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Advanced lookup and insertion functions for associative containers

Advanced lookups
Advanced insertions
Positional insertions

Boost.Intrusive associative containers offer an interface similar to STL associative containers. However, STL's ordered and unordered simple associative containers (std::set, std::multiset, std::unordered_set and std::unordered_multiset) have some inefficiencies caused by the interface in several search, insertion or erasure functions (equal_range, lower_bound, upper_bound, find, insert, erase...): the user can only operate with value_type objects or (starting from C++11), heterogeneous comparison lookups which are not flexible enough as key_compare shall support the comparison between the provided key and value_type, which precludes the use of user-defined comparison objects that can partition the search in a compatible but advanced way.

To solve these problems, Boost.Intrusive containers offers functions where a key type different from key_type and a comparison object are provided by the user. This applies to: * equal_range * lower_bound * upper_bound * count * find * erase

Requirements for such functions are:

  • For unordered container the provided comparison and hashing function with the given key shall induce the same hash and equivalence as key_compare and hasher.
  • For ordered associative containers, lookup and erasure functions, the container to be searched shall be partitioned in regards to the supplied comparison object and key.

For more details, see Requires clauses of such functions in the reference.

Imagine that the object to be searched is quite expensive to construct (called Expensive in the example):

#include <boost/intrusive/set.hpp>
#include <boost/intrusive/unordered_set.hpp>
#include <cstring>

using namespace boost::intrusive;

// Hash function for strings
struct StrHasher
{
   std::size_t operator()(const char *str) const
   {
      std::size_t seed = 0;
      for(; *str; ++str)   boost::hash_combine(seed, *str);
      return seed;
   }
};

class Expensive : public set_base_hook<>, public unordered_set_base_hook<>
{
   std::string key_;
   // Other members...

   public:
   Expensive(const char *key)
      :  key_(key)
      {}  //other expensive initializations...

   const std::string & get_key() const
      {  return key_;   }

   friend bool operator <  (const Expensive &a, const Expensive &b)
      {  return a.key_ < b.key_;  }

   friend bool operator == (const Expensive &a, const Expensive &b)
      {  return a.key_ == b.key_;  }

   friend std::size_t hash_value(const Expensive &object)
      {  return StrHasher()(object.get_key().c_str());  }
};

// A set and unordered_set that store Expensive objects
typedef set<Expensive>           Set;
typedef unordered_set<Expensive> UnorderedSet;

// Search functions
Expensive *get_from_set(const char* key, Set &set_object)
{
   Set::iterator it = set_object.find(Expensive(key));
   if( it == set_object.end() )     return 0;
   return &*it;
}

Expensive *get_from_uset(const char* key, UnorderedSet &uset_object)
{
   UnorderedSet::iterator it = uset_object.find(Expensive (key));
   if( it == uset_object.end() )  return 0;
   return &*it;
}

If "key" c-string is quite long Expensive has to construct a std::string using heap memory. Like Expensive, many times the only member taking part in ordering issues is just a small part of the class. E.g.: with Expensive, only the internal std::string is needed to compare the object.

In both containers, if we call get_from_set/get_from_unordered_set in a loop, we might get a performance penalty, because we are forced to create a whole Expensive object to be able to find an equivalent one.

Sometimes the problem is not only performance-related, as we might not have enough information to construct the object but we might have enough information to find the object. In this case, a name is enough to search Expensive objects in the container but constructing an Expensive object might require more information that the searcher might not have.

To solve this, we can use the functions that take any type comparable with the value and a the 'compatible' comparison object (and hash, when the associative container is unordered) Let's see optimized search function:

// These compare Expensive and a c-string
struct StrExpComp
{
   bool operator()(const char *str, const Expensive &c) const
   {  return std::strcmp(str, c.get_key().c_str()) < 0;  }

   bool operator()(const Expensive &c, const char *str) const
   {  return std::strcmp(c.get_key().c_str(), str) < 0;  }
};

struct StrExpEqual
{
   bool operator()(const char *str, const Expensive &c) const
   {  return std::strcmp(str, c.get_key().c_str()) == 0;  }

   bool operator()(const Expensive &c, const char *str) const
   {  return std::strcmp(c.get_key().c_str(), str) == 0;  }
};

// Optimized search functions
Expensive *get_from_set_optimized(const char* key, Set &set_object)
{
   Set::iterator it = set_object.find(key, StrExpComp());
   if( it == set_object.end() )   return 0;
   return &*it;
}

Expensive *get_from_uset_optimized(const char* key, UnorderedSet &uset_object)
{
   UnorderedSet::iterator it = uset_object.find(key, StrHasher(), StrExpEqual());
   if( it == uset_object.end() )  return 0;
   return &*it;
}

A similar issue happens with insertions in simple ordered and unordered associative containers with unique keys (std::set and std::unordered_set). In these containers, if a value is already present, the value to be inserted is discarded. With expensive values, if the value is already present, we can suffer efficiency problems.

set and unordered_set-like containers have insertion functions (insert_check, insert_unique_check,...) to check efficiently, without constructing the value, if a value is present or not and if it's not present, a function to insert it immediately (insert_commit) without any further lookup. Requirements for functions that check the existence of such value are:

  • For unordered container the provided comparison and hashing function with the given key shall induce the same hash and equivalence as key_compare and hasher.
  • For ordered associative containers, the provided comparison function with the given key, shall induce the same strict weak order as key_compare.

