# More on HashCompare

There are several ways to make the `HashCompare` argument for `concurrent_hash_map` work for your own types.

- Specify the `HashCompare` argument explicitly

- Let the `HashCompare` default to `tbb_hash_compare<Key>` and do one of the following:

  - Define a specialization of template `tbb_hash_compare<Key>`.

  - Define an overload of function `tbb_hasher(Key)`.

  - Rely on the definitions of `tbb_hasher(Key)` provided by the library.

Function `tbb_hasher` is predefined for the following types:

- Types convertible to `size_t`, such as integral types.

- Pointer types.

- Instances of `std::basic_string`.

- `std::pair<Key1,Key2>`, where `tbb_hasher(Key)` and `tbb_hasher(Key2)` are defined.

For example, if you have keys of type `Foo`, and `operator==` is defined for `Foo`, you just have to provide a definition of `tbb_hasher` as shown below:

    size_t tbb_hasher(const Foo& f) {
        size_t h = ...compute hash code for f...
        return h;
    };

In general, the definition of `tbb_hash_compare<Key>` or `HashCompare` must provide two signatures:

- A method `hash` that maps a `Key` to a `size_t`

- A method `equal` that determines if two keys are equal

The signatures go together in a single class because if two keys are equal, then they must hash to the same value, otherwise the hash table might not work. You could trivially meet this requirement by always hashing to `0`, but that would cause tremendous inefficiency. Ideally, each key should hash to a different value, or at least the probability of two distinct keys hashing to the same value should be kept low.

The methods of `HashCompare` should be `static` unless you need to have them behave differently for different instances. If so, then you should construct the `concurrent_hash_map` using the constructor that takes a `HashCompare` as a parameter. The following example is a variation on an earlier example with instance-dependent methods. The instance performs both case-sensitive or case-insensitive hashing, and comparison, depending upon an internal flag `ignore_case`.

    // Structure that defines hashing and comparison operations
    class VariantHashCompare {
        // If true, then case of letters is ignored.
        bool ignore_case;
    public:
        size_t hash(const string& x) const {
            size_t h = 0;
            for(const char* s = x.c_str(); *s; s++)
                h = (h*16777179)^*(ignore_case?tolower(*s):*s);
            return h;
        }
        // True if strings are equal
        bool equal(const string& x, const string& y) const {
            if( ignore_case )
                strcasecmp(x.c_str(), y.c_str())==0;
            else
                return x==y;
        }
        VariantHashCompare(bool ignore_case_) : ignore_case(ignore_case_) {}
    };

    typedef concurrent_hash_map<string,int, VariantHashCompare> VariantStringTable;

    VariantStringTable CaseSensitiveTable(VariantHashCompare(false));
    VariantStringTable CaseInsensitiveTable(VariantHashCompare(true));

The directory `examples/concurrent_hash_map/count_strings` contains a complete example that uses `concurrent_hash_map` to enable multiple processors to cooperatively build a histogram.

Parent topic: concurrent_hash_map