15.6.2. set Associative Container

The set associative container (from header <set>) is used for fast storage and retrieval of unique keys. The implementation of a set is identical to that of a multiset, except that a set must have unique keys. Therefore, if an attempt is made to insert a duplicate key into a set, the duplicate is ignored; because this is the intended mathematical behavior of a set, we do not identify it as a common programming error. A set supports bidirectional iterators (but not random-access iterators). If the order of the keys is not important, you can use unordered_set (header <unordered_set>) instead. Figure 15.16 demonstrates a set of doubles.

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 1   // Fig. 15.16: fig15_16.cpp
 2   // Standard Library set class template.
 3   #include <iostream>
 4   #include <array>
 5   #include <set>
 6   #include <algorithm>
 7   #include <iterator> // ostream_iterator
 8   using namespace std;
 9
10   int main()
11   {
12      const size_t SIZE = 5;
13      array< double, SIZE > a = { 2.1, 4.2, 9.5, 2.1, 3.7 };
14      set< double, less< double > > doubleSet( a.begin(), a.end() );
15      ostream_iterator< double > output( cout, " " );
16
17      cout << "doubleSet contains: ";
18      copy( doubleSet.begin(), doubleSet.end(), output );
19
20      // insert 13.8 in doubleSet; insert returns pair in which
21      // p.first represents location of 13.8 in doubleSet and  
22      // p.second represents whether 13.8 was inserted         
23      auto p = doubleSet.insert( 13.8 ); // value not in set   
24      cout << " " << *( p.first )
25         << ( p.second ? " was" : " was not" ) << " inserted";
26      cout << " doubleSet contains: ";
27      copy( doubleSet.begin(), doubleSet.end(), output );
28
29      // insert 9.5 in doubleSet                          
30      p = doubleSet.insert( 9.5 ); // value already in set
31      cout << " " << *( p.first )
32         << ( p.second ? " was" : " was not" ) << " inserted";
33      cout << " doubleSet contains: ";
34      copy( doubleSet.begin(), doubleSet.end(), output );
35      cout << endl;
36   } // end main

doubleSet contains: 2.1 3.7 4.2 9.5

13.8 was inserted
doubleSet contains: 2.1 3.7 4.2 9.5 13.8

9.5 was not inserted
doubleSet contains: 2.1 3.7 4.2 9.5 13.8

Fig. 15.16. Standard Library set class template.

Line 14 creates a set of doubles ordered in ascending order, using the function object less<double>. The constructor call takes all the elements in array a and inserts them into the set. Line 18 uses algorithm copy to output the contents of the set. Notice that the value 2.1—which appeared twice in array—appears only once in doubleSet. This is because container set does not allow duplicates.

Line 23 defines and initializes a pair to store the result of a call to set function insert. The pair returned consists of a const_iterator pointing to the item in the set inserted and a bool value indicating whether the item was inserted—true if the item was not in the set; false if it was.

Line 23 uses function insert to place the value 13.8 in the set. The returned pair, p, contains an iterator p.first pointing to the value 13.8 in the set and a bool value that’s true because the value was inserted. Line 30 attempts to insert 9.5, which is already in the set. The output shows that 9.5 was not inserted again because sets don’t allow duplicate keys. In this case, p.first in the returned pair points to the existing 9.5 in the set.

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