15.6.4. map Associative Container

The map associative container (from header <map>) performs fast storage and retrieval of unique keys and associated values. Duplicate keys are not allowed—a single value can be associated with each key. This is called a one-to-one mapping. For example, a company that uses unique employee numbers, such as 100, 200 and 300, might have a map that associates employee numbers with their telephone extensions—4321, 4115 and 5217, respectively. With a map you specify the key and get back the associated data quickly. Providing the key in a map’s subscript operator [] locates the value associated with that key in the map. Insertions and deletions can be made anywhere in a map. If the order of the keys is not important, you can use unordered_map (header <unordered_map>) instead.

Figure 15.18 demonstrates a map and uses the same features as Fig. 15.17 to demonstrate the subscript operator. Lines 27–28 use the subscript operator of class map. When the subscript is a key that’s already in the map (line 27), the operator returns a reference to the associated value. When the subscript is a key that’s not in the map (line 18), the operator inserts the key in the map and returns a reference that can be used to associate a value with that key. Line 27 replaces the value for the key 25 (previously 33.333 as specified in line 16) with a new value, 9999.99. Line 28 inserts a new key–value pair in the map (called creating an association).

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 1   // Fig. 15.18: fig15_18.cpp
 2   // Standard Library class map class template.
 3   #include <iostream>
 4   #include <map> // map class-template definition
 5   using namespace std;
 6
 7   int main()
 8   {
 9      map< int, double, less< int > > pairs;
10
11      // insert eight value_type objects in pairs          
12      pairs.insert( make_pair( 15, 2.7 ) );                
13      pairs.insert( make_pair( 30, 111.11 ) );             
14      pairs.insert( make_pair( 5, 1010.1 ) );              
15      pairs.insert( make_pair( 10, 22.22 ) );              
16      pairs.insert( make_pair( 25, 33.333 ) );             
17      pairs.insert( make_pair( 5, 77.54 ) ); // dup ignored
18      pairs.insert( make_pair( 20, 9.345 ) );              
19      pairs.insert( make_pair( 15, 99.3 ) ); // dup ignored
20
21      cout << "pairs contains: Key Value ";
22
23      // walk through elements of pairs
24      for ( auto mapItem : pairs )
25         cout << mapItem.first << ' ' << mapItem.second << ' ';
26
27      pairs[ 25 ] = 9999.99; // use subscripting to change value for key 25
28      pairs[ 40 ] = 8765.43; // use subscripting to insert value for key 40
29
30      cout << " After subscript operations, pairs contains: Key Value ";
31
32      // use const_iterator to walk through elements of pairs
33      for ( auto mapItem : pairs )
34         cout << mapItem.first << ' ' << mapItem.second << ' ';
35
36      cout << endl;
37   } // end main

pairs contains:
Key     Value
5       1010.1
10      22.22
15      2.7
20      9.345
25      33.333
30      111.11

After subscript operations, pairs contains:
Key     Value
5       1010.1
10      22.22
15      2.7
20      9.345
25      9999.99
30      111.11
40      8765.43

Fig. 15.18. Standard Library map class template.

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