Are boost multi_index extracted keys cached?

Question

I am using boost::multi_index with a data type I'd like to index based on its size. However, the size() member function of this data type is expensive to execute. Does multi_index cache the values it gets from its key extractors?

For example, if I created a multi_index container with an ordered index with a member function key (element.size()), and inserted an element whose size put it somewhere in the middle of the container, would the container re-invoke the size() member function on all the elements it visits while traversing its internal data structure before finding the right insertion point?

Answer 1

Well, the documentation for member function indexers says they call the referenced member function: http://www.boost.org/doc/libs/1_46_0/libs/multi_index/doc/reference/key_extraction.html#key_extractors

But when in doubt, profile:

#include <boost/multi_index_container.hpp>
#include <boost/multi_index/mem_fun.hpp>
#include <boost/multi_index/indexed_by.hpp>
#include <boost/multi_index/hashed_index.hpp>
#include <boost/multi_index/ordered_index.hpp>
#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

namespace bmi = boost::multi_index;

int g_calls = 0;
struct A
{
  explicit A(int sz) : m_size(sz) { }
  int size() const { ++g_calls; return m_size; }
private:
  int m_size;
};

typedef boost::multi_index_container<
  A*,
  bmi::indexed_by<
    bmi::ordered_non_unique<
      BOOST_MULTI_INDEX_CONST_MEM_FUN(A,int,A::size)
    >
  >
> container_t;

int main()
{
  container_t cont;
  int n = 100;
  vector<int> o(2*n+1);
  for( int i = 0; i != 2*n+1; ++i )
    o[i] = i;
  random_shuffle(o.begin(), o.end());

  for( int i = 0; i != n; ++i )
    cont.insert(new A(o[i]));
  cout << "Calls to A::size(): "<< g_calls << endl;
  for( int i = n+1; i <= 2*n; ++i )
    cont.insert(new A(o[i]));
  cout << "Calls to A::size(): "<< g_calls << endl;
  cont.insert(new A(o[n]));
  cout << "Calls to A::size(): "<< g_calls << endl;
  for( int i = 0; i != o.size(); ++i )
    cont.find(o[i]);
  cout << "Calls after calling find " << o.size() << " times: "<< g_calls << endl;
  return 0;
}

Gives the following output (using Boost 1.46):

Calls to A::size(): 629
Calls to A::size(): 1465
Calls to A::size(): 1474
Calls after calling find 201 times: 3262

So, it appears the answer is no, it doesn't cache values.