11.3.4. Subscripting a map

The map and unordered_map containers provide the subscript operator and a corresponding at function (§ 9.3.2, p. 348), which are described in Table 11.6 (overleaf). The set types do not support subscripting because there is no “value” associated with a key in a set. The elements are themselves keys, so the operation of “fetching the value associated with a key” is meaningless. We cannot subscript a multimap or an unordered_multimap because there may be more than one value associated with a given key.

Table 11.6. Subscript Operation for map and unordered_map

Like the other subscript operators we’ve used, the map subscript takes an index (that is, a key) and fetches the value associated with that key. However, unlike other subscript operators, if the key is not already present, a new element is created and inserted into the map for that key. The associated value is value initialized (§ 3.3.1, p. 98).

For example, when we write

map <string, size_t> word_count; // empty map
// insert a value-initialized element with key Anna; then assign 1 to its value
word_count["Anna"] = 1;

the following steps take place:

• word_count is searched for the element whose key is Anna. The element is not found.

• A new key-value pair is inserted into word_count. The key is a const string holding Anna. The value is value initialized, meaning in this case that the value is 0.

• The newly inserted element is fetched and is given the value 1.

Because the subscript operator might insert an element, we may use subscript only on a map that is not const.

Using the Value Returned from a Subscript Operation

Another way in which the map subscript differs from other subscript operators we’ve used is its return type. Ordinarily, the type returned by dereferencing an iterator and the type returned by the subscript operator are the same. Not so for maps: when we subscript a map, we get a mapped_type object; when we dereference a map iterator, we get a value_type object (§ 11.3, p. 428).

In common with other subscripts, the map subscript operator returns an lvalue (§ 4.1.1, p. 135). Because the return is an lvalue, we can read or write the element:

cout << word_count["Anna"]; // fetch the element indexed by Anna; prints 1
++word_count["Anna"];       // fetch the element and add 1 to it
cout << word_count["Anna"]; // fetch the element and print it; prints 2

The fact that the subscript operator adds an element if it is not already in the map allows us to write surprisingly succinct programs such as the loop inside our word-counting program (§ 11.1, p. 421). On the other hand, sometimes we only want to know whether an element is present and do not want to add the element if it is not. In such cases, we must not use the subscript operator.