Specify the minimum number of buckets when constructing a boost::unordered_map

Question

I am trying to use boost::unordered_map to cache some values. I try to specify minimum number of buckets in the constructor:

#include <boost/unordered_map.hpp>
typedef boost::unordered_map<float, float> Mycache;
Mycache cache((std::size_t)25165843,
              boost::hash<float>(),
              std::equal_to<float>(),
              std::allocator<std::pair<float const, float> >());

But when I display information about my unordered_map at the end of program:

g++:

unordered_map.size(): 15861612
unordered_map.load_factor: 10.0845
unordered_map.bucket_count: 1572869
unordered_map.max_size: 1572868
unordered_map.max_load_factor: 1
unordered_map.max_bucket_count: 1572869

vc++:

unordered_map.size(): 13916119
unordered_map.load_factor: 8.8476
unordered_map.bucket_count: 1572869
unordered_map.max_size: 1572868
unordered_map.max_load_factor: 1
unordered_map.max_bucket_count: 1572869

How do I specify the minimum number of buckets ?

Answer 1

boost::unordered_map::max_bucket_count() returns the implementation-dependent limit on the bucket count of an unordered_map. You appear to have exceeded this limit with your constructor parameter. Note that while MSDN defines this to be the max buckets "currently" permitted (whatever that means), the C++0x spec defines it to be the most buckets the map can ever have.

I've never used the class, and I can't see anything in the draft C++0x spec to explain why the constructor is silently creating an object which doesn't do what you told it to.

I also don't know what the motivation might be behind the value 1572869, other than that it's a largeish prime.

Answer 2

The other answer is correct about the standard, but the small max_bucket_count is actually a bug in Boost 1.38, any other version will let you use more buckets.