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you can define your own hash function, equal comparison function, and allocator function, respectively. These parameters can usually be ignored since they have default values. We recommend you keep those default values. For example, it’s not trivial to implement your own good hash function. The most important parameters are the first two. As with maps, C++11 uniform initialization can be used to initialize the unordered_map with elements as shown in the following example:

unordered_map m = {

{1, "Item 1"},

{2, "Item 2"},

{3, "Item 3"},

{4, "Item 4"}

};

for (auto& p : m)

cout << p.first << " = " << p.second << endl;

Code snippet from unordered_map\unordered_map.cpp

The following table summarizes the differences between a map and an unordered_map.

OPERATION map unordered_map

at() x x

begin() x x

bucket() x

bucket_count() x

bucket_size() x

cbegin() x x

cend() x x

clear() x x

count() x x

crbegin() x

crend() x

emplace() x x

emplace_hint() x x

empty() x x

end() x x

equal_range() x x

erase() x x

find() x x

insert() x x

iterator / const_iterator x x

load_factor() x

local_iterator / const_local_iterator x

lower_bound() x

max_bucket_count() x

max_load_factor() x

operator[] x x

rbegin() x

rehash() x

rend() x

reserve() x

reverse_iterator / const_reverse_iterator x

size() x x

swap() x x

upper_bound() x

As with a normal map, all keys in an unordered_map should be unique. The preceding table includes a number of hash specific methods. For example, load_factor() returns the average number of elements per bucket to give you an indication on the number of collisions. The bucket_count() method returns the number of buckets in the container. It also provides a local_iterator and const_local_iterator allowing you to iterate over the elements in a single bucket; but, these may not be used to iterate across buckets. The bucket(key) method returns the index of the bucket that contains the given key; begin(n) returns a local_iterator referring to the first element in the bucket with index n, and end(n) returns a local_iterator referring to one-past-the-last element in the bucket with index n. The example in the next section will make things more clear on how to use these methods.

unordered_map Example: Phone Book

The following example uses an unordered_map to represent a phone book. The name of a person will be the key while the phone number will be the value associated with that key.

template

void printMap(const T& m)

{

for (auto& p : m) {

cout << p.first << "(Phone: " << p.second << ")" << endl;

}

cout << "-------" << endl;

}

int main()

{

// Create a hash table.

unordered_map um;

um.insert({

{"Marc G.", "123-456789"},

{"Zulija N.", "987-654321"},

{"Scott K.", "654-987321"} });

printMap(um);

// Add/remove some phone numbers.

um.insert(make_pair("John D.", "321-987654"));

um["Johan G."] = "963-258147";

um["Freddy K."] = "999-256256";

um.erase("Freddy K.");

printMap(um);

// Find the bucket index for a specific key.

int bucket = um.bucket("Marc G.");

cout << "Marc G. is in bucket " << bucket

<< " which contains the following "

<< um.bucket_size(bucket) << " elements:" << endl;

// Get begin and end iterators for the elements in this bucket.

// 'auto' is being used here. The compiler will derive the type

// of both iterators as

// unordered_map::const_local_iterator

auto liter = um.cbegin(bucket);

auto literEnd = um.cend(bucket);

while (liter != literEnd) {

cout << "\t" << liter->first << "(Phone: "

<< liter->second << ")" << endl;

++liter;

}

cout << "-------" << endl;

// Print some statistics about the hash table

cout << "There are " << um.bucket_count() << " buckets." << endl;

cout << "Average number of elements in a bucket is "

<< um.load_factor() << endl;

return 0;

}

Code snippet from PhoneBook\PhoneBook.cpp

This example uses a lot of C++11 features: unordered_map, range-based for loop, auto and initializer_lists. The output of this code is as follows:

Zulija N.(Phone: 987-654321)

Marc G.(Phone: