Intel Threading Building Blocks

Switching from an STL map

Now we’ll change the definition of the table from an STL map to a Threading Building Blocks concurrent_hash_map. First, we include the Intel Threading Building Blocks concurrent_hash_map class at line 1 (Example 11-28) instead of the STL map class that was at line 2 in the original listing (Example 11-27). Next, we define a traits class for the map, which shows how the map will perform its two central functions: deriving the hash value and determining whether two values being compared are equal. MyHashCompare, defined at line 4, provides the hash function to use in mapping, as well as a function that evaluates the equality of two keys.

The typedef on line 19 of the new example replaces the typedef on line 16 in the original listing for the StringTable type.

Example 11-28. Converting to use concurrent_hash_map

 1 #include <tbb/concurrent_hash_map.h>
 2
 3 //! Structure that defines hashing and comparison operations for user's type.
 4 struct MyHashCompare {
 5
 6     static size_t hash( const string& x ) {
 7         size_t h = 0;
 8         for( const char* s = x.c_str( ); *s; s++ )
 9             h = (h*17)^*s;
10         return h;
11     }
12     //! True if strings are equal
13         static bool equal( const string& x, const string& y ) {
14           return x==y;
15         }
16     };
17
18 //! maps strings to ints.
19 typedef concurrent_hash_map<string,int,MyHashCompare> StringTable;

In the next version (Example 11-29), we remove the coarse-grained lock code from Tally::operator() and use an accessor instead to gain exclusive access ...