#include <set>
    size_type count( const key_type& key ) const;

The function count() returns the number of occurrences of key in the set, which is always 0 or 1. count() should run in logarithmic time.

For example, the following code uses count() to determine if elements are contained in the set:

     // Create a set of characters
     set<char> charSet;
     const char* s = "Hello There";
     for( int i=0; i < strlen(s); i++ ) {
       charSet.insert( s[i] );
     // Display the set
     cout << charSet.count('A');
     cout << charSet.count('T');
     // output is "01" (the characters in the set are " HTehlor")