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++?

Please tell me that you are protecting against SQL injection somewhere. — D.Shawley, Aug 18 '13 at 16:40
@D.Shawley I don't at the moment. But it is about saves of a computer game. I code the game and there is no way to inject SQL from outside of the application code at all. — danijar, Aug 18 '13 at 16:44
I just cringe any time that I see building SQL strings by raw string concatenation. — D.Shawley, Aug 18 '13 at 19:50
@D.Shawley That is totally understandable in a context where requests over networks can cause database accesses. — danijar, Aug 18 '13 at 21:55
It's not always data from a untrusted source. I've had applications crash based on things like setting the player's name to stuff that isn't SQL safe. — D.Shawley, Aug 19 '13 at 01:58
@danijar Do it anyway. You don't want "INSERT BIG COMPANY NAME HERE HACKED BECAUSE DIDN'T THINK DATABASE WARRANTED SQL PROTECTION" — noɥʇʎԀʎzɐɹƆ, Dec 15 '15 at 02:37

score 40 · Accepted Answer · answered Aug 18 '13 at 16:48

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.

score 17 · Answer 2 · edited May 23 '17 at 12:25

17

Why not build them together?

for(auto i : map)
{
    if(first) first = false;
    else{
        keys += ", ";
        query += ", ";
    }
    keys += i.first;

    values += i.second;
}

std::string query = "INSERT INTO table (" + keys + ") VALUES (" + values ")";

Looks nicer too imo.

Please note, if this section is performance critical, you could consider optimizing the string building process with std::stringstream as shown here, though it is not clear how much that might help

edited May 23 '17 at 12:25

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answered Aug 18 '13 at 16:37

Karthik T

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Consider using `ostringstream` instead, but this is the better approach. – D.Shawley Aug 18 '13 at 16:39
@D.Shawley yes probably if this is a performance intensive section. Else this might be better purely on readability grounds, either way I was mainly focusing on the fix to his problem. – Karthik T Aug 18 '13 at 16:41
Great! I could even cut that down, because the first key and value is given. `std::string keys = "id", values = to_string(Id); for(auto i : serialized) keys += ", " + i.first, values += ", " + i.second;`. – danijar Aug 18 '13 at 16:50

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

2 Answers2