A concurrent_hash_map<Key,T,HashCompare> is a hash table that permits concurrent accesses. The table is a map from a key to a type T. The HashCompare traits type defines how to hash a key and how to compare two keys.

Example 5-4 builds a concurrent_hash_map in which the keys are strings and the corresponding data is the number of times each string occurs in the array Data.

#include "tbb/concurrent_hash_map.h"
#include "tbb/blocked_range.h"
#include "tbb/parallel_for.h"
#include <string>

using namespace tbb;
using namespace std;

// Structure that defines hashing and comparison operations for user's type.
struct MyHashCompare {
    static size_t hash( const string& x ) {
        size_t h = 0;
        for( const char* s = x.c_str(); *s; ++s )
            h = (h*17)^*s;
        return h;
    }
    //! True if strings are equal
    static bool equal( const string& x, const string& y ) {
        return x==y;
    }
};

// A concurrent hash table that maps strings to ints.
typedef concurrent_hash_map<string,int,MyHashCompare> StringTable;

// Function object for counting occurrences of strings.
struct Tally {
    StringTable& table;
    Tally( StringTable& table_ ) : table(table_) {}
    void operator()( const blocked_range<string*> range ) const {
        for( string* p=range.begin(); p!=range.end(); ++p ) {
            StringTable::accessor a;
            table.insert( a, *p );
            a->second += 1;
        }
    }
};

const size_t N = 1000000;

string Data[N];

void CountOccurrences() {
    // Construct empty table.
    StringTable table;

    // Put occurrences into the table
      parallel_for( blocked_range<string*>( Data, Data+N, 100 ),
                                                  Tally(table) );

    // Display the occurrences
    for( StringTable::iterator i=table.begin(); i!=table.end(); ++i )
        printf("%s %d
",i->first.c_str(),i->second);
}

A concurrent_hash_map acts as a container of elements of type std::pair<const Key,T>. Typically, when accessing a container element, you are interested in either updating it or reading it. The template class concurrent_hash_map supports these two operations with the accessor and const_accessor classes, respectively, which act as smart pointers.

An accessor represents update (write) access. As long as it points to an element, all other attempts to look up that key in the table block until the accessor is done. A const_accessor is similar, except that it represents read-only access. Therefore, multiple const_accessors can point to the same element at the same time. This feature can greatly improve concurrency in situations where elements are frequently read and infrequently updated.

The find and insert methods take an accessor or const_accessor as an argument. The choice tells concurrent_hash_map whether you are asking for update or read-only access, respectively. Once the method returns, the access lasts until the accessor or const_accessor is destroyed.

Because having access to an element can block other threads, try to shorten the lifetime of the accessor or const_accessor. To do so, declare it in the innermost block possible. To release access even sooner than the end of the block, use the release method.

Example 5-5 is a rework of the loop body that uses release instead of waiting for the destruction of the accessor for the lock to be released.

StringTable accessor a;
for( string* p=range.begin(); p!=range.end(); ++p ) {
    table.insert( a, *p );
    a->second += 1;
    a.release();
}

The method remove(key) can also operate concurrently. It implicitly requests write access. Therefore, before removing the key, it waits on any other extant accesses on the key.