first) first = false;     else query += ", ";     query += i.first; } query += ") ";  query += "VALUES ("; first = true; for(auto i : map) {     if(first) first = false;     else query += ", ";     query += i.second; } query += ");" 

In the example above, the resulting string should be in that form. Therefore, it is important that both times, the order of iteration is the same.

INSERT INTO table (key1, key2, key3) VALUES (value1, value2, value3); 

Is this guaranteed in C++?

Answer 1

The iteration order of unordered associative containers can only change when rehashing as a result of a mutating operation (as described in C++11 23.2.5/8). You are not modifying the container between iterations, so the order will not change.

Although the specification doesn't explicitly state that rehashing can't occur at any other time, doing so would invalidate all iterators over the container, making any iteration at all impossible.