11.2.1. Defining an Associative Container

As we’ve just seen, when we define a map, we must indicate both the key and value type; when we define a set, we specify only a key type, because there is no value type. Each of the associative containers defines a default constructor, which creates an empty container of the specified type. We can also initialize an associative container as a copy of another container of the same type or from a range of values, so long as those values can be converted to the type of the container. Under the new standard, we can also list initialize the elements:

map<string, size_t> word_count;  // empty
// list initialization
set<string> exclude = {"the", "but", "and", "or", "an", "a",
                       "The", "But", "And", "Or", "An", "A"};
// three elements; authors maps last name to first
map<string, string> authors = { {"Joyce", "James"},
                                {"Austen", "Jane"},
                                {"Dickens", "Charles"} };

As usual, the initializers must be convertible to the type in the container. For set, the element type is the key type.

When we initialize a map, we have to supply both the key and the value. We wrap each key–value pair inside curly braces:

{key, value}

to indicate that the items together form one element in the map. The key is the first element in each pair, and the value is the second. Thus, authors maps last names to first names, and is initialized with three elements.

Initializing a multimap or multiset

The keys in a map or a set must be unique; there can be only one element with a given key. The multimap and multiset containers have no such restriction; there can be several elements with the same key. For example, the map we used to count words must have only one element per given word. On the other hand, a dictionary could have several definitions associated with a particular word.

The following example illustrates the differences between the containers with unique keys and those that have multiple keys. First, we’ll create a vector of ints named ivec that has 20 elements: two copies of each of the integers from 0 through 9 inclusive. We’ll use that vector to initialize a set and a multiset:

// define a vector with 20 elements, holding two copies of each number from 0 to 9
vector<int> ivec;
for (vector<int>::size_type i = 0; i != 10; ++i) {
    ivec.push_back(i);
    ivec.push_back(i);  // duplicate copies of each number
}
// iset holds unique elements from ivec; miset holds all 20 elements
set<int> iset(ivec.cbegin(), ivec.cend());
multiset<int> miset(ivec.cbegin(), ivec.cend());
cout << ivec.size() << endl;    // prints 20
cout << iset.size() << endl;    // prints 10
cout << miset.size() << endl;   // prints 20

Even though we initialized iset from the entire ivec container, iset has only ten elements: one for each distinct element in ivec. On the other hand, miset has 20 elements, the same as the number of elements in ivec.