What's the difference between std::multimap<key, value> and std::map<key, std::set<value> >

Question

I found that they have one key and multiple values which is unique.

Solution 1

The multimap stores pairs of (key, value) where both key and value can appear several times.

The map<key, set<value>> will only store each value once for a specific key. To do that, it will have to be able to compare the values, not just the keys.

It depends on your application if the values that compare equal are equivalent, or if you wish to store them separately anyway. Perhaps they contain fields that are different but do not take part in the comparison for the set.

Solution 2

A std::map is an associative container, that allows you to have a unique key associated with your type value. For example,

void someFunction()
{
    typedef std::map<std::string, int> MapType;
    MapType myMap;

    // insertion
    myMap.insert(MapType::value_type("test", 42));
    myMap.insert(MapType::value_type("other-test", 0));

    // search
    auto it = myMap.find("test");
    if (it != myMap.end())
        std::cout << "value for " << it->first << " is " << it->second << std::endl;
    else
        std::cout << "value not found" << std::endl;
}

A std::multimap is equal to a std::map, but your keys are not unique anymore. Therefore you can find a range of items instead of just find one unique item. For example,

void someFunction()
{
    typedef std::multimap<std::string, int> MapType;
    MapType myMap;

    // insertion
    myMap.insert(MapType::value_type("test", 42));
    myMap.insert(MapType::value_type("test", 45));
    myMap.insert(MapType::value_type("other-test", 0));

    // search
    std::pair<auto first, auto second> range = myMap.equal_range("test");
    for (auto it = range.first; it != range.second; ++it)
        std::cout << "value for " << it->first << " can be " << it->second << std::endl;
}

The std::set is like an std::map, but it is not storing a key associated to a value. It stores only the key type, and assures you that it is unique within the set.

You also have the std::multiset, that follows the same pattern.

All these containers provide an O(log(n)) access with their find / equal_range.

Solution 3

map::insert

Because map containers do not allow for duplicate key values, the insertion operation checks for each element inserted whether another element exists already in the container with the same key value if so, the element is not inserted and its mapped value is not changed in any way.

on the other hand

multimap::insert 

can insert any number of items with the same key.

http://www.cplusplus.com/reference/stl/map/
http://www.cplusplus.com/reference/stl/multimap/

Solution 4

The latter requires that the values can be ordered (either via operator< or a comparison-function), the former does not.

View more solutions

Comments

[大宝剑]

So, a std::multimap<key, value> is like a std::map<key, std::multiset<value> >, the difference between them is that the later's values are sorted. Is that right?

[johnbakers]

It would appear operator< works the same on either map or multimap? en.cppreference.com/w/cpp/container/map/operator_cmp

[Björn Pollex]

Yes, but my answer referred to the ordering of the values. Suppose you have a type T that as no ordering. You can use it to create an std::multimap<U, T>, but you cannot use to the create an std::map<U, std::set<T> >.

[vancexu]

In multimap function, this line std::pair<auto first, auto second> range = myMap.equal_range("test"); doesn't work, because error: 'auto' not allowed in template argument. Use const auto range = myMap.equal_range("test") instead.

[Rndp13]

Good link on the both the difference and how it works internally. link

[lolololol ol]

mapType? Shouldn't it be MapType on line 4?

[463035818_is_not_a_number]

not sure who was first, but one is obviously a copy paste of the other : cppbuzz.com/What-is-difference-between-map-and-multimap

[typedef]

ahah, cppbuzz is scraping StackOverflow or what?, I wrote this answer myself years ago when I was still daily coding in c++. And there is indeed a typo line 4, thanks @lololololol

[typedef]

(and their copy/paste failed, they do not even display types in template std::map declaration : std::map<std::string, int>)

[Yixing Liu]

No, std::multimap<key, value> allows the same key to appear multiple times whereas std::map<key, whatever> requires the uniqueness of key.

[L. F.]

@typedef Someone else scraping your work is a serious problem. In such cases, the correct action is to use the "contact us" form. See meta.stackexchange.com/a/200178.