To sum up, insert_check is similar to a normal insert but:

  • insert_check can be used with arbitrary keys
  • if the insertion is possible (there is no equivalent value) insert_check collects all the needed information in an insert_commit_data structure, so that insert_commit:
    • does not execute further comparisons
    • can be executed with constant-time complexity
    • has no-throw guarantee.

These functions must be used with care, no other insertion or erasure must be executed between an insert_check and an insert_commit pair. Otherwise, the behaviour is undefined.

See set and unordered_set-like containers' reference for more information about insert_check and insert_commit.

With multiple ordered and unordered associative containers (multiset and unordered_multiset) there is no need for these advanced insertion functions, since insertions are always successful.

For example, using the same Expensive class, this function can be inefficient:

// Insertion functions
bool insert_to_set(const char* key, Set &set_object)
{
   Expensive *pobject = new Expensive(key);
   bool success = set_object.insert(*pobject).second;
   if(!success)   delete pobject;
   return success;
}

bool insert_to_uset(const char* key, UnorderedSet &uset_object)
{
   Expensive *pobject = new Expensive(key);
   bool success = uset_object.insert(*pobject).second;
   if(!success)   delete pobject;
   return success;
}

If the object is already present, we are constructing an Expensive that will be discarded, and this is a waste of resources. Instead of that, let's use insert_check and insert_commit functions:

// Optimized insertion functions
bool insert_to_set_optimized(const char* key, Set &set_object)
{
   Set::insert_commit_data insert_data;
   bool success = set_object.insert_check(key, StrExpComp(), insert_data).second;
   if(success) set_object.insert_commit(*new Expensive(key), insert_data);
   return success;
}

bool insert_to_uset_optimized(const char* key, UnorderedSet &uset_object)
{
   UnorderedSet::insert_commit_data insert_data;
   bool success = uset_object.insert_check
      (key, StrHasher(), StrExpEqual(), insert_data).second;
   if(success) uset_object.insert_commit(*new Expensive(key), insert_data);
   return success;
}

Some ordered associative containers offer low-level functions to bypass ordering checks and insert nodes directly in desired tree positions. These functions are provided for performance reasons when values to be inserted in the container are known to fulfill order (sets and multisets) and uniqueness (sets) invariants. A typical usage of these functions is when intrusive associative containers are used to build non-intrusive containers and the programmer wants to speed up assignments from other associative containers: if the ordering and uniqueness properties are the same, there is no need to waste time checking the position of each source value, because values are already ordered: back insertions will be much more efficient.

Note: These functions don't check preconditions so they must used with care. They are low-level operations that will break container invariants if ordering and uniqueness preconditions are not assured by the caller.

Let's see an example:

#include <boost/intrusive/set.hpp>
#include <vector>
#include <functional>
#include <cassert>

using namespace boost::intrusive;

//A simple class with a set hook
class MyClass : public set_base_hook<>
{
   public:
   int int_;

   MyClass(int i) :  int_(i) {}
   friend bool operator< (const MyClass &a, const MyClass &b)
      {  return a.int_ < b.int_;  }
   friend bool operator> (const MyClass &a, const MyClass &b)
      {  return a.int_ > b.int_;  }
};

int main()
{
   //Create some ORDERED elements
   std::vector<MyClass> values;
   for(int i = 0; i < 100; ++i)  values.push_back(MyClass(i));

   {  //Data is naturally ordered in the vector with the same criteria
      //as multiset's comparison predicate, so we can just push back
      //all elements, which is more efficient than normal insertion
      multiset<MyClass> mset;
      for(int i = 0; i < 100; ++i)  mset.push_back(values[i]);

      //Now check orderd invariant
      multiset<MyClass>::const_iterator next(mset.cbegin()), it(next++);
      for(int i = 0; i < 99; ++i, ++it, ++next) assert(*it < *next);
   }
   {  //Now the correct order for the set is the reverse order
      //so let's push front all elements
      multiset<MyClass, compare< std::greater<MyClass> > > mset;
      for(int i = 0; i < 100; ++i)  mset.push_front(values[i]);

      //Now check orderd invariant
      multiset<MyClass, compare< std::greater<MyClass> > >::
         const_iterator next(mset.cbegin()), it(next++);
      for(int i = 0; i < 99; ++i, ++it, ++next) assert(*it > *next);
   }
   {  //Now push the first and the last and insert the rest
      //before the last position using "insert_before"
      multiset<MyClass> mset;
      mset.insert_before(mset.begin(), values[0]);
      multiset<MyClass>::const_iterator pos =
         mset.insert_before(mset.end(), values[99]);
      for(int i = 1; i < 99; ++i)  mset.insert_before(pos, values[i]);

      //Now check orderd invariant
      multiset<MyClass>::const_iterator next(mset.cbegin()), it(next++);
      for(int i = 0; i < 99; ++i, ++it, ++next) assert(*it < *next);
   }

   return 0;
}

For more information about advanced lookup and insertion functions see associative containers' documentation (e.g. set, multiset, unordered_set and unordered_multiset references).


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