cds::container::feldman_hashmap::make_traits< Options > Struct Template Reference

Metafunction converting option list to feldman_hashmap::traits. More...

#include <cds/container/details/feldman_hashmap_base.h>

Public Types
typedef implementation_defined	type
	Metafunction result.

Detailed Description

template<typename... Options>
struct cds::container::feldman_hashmap::make_traits< Options >

Metafunction converting option list to feldman_hashmap::traits.

Supported Options are:

• opt::hash - a hash functor, default is std::hash FeldmanHashMap may use any hash functor converting a key to fixed-sized bit-string, for example, SHA1, SHA2, MurmurHash, CityHash or its successor FarmHash.

If you use a fixed-sized key you can use it directly instead of a hash. In such case traits::hash should be specified as opt::none. However, if you want to use the hash values or if your key type is not fixed-sized you must specify a proper hash functor in your traits. For example: fixed-sized key - IP4 address map

// Key - IP address
struct ip4_address {
    uint8_t ip[4];
};

// IP compare
struct ip4_cmp {
    int operator()( ip4_address const& lhs, ip4_address const& rhs ) const
    {
        return memcmp( &lhs, &rhs, sizeof(lhs));
    }
};

// Value - statistics for the IP address
struct statistics {
    // ...
};

// Traits
// Key type (ip4_addr) is fixed-sized so we may use the map without any hash functor
struct ip4_map_traits: public cds::container::multilevl_hashmap::traits
{
    typedef ip4_cmp  compare;
};

// IP4 address - statistics map
typedef cds::container::FeldmanHashMap< cds::gc::HP, ip4_address, statistics, ip4_map_traits > ip4_map;

variable-size key requires a hash functor: URL map

// Value - statistics for the URL
struct statistics {
    // ...
};

// Traits
// Key type (std::string) is variable-sized so we must provide a hash functor in our traits
// We do not specify any comparing predicate (less or compare) so <tt> std::less<std::string> </tt> will be used by default
struct url_map_traits: public cds::container::multilevl_hashmap::traits
{
    typedef std::hash<std::string> hash;
};

// URL statistics map
typedef cds::container::FeldmanHashMap< cds::gc::HP, std::string, statistics, url_map_traits > url_map;

• feldman_hashmap::hash_size - the size of hash value in bytes. By default, the size of hash value is sizeof( hash_type ) where hash_type is type of hash() result or sizeof( key ) if you use fixed-sized key.

Sometimes that size is wrong, for example, for that 6-byte key:

struct key_type {
    uint32_t    key;
    uint16_t    subkey;
};

static_assert( sizeof( key_type ) == 6, "Key type size mismatch" );

Here sizeof( key_type ) == 8 so static_assert will be thrown.

For that case you can specify hash_size explicitly.

Value 0 means auto-calculated sizeof( key_type ).

• opt::allocator - item allocator Default is CDS_DEFAULT_ALLOCATOR
• opt::node_allocator - array node allocator. Allocator for array nodes. The allocator is used for creating headNode and arrayNode when the set grows. Default is CDS_DEFAULT_ALLOCATOR
• opt::compare - hash comparison functor. No default functor is provided. If the option is not specified, the opt::less is used.
• opt::less - specifies binary predicate used for hash comparison. If less and compare are not specified, memcmp() -like bit-wise hash comparator is used because the hash value is treated as fixed-sized bit-string.
• opt::back_off - back-off strategy used. If the option is not specified, the cds::backoff::Default is used.
• opt::item_counter - the type of item counting feature. The item counting is an important part of FeldmanHashSet algorithm: the empty() member function depends on correct item counting. Therefore, atomicity::empty_item_counter is not allowed as a type of the option.

Default is atomicity::item_counter. To avoid false sharing you can aldo use atomicity::cache_friendly_item_counter

• opt::memory_model - C++ memory ordering model. Can be opt::v::relaxed_ordering (relaxed memory model, the default) or opt::v::sequential_consistent (sequentially consisnent memory model).
• opt::stat - internal statistics. By default, it is disabled (feldman_hashmap::empty_stat). To enable it use feldman_hashmap::stat
• opt::rcu_check_deadlock - a deadlock checking policy for RCU-based FeldmanHashSet Default is opt::v::rcu_throw_deadlock

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The documentation for this struct was generated from the following file:

• cds/container/details/feldman_hashmap_base.